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Flowering sex ratios of d.ioecious plants are commonly male-biased but rarely female-biased. While greater costs of reproduction from females have been repeatedly demonstrated and explain male biases, male reproductive costs almost never exceed female costs, making the origins of female biases enigmatic. I investigated the seagrasses Phyllospadix scouleri and P. serrulatus (surfgrasses), which have some of the most extreme female-biased sex ratios documented (>90% female), to identify the mechanisms driving sex ratio bias. I developed sex-linked amplified fragment length polymorphism (AFLP) markers and applied them to three P. scouleri life stages at four sites to determine when during the life cycle sex ratio bias arises. Sex ratios were even among seedlings but became more female-biased at later life stages, indicating that sex ratios were driven by male-biased mortality. To identify when during the life cycle sex ratio bias developed, I examined sex differences in survival among seedlings and three aspects of reproductive costs that could potentially generate biased sex ratios under field conditions. No differences in seedling survival between the sexes were detected, and there was no evidence of substantial sex differences in costs of reproduction. I found no support for a trade-off between current and future reproduction or between reproductive investment and growth. Thus, costs of reproduction appear unlikely to drive sex ratio bias in surfgrass. Instead, small sex differences in growth and survival spread across the life cycle appear to be responsible for female-biased sex ratios and suggest that life history trade-offs other than reproductive costs drive sex ratio bias. Key words: cost of reproduction; dioecy; marine angiosperm; Olympic Peninsula, Washington, USA; Phyllospadix scouleri; Phyllospadix serrulatus; seagrass; seedling survivorship; sex-linked genetic marker; sex ratio.

Theory suggests that iteroparity may confer greater fitness than semelparity in situations in which temporal environmental variation is high and unpredictable. Variable age-specific mortality, density dependence, and other factors may also favor iteroparity over semelparity. Here, we empirically test the adaptive benefits of greater numbers of reproductive years in a study of reproductive schedules in an experimental population of a short-lived polycarpic perennial, Plantago lanceolata. A large experimental population was established that included four cohorts with similar genetic structure. Individuals were censused for mortality, size, and reproduction for seven years. Plants experienced variable numbers of reproductive years, but one or two years were most common (~46.7% of the population reproduced only once). The probability of flowering at least once prior to death was determined strongly by extrinsic, environmental or intrinsic but environmentally influenced variables, including early-life size, cohort, and block, but also varied with a number of interactions involving paternal lineage. Maternal effects explained small but significant components of the variance in the number of reproductive years among individuals in each cohort, while paternal effects were significant in only two cohorts. Number of reproductive years contributed significantly to fitness in this system, more so than all other variables tested, although most of the variation in relative fitness may be attributed ultimately to environmental influences. We suggest that the high proportion of each cohort composed of plants reproducing only once may be due to environmental constraints on either growth or size. Such environmental influences, particularly on early life size, may result in small but important indirect effects on fitness. Key words: inflorescence; iteroparity; Plantago lanceolata; reproductive schedule; reproductive timing; semelparity; Virginia, USA.

Development of theory has outpaced experimental tests for most maintenance of diversity mechanisms. Here we demonstrate how data from biodiversity--ecosystem functioning experiments can be used to determine the mechanisms that maintain plant species diversity. We hypothesized that grassland plant diversity is maintained by two classes of mechanisms: (1) equalizing mechanisms, which reduce asymmetric competition by reducing differences in monoculture biomass production among species in mixture, and (2) species interaction mechanisms, which increase overyielding by increasing niche partitioning and facilitation among species in mixture. Specifically, equalizing mechanisms reduce the coefficient of variation in monoculture biomass production among species in mixture. Species interaction mechanisms increase species overyielding in mixture, especially for low-biomass species. We tested these predictions with a seven-year data set from an experiment that varied grassland plant species evenness and richness. We used path analysis to model effects of these mechanisms on annual and multiyear changes in diversity. We found that diversity was frequently maintained by species interaction mechanisms and was infrequently maintained by equalizing mechanisms. Species interaction mechanisms maintained diversity by allowing the species that produced the least biomass in monoculture to benefit the most from species interactions in mixture. Equalizing mechanisms infrequently maintained diversity because asymmetric competition infrequently resulted in competitive exclusion. We propose that this mechanistic framework be used to better understand the specific processes that influence diversity. Key words: biodiversity; coexistence; complementarity; ecosystem function; net biodiversity effect; productivity.

A well-known, but largely untested, prediction in plant reproductive ecology is that dioecious taxa should produce larger, more, higher-quality, or better-defended seeds than cosexual taxa. Using a data set composed of 972 species in 104 families, representing the flora of the Tambopata Wildlife Reserve (Madre de Dios, Peru), we evaluated the first component of this prediction, examining ecological and evolutionary relationships between breeding system and mean seed size with two kinds of tests. First, we conducted cross-species analyses to determine whether species with different breeding systems differed significantly with respect to mean individual seed size. Second, we used a hypothesized phylogeny to identify pairs of the most closely related taxa or clades within the Tambopata community that differed with respect to breeding system. Comparing pair members allowed us to determine whether evolutionary divergence in breeding system (between taxa with unisexual vs. cosexual individuals) was consistently associated with evolutionary change in seed size. In both analyses, we controlled for potentially confounding effects of growth form by examining these relationships within woody and nonwoody taxa. Cross-species analyses revealed that dioecious species produced larger seeds than cosexual species among woody species, shrubs, lianas (each growth form analyzed separately), and all species pooled, but not among trees. Phylogenetically independent contrasts upheld the significant association between breeding system and seed size among woody taxa, lianas, and all taxa pooled, but not among shrubs. We discuss the implications of our findings for evolutionary hypotheses regarding associations between dioecy and seed size. Key words: angiosperms, breeding system, dioecy; monoecy; plant growth forms; rain forest; seed size.

Intraspecific variation in resistance to herbivory among genders and life-history phases of primary producers can significantly alter the ecological and evolutionary consequences of plant herbivore interactions. Seaweeds (macroalgae) with complex life histories have multiple distinct phases with associated variation in traits that can potentially lead to differences in resistance to consumers and provide a unique system in which to simultaneously test the effects of sex and life-history stage on herbivory. We tested the susceptibility to grazing of the three life-history stages and separate sexes of the chemically defended red alga Asparagopsis armata against the sea hare Aplysia parvula, and we related this to the plant quality traits of different stages and genders. Differences in nutrient content and halogenated secondary metabolites between life-history phases were highly sex dependent. Male gametophytes had a low concentration of secondary metabolites and the highest nutrient content. The highest secondary metabolite content was found within the female gametophyte, in the wall of the reproductive structures (cystocarps) that contain the microscopic carposporophyte phase. Feeding choices by A. parvula were consistent with differences in algal quality and defense and resulted in the haploid male gametophytes being the most preferred food type. The diploid carposporophyte found inside the chemically rich cystocarps was the least consumed life-history stage. Selective herbivory of male gametophytes by A. parvula is consistent with an observed shift in gametophyte sex ratio in the field from unity at the beginning of the reproductive season to female bias at the end. The variation in susceptibility to herbivory found between sex and life-history stages of A. armata represents the first example of sex-biased consumption in seaweeds and may contribute to the maintenance of complex life histories such as those found in red algae. Key words: algae; Aplysia parvula; Asparagopsis armata; chemical defense; dioecy; herbivory; heteromorphic life cycle; life history; plant-herbivore interaction.

Invasive species are a global threat to biodiversity and the functioning of natural ecosystems. Here, we report on a two-year experiment aimed at elucidating the combined and relative effects of three key controls on plant invasions: propagule supply, soil nitrogen (N) availability, and herbivory by native insects. We focus on the exotic species Lespedeza cuneata, a Rank 1 invasive species. Propagule supply and soil N-availability interacted to control the density and foliar cover of L. cuneata. In low N plots, density and foliar cover of L. cuneata were higher in the propagule addition plots than in the plots to which propagules were not added. Surprisingly, this interaction was significant only when the abundance of herbivores was experimentally reduced. This experiment provides evidence that native insect herbivores mediate the interactive effects of propagule supply and resources on invasion by a widespread invasive plant species. Key words: biological invasion; dispersal; herbivory; invasive species; Lespedeza cuneata; nutrients: plant-insect interactions.

The relationship between net primary productivity and biological diversity has been a central topic in ecology for several decades. The unimodal ('hump-back') relationship has been the most widely accepted for plants with the decrease in diversity at high productivity usually attributed to competitive exclusion. However, the relatively small species pool size under high productivity conditions may account for this pattern as well. Small species pool sizes for highly productive habitats are characteristic of temperate regions, where productive habitats for speciation and species migration have historically been rare. In contrast, productive habitats in the tropics have been relatively common during evolutionary history, resulting in large species pools. We hypothesize that evolutionary history contributes to the observed productivity--diversity relationship of plants, and that the productivity--diversity relationship differs between temperate and tropical regions. We investigated the productivity--diversity relationship patterns from 163 case studies throughout the world. Latitude described ~80% of the variation in the shape of the relationships. The unimodal relationship was found to dominate in the temperate zone, whereas the positive relationship was significantly more common in the tropics. We detected no influence due to methods of productivity measurement, but unimodal or positive productivity--diversity relationships were more likely within larger ranges of productivity. The length of the productivity gradient did not affect the latitudinal influence. In summary, the shape of the productivity--diversity relationship differs between temperate and tropical regions and the different evolutionary history of the local species pools is a probable cause for the difference. Key words: biodiversity conservation; generality of ecological rules; global diversity patterns; latitudinal gradient; macroecology; meta-study; plant biomass; plant species richness; primary productivity; productivity--diversity relationship; species pool; tropical ecology.

Central to the ecology and evolution of a broad range of plants is understanding why they routinely have submaximal reproduction manifested as low seed: ovule and fruit : flower ratios. We know much less about the processes responsible for low seed : ovule ratios than we do for fruit : flower ratios. Current hypotheses for low seed : ovule ratios are largely drawn from those for fruit : flower ratios, including proximate (ecological) causes of pollen limitation, resource limitation, and pollen quality, as well as the ultimate (evolutionary) hypothesis of 'bet hedging' on stochastic pollination. Yet, such mechanisms operating on fruit : flower ratios at the whole-plant level may not best explain low seed : ovule ratios at the individual-flower level. We tested each of these proximate and ultimate causes for low seed : ovule ratios using the specialized pollination mutualism between senita cacti (Pachycereus schottii) and senita moths (Upiga virescens). Seed : ovule ratios were consistently low (~0.61). Such excess ovule production by senita likely has a strong genetic component given the significant differences among plants in ovule number and the consistency in ovule production by plants within and among flowering seasons. Excess ovule production and low seed : ovule ratios could not be explained by pollen limitation, resource limitation, pollen quality, or bet hedging. Nevertheless, phenotypic selection analyses did show significant selection gradients for increased ovule number, suggesting that other evolutionary processes may be responsible for excess ovule production and low seed : ovule ratios. In contrast, low fruit : flower ratios at the whole-plant level were explained by an apparent equilibrium between pollen and resource limitation. Thus, mechanisms responsible for low fruit : flower ratios at the whole-plant level are not necessarily in accord with those of low seed : ovule ratios at the individual-flower level. This suggests that we may need to adopt a more pluralistic approach to seed : ovule ratios and consider alternative hypotheses, including a greater array of proximate and ultimate causes. Initial results of this study suggest that floral allometry, selection on correlated floral traits, stigma clogging with pollen grains, and style clogging with pollen tubes may provide promising avenues for understanding low seed : ovule ratios. Key words: bet hedging; Cactaceae; floral allometry; fruit : flower ratio; Pachycerus schottii; phenotypic selection; pollen limitation; pollen load; pollen quality; resource limitation; seed : ovule ratio; senita.

Pollination commonly limits seed production, as addition of pollen to stigmas often increases fecundity. This response is usually interpreted as evidence that plants' stigmas receive too few pollen grains to maximize ovule fertilization (quantity limitation); however, many genetic studies demonstrate that poor-quality pollen can also reduce seed production (quality limitation). We explore both aspects of pollen limitation theoretically with a dose-response model that incorporates a saturating negative-exponential relation of seed production to pollen receipt. This relation depends on aspects of ovule production, pollen import, pollen-pistil interactions and seed development, all of which can contribute to pollen limitation. Our model reveals that quantity limitation is restricted to the lowest range of pollen receipt, for which siring success per pollen grain is high, whereas quality limitation acts throughout the range of pollen receipt if plants do not import the highest-quality pollen. In addition to pollinator availability and efficiency, quantity limitation is governed by all post-pollination aspects of seed production. In contrast, quality limitation depends on the difference in survival of embryos sired by naturally delivered pollen vs. by pollen of maximal quality. We briefly illustrate the distinction between these two components of pollen limitation with results from the mistletoe Tristerix corymbosus. Our model also shows that the standard pollen-supplementation technique neither estimates the total intensity of pollen limitation nor distinguishes between its quantity and quality components. As an alternative, we propose a methodological protocol that requires both measurement of seed production following excess pollination with only outcross pollen and quantification of the dose-response relation of seed output to pollen receipt. This method estimates both the total extent of pollen limitation and its two components. Finally, we consider the influences on quantity and quality limitation, which reveals that quantity limitation probably occurs much less often than has been inferred from pollen-supplementation experiments. These interpretations suggest that an expanded perspective that recognizes the fecundity consequences of pollination with poor-quality pollen would promote ecological understanding of pollen limitation. Key words: plant reproduction; pollen limitation; pollen quality; pollination; seed production; supplemental pollination; Tristerix corymbosus.

Given the occurrence of mixed mating systems among plants, a general mechanism explaining the evolution and maintenance of this condition is needed. Although numerous theoretical models predict mixed mating to be evolutionarily stable, conditions favoring intermediate selfing are often stringent and have limited applicability. Here we investigated the role of vegetative herbivory, a ubiquitous biotic factor limiting plant reproduction, in the mating system expression of Impatiens capensis (Balsaminaceae), a species with an obligate mixed-mating system (individuals produce both selfing, cleistogamous, and facultatively outcrossing, chasmogamous flowers). Herbivory reduced proportional chasmogamous reproduction partially, but not entirely, through a reduction in plant size and the strength of this effect varied among replicates. Herbivory decreased geitonogamous selfing in chasmogamous flowers via several mechanisms including reduced chasmogamous flower display size and pollinator visitation rate and altered pollinator composition. Overall, herbivory caused a decrease in whole-plant outcrossing, indicating that the effects of herbivory on proportional chasmogamous reproduction, which favor selfing, outweigh the effects on chasmogamous outcrossing rate, which favor outcrossing. Not only do our findings unravel the mechanisms underlying herbivore-mediated changes in the mating system, but they also point to the role of natural enemies in contributing to the maintenance of a mixed mating system. Key words: Impatiens capensis; inbreeding; mating system; outcrossing; plant-pollinator interactions; vegetative herbivory.

Population context should influence pollination success and selection on floral display in animal-pollinated plants because attraction of pollinators depends not only on the characteristics of individual plants, but also on the attractiveness of co-occurring conspecifics. The insect-pollinated herb Primula farinosa is polymorphic for inflorescence height. Natural populations may include both long-scaped plants, which present their flowers well above the soil surface, and short-scaped plants, with their flowers positioned close to the ground. We experimentally tested whether seed production in short-scaped P. farinosa varied with local morph frequency and surrounding vegetation height. In tall vegetation, short-scaped plants in polymorphic populations produced more fruit and tended to produce more seeds than short-scaped plants did in monomorphic populations. In low vegetation, population composition did not significantly affect fruit and seed output of short-scaped plants. The results suggest that long-scaped plants facilitate short-scaped plants in terms of pollinator attraction and that the facilitation effect is contingent on the height of the surrounding vegetation. The documented facilitation should contribute to the maintenance of the scape length polymorphism in ungrazed areas where litter accumulates and vegetation grows tall. Key words: alternative reproductive strategies; facilitation; floral display; plant height; pollination; positive interactions; predation; Primula farinosa.

Pollination by animals is critical to sexual reproduction of most angiosperms. However, little is known about variation in pollination service to single plant species. We report results of a long-term study Ipomopsis aggregata, a semelparous montane herb whose flowers are visited by hummingbird and insect pollinators as well as 'floral larcenists.' We censused flower visitors over seven summers at permanent study sites separated by several hundred meters, and counted pollen delivered to flowers on a subset of plants observed for visitation. The species composition of the community of visitors varied significantly across years and within the flowering season; sites varied significantly only in the magnitude of parallel annual changes in the visitor community. Rates of flower visitation fluctuated over an order of magnitude or more. Variation in mean stigma pollen load among plants flowering in the same site and year was explained by a causal path model in which visitation rates by pollinators and larcenists had linear positive and negative effects, respectively. A simplified model including only pollinators explained almost as much variance as did the full model. However, qualitatively different parameter estimates were produced by an analogous causal model based on population means across site-year combinations. Discrepant results from within- and between-population levels of analysis suggest that pollen receipt is influenced by environmental factors that vary among sites and years, as well as by pollinator visit rates. We present a heuristic causal model that includes such factors, and we note its implications for ecological and evolutionary studies of pollination. Key words: field study; floral larcenists; floral visitation rates; flower-visitor community; hummingbirds; insects; Ipomopsis aggregata; long-term study; path analysis; plant population dynamics, pollen delivery; pollination services; structural equation modeling.

We analyzed seed production of mountain beech (Nothofagus solandri var. cliffortioides) forest along an elevational gradient in New Zealand from 1020 to 1370 m (treeline) for the years 1973-2002. We used seed production data from nine elevations and a site- and species-specific net carbon (C) availability model from two elevations (1050 m and 1340 m) to examine how three variables (temperature, soil moisture, and net C availability) during three key periods (resource priming, flowering primordia development, and flowering) influence seed production. These three strongly interrelated variables have all been considered determinants of seed production but have not previously been tested together in a single analysis. Seed production increased over the 30-year period, with the greatest increases at high elevations; this increase was driven by a greater frequency of intermediate-sized seeding years. We then determined how temperature, soil moisture, and net C availability determined seeding, and examined whether temporal trends in the seeding data could be linked to similar temporal trends in temperature, soil moisture, or net C availability. High seed production was related to cool summers with high soil moisture during resource priming, warm summers during flower primordia development, and low net C availability during flowering. Positive temporal trends in temperatures during the period of flower primordia development accounted for the increase in seed production, suggesting that increasing temperatures are promoting more frequent seed production at high elevations. Key words: carbon dioxide; evergreen; global change; masting; montane; mountain beech; New Zealand; reproduction; respiration; soil moisture; temperature; warming.

Current understanding of the extent, causes for, and consequences of variation in nutrient composition in plants is limited. Important questions to be addressed include to what extent nutrients covary, how flexible nutrient ratios are within a population or species, how reproduction influences nutrient ratios, and how much the ratios of nutrients to mass and nutrients to each other change through ontogeny. This information is needed to assess the physiological and ecological consequences of plant nutrient composition and to what extent plants function as balanced systems in acquisition and allocation of resources. We studied the variation in nutrient stoichiometry (i.e., the ratio between contents of different nutrients within a plant) in relation to three factors: (1) environmental availability of nitrogen, (2) plant size, and (3) reproductive status. We investigated these questions in 11 populations of the carnivorous plant Pinguicula vulgaris in northern Scandinavia. Dry mass and N and P content were measured for reproductive and vegetative portions of flowering individuals and for winter buds corresponding to four reproductive states: control reproductive individuals, experimentally vegetative individuals (from which flower buds were removed), adult vegetative individuals, and individuals below the threshold size for reproduction. [N], [P], and to a lesser extent, N and P content were positively related to soil N, but not to prey capture. Nutrient stoichiometry was also size dependent; in general, small plants were relatively enriched in N and relatively depleted in P compared to larger plants. Reproductive status affected not only size, but also nutrient stoichiometry of the resulting winter bud. Winter buds derived from reproductive individuals had a higher [N] and lower [P] than those of the different types of nonreproductive individuals. Our findings indicate that studies of nutrient stoichiometry in plants must go beyond links between environmental and plant nutrient concentrations to consider internal processes such as growth and reproduction. Key words: carnivorous plant; Pinguicula vulgaris; plant nutrient stoichiornetry; plant size; reproductive status; Scandinavia; size dependence; soil nutrient availability.

Sympatric plant species with similar flowering phenologies and floral morphologies may compete for pollination, and as a consequence potentially influence each other's reproductive success and mating system. Two likely competitors are Mimulus ringens and Lobelia siphilitica, which co-occur in wet meadows of central and eastern North America, produce blue zygomorphic flowers, and share several species of bumble bee pollinators. To test for effects of competition for pollination, we planted experimental arrays of Mimulus ringens, each consisting of genets with unique combinations of homozygous marker genotypes. In two arrays we planted mixtures of Mimulus and Lobelia, and in two additional arrays we planted Mimulus without a competitor for pollination. Bumble bee pollinators frequently moved between Mimulus and Lobelia flowers in the mixed-species arrays, with 42% of plant-to-plant movements being interspecific transitions. Pollinator movements between species were associated with a reduction in the amount of conspecific pollen arriving on Mimulus stigmas. The presence of Lobelia led to a significant 37% reduction in the mean number of Mimulus seeds per fruit. In addition, Mimulus had a significantly lower rate of outcrossing in the mixed-species arrays (0.43) than in the 'pure' arrays (0.63). This is the first study to demonstrate that competition for pollination directly influences outcrossing rates. Our work suggests that in self-compatible populations with genetic load, competition for pollination may not only reduce seed quantity, but may also lower seed quality. Key words: Bombus fervidus; competition for pollination; field experiment; improper pollen transfer; Lobelia siphilitica; mating system; Mimulus ringens; outcrossing rate; seeds per fruit; seed set; pollen loss; visitation rate.

Optimal Defense Theory predicts that plants exposed to herbivory should allocate more resources to produce costly secondary metabolites in tissues with higher fitness values. To increase plant resistance, the secondary metabolites must have a negative impact on the preference and/or performance of herbivores. We tested the hypotheses that induction of secondary metabolites (phlorotannins) in a brown seaweed in response to grazing by herbivorous gastropods will differ between seaweed tissues with different fitness values (basal stipes and annual shoots), and that the subsequent change in food value will affect the fitness (growth and fecundity) of the gastropods. Induction of phlorotannins was significant in both tissue types but was more pronounced in basal stipes, which have a higher fitness value. Basal tissues also had significantly higher constitutive defense levels than did apical tissues. No effects of algal tissue type or grazing history on the growth rate of the gastropods were detected. However, the number of viable eggs was significantly lower for gastropods feeding on basal shoots, and there was a significantly lower proportion of viable eggs produced by gastropods that were offered previously grazed seaweed tissues. The results show that induced resistance, and its variation among different plant parts, can have significant negative effects on herbivore performance that may reduce future herbivore pressure and thus enhance plant fitness. Key words: Ascophyllum nodosum; chemical defense; consumption; egg production; fecundity; fitness; growth; Littorina obtusata; phlorotannins; reproduction; resistance; within-plant variation.

Variable seed production may have important consequences for recruitment but is poorly documented for frugivore-dispersed tropical trees. Recruitment limitation may also be a critical spatial process affecting forest dynamics, but it is rarely assessed at the scale of individual trees. Over an 11-yr period, we studied the consequences of variable seed production for initial seedling recruitment in three shade-tolerant tree species (Quararibea asterolepis, Tetragastris panamensis, and Trichilia tuberculata) on Barro Colorado Island, Panama. We measured annual seed production for a permanent sample of focal trees and censused annual new seedling establishment within a restricted dispersal neighborhood of each tree. We analyzed temporal and spatial variability in recruitment and compared recruitment limitations among species. Annual seed production in Quararibea and Tetragastris fluctuated widely, whereas Trichilia had more consistent seed production across years. Within species, trees were synchronized in seed production, and synchrony appeared partly influenced by El Nino climatic events. Seedling recruitment varied temporally and spatially. At the population scale, all species had greater recruitment in productive seed years, and individual trees had differing contributions to this temporal pattern. At the focal-tree scale, recruitment patterns differed among species. Quararibea and Tetragastris trees with higher average seed production recruited more seedlings in their local neighborhoods than did less fecund trees. In contrast, Trichilia seedling recruitment did not increase with seed crop size, suggesting negative density dependence. At the tree neighborhood scale, no species showed evidence of density dependence in first-year survival. Comparatively, recruitment was more seed-limited in Quararibea and Tetragastris and more establishment-limited in Trichilia. Overall, our results indicated that: (1) variable seed production influenced seedling recruitment both temporally and spatially, and (2) species differed in limitations to recruitment. Key words: Barro Colorado Island, Panama; density dependence; Quararibea asterolepis; recruitment limitation; seed production; seedling demography; Tetragastris panamensis; Trichilia tuberculata; tropical tree demography.

Propagule dispersal in seaweeds is a process influenced by a variety of biological and physical factors, the complexity of which has hindered efforts to understand colonization, persistence, post-disturbance recovery, and dynamics of algal populations in general. In view of this limitation, we employ here modifications to an existing turbulent-transport model to explore the mechanics of nearshore macroalgal spore dispersal and its relationship to coastal hydrodynamic conditions. Our modeling efforts focus on four example species of seaweed whose reproductive propagules span a wide range in sinking speed and height of release above the sea floor: the giant kelp Macrocystis pyrifera, the erect fucoid Sargassum muticum, the small filamentous brown alga Ectocarpus siliculosus, and the flaccid red alga Sarcodiotheca gaudichaudii. Results indicate that both propagule sinking speed and release height can affect dispersal distance substantially, but that the influence of these biological parameters is modulated strongly by the intensity of turbulence as dictated by waves and currents. In rapid flows with larger waves, it is primarily fluid dynamic processes, in particular current velocities, that determine dispersal distance. Additional simulations suggest that patterns of spore dispersal are highly skewed, with most propagules encountering the sea floor within a few meters to hundreds of meters of their parents, but with a sizeable fraction of spores also dispersing as far as kilometers. Such model predictions imply a much greater potential for longer range dispersal than has typically been assumed, a finding with important implications for understanding the demographics of algal populations and for predicting levels of connectivity among them. Key words: Ectocarpus siliculosus; eddy diffusivity; flow; macroalgae; Macrocystis pyrifera; propagule; release height; Sarcodiotheca gaudichaudii; Sargassum muticum; sinking speed; turbulence; vertical mixing.

Masting, the intermittent production of large flower or seed crops by a population of perennial plants, can enhance the reproductive success of participating plants and drive fluctuations in seed-consumer populations and other ecosystem components over large geographic areas. The spatial and taxonomic extent over which masting is synchronized can determine its success in enhancing individual plant fitness as well as its ecosystem-level effects, and it can indicate the types of proximal cues that enable reproductive synchrony. Here, we demonstrate high intra- and intergeneric synchrony in mast seeding by 17 species of New Zealand plants from four family across >150 000 [km.sup.2]. The synchronous species vary ecologically (pollination and dispersal modes) and are geographically widely separated, so intergeneric synchrony seems unlikely to be adaptive per se. Synchronous fruiting by these species was associated with anomalously high temperatures the summer before seedfall, a cue linked with the La Nina phase of El Nino--Southern Oscillation. The lone asynchronous species appears to respond to summer temperatures, but with a 2-yr rather than 1-yr time lag. The importance of temperature anomalies as cues for synchronized masting suggests that the timing and intensity of masting may be sensitive to global climate change, with widespread effects on taxonomically disparate plant and animal communities. Key words: abiotic factors; Chionochloa; cross-correlation; Dacrydium; El Nino--Southern Oscillation; Elaeocarpus; masting; New Zealand; Nothofagus; Phormium; synchrony; temperature.

Plant reproductive strategies may evolve from fitness trade-offs between seed dispersal and seedling establishment. Small crops of large seeds generally are not dispersed as far as large crops of small seeds, but large seeds contain more reserves for seedling establishment. However, selection pressures imposed on these traits may vary among environments. This study examined the extent and consequences of environmental variation in the seed dispersal strategy of a fleshy-fruited plant, the common chokecherry (Prunus virginiana). From 1996 to 1998, P. virginiana fruit crop and seed traits were measured for plants sampled in moist riparian or dry slope habitats in western Montana. The consequences of variation in P. virginiana location, seed size, and seed number for seed dispersal were estimated from 1996 to 1999 by collecting seeds in seed traps. In all years, P. virginiana produced larger crops of smaller seeds in riparian than in slope habitats. Reproductive biomass was lower for plants on moisture-limited slopes than for similarly sized plants in riparian habitats. However, P. virginiana on slopes increased seed size rather than seed number in a year with relatively high levels of precipitation, suggesting that large offspring size may play a more important role than seed number in this environment. Environmental variation in P. virginiana reproductive traits may contribute to differences in seed dispersal patterns. Frugivores removed greater numbers of seeds from plants with large fruit crops and preferentially consumed relatively small seeds within sites (particularly in slope habitats). In riparian habitats, seed deposition by frugivores was greater away from rather than under P. virginiana canopies. In contrast, seed deposition on slopes was greatest under canopies. Thus, the production of many small seeds may be an adaptive response to increase seed dispersal in the riparian environment, where dispersal may be more important than seed size for successful offspring recruitment. Key words: common chokecherry; environmental variation; fruit removal; plant reproductive strategies; Prunus virginiana; seed dispersal; seed size vs. number.

Competition for pollination has been hypothesized to select for the divergence of floral traits between species and populations. A primary prediction of this hypothesis is that the strength of competition for pollination, mediated by variation in plant abundance, should directly influence the strength of selection on floral traits. To test this prediction, I examined the relationships between multiple components of plant abundance and pollination, reproductive success, and phenotypic selection via female fitness on four floral traits in artificial and natural populations of the hummingbird-pollinated Ipomopsis aggregata. In the artificial arrays, I manipulated the absolute density and interspersion of neighboring I. aggregata and a competitor for pollination (Castilleja linariaefolia). I also measured natural variation in the absolute and relative density of these two species within a 2.5-m radius of focal I. aggregata plants in three natural populations. The strength of competition for pollination in the I. aggregata--C. linariaefolia system was only weakly influenced by local plant abundance. Both the absolute density and interspersion of plants in the arrays significantly influenced at least one component of I. aggregata's pollination and reproductive success, but the effects were not consistent across these components. For example, the treatment groups that received more conspecific pollen were not the same ones that set more seeds/fruit. Within the natural populations, variation in relative and absolute plant density influenced two components of I. aggregata's pollination and reproductive success, but only in one to two populations per component. As would be expected from these inconsistent effects of plant abundance on pollination and reproduction, the strength of selection on floral traits of I. aggregata in both the arrays and natural populations was also only weakly dependent on abundance. Previous studies have indicated (1) that self-incompatible species such as I. aggregata should experience strong, abundance-dependent effects on pollination, and (2) that variation in plant abundance at a local scale should have a stronger effect on pollination than variation at a larger spatial scale. In contrast, my results suggest that the effect of plant abundance on pollination cannot be easily predicted from simple diagnostic traits such as breeding system or spatial scale. Key words: Castilleja linariaefolia; competition; density; floral traits; Ipomopsis aggregata; natural selection; phenotypic selection; pollination.

The effects of nectar robbers on plants and their mutualistic pollinators are poorly understood due, in part, to the paucity of studies examining male reproductive success in nectar-robbed plants. Here we measured the effects of a nectar-robbing bumblebee, Bombus occidentalis, on realized male reproductive success (seeds sired) in a hummingbird-pollinated plant, Ipomopsis aggregata. To determine the effects of nectar robbing on paternity, we used a series of experimental populations of plants containing a known allozyme marker. In each population, we experimentally controlled the levels of nectar robbing on each I. aggregata plant by cutting a hole in the corolla with dissecting scissors and removing nectar with a micro-capillary tube. We measured hummingbird-pollinator foraging behavior and fruit and seed production (maternal function) for each plant. We then genotyped seeds for the allozyme marker to determine the number of seeds sired by plants with known levels of robbing. Heavy nectar robbing (> 80% of flowers robbed) significantly reduced the number of seeds sired, as well as the number of seeds produced due to pollinator avoidance of heavily robbed plants. Total plant reproduction, both male and female contributions, were reduced by 50% due to high levels of robbing. To date, no other studies have measured the effects of nectar robbing on realized male function (number of seeds sired). Ours is the first study to demonstrate that robbing can simultaneously decrease realized male reproductive success as well as female reproductive success, and that the effects are incurred indirectly through pollinator avoidance of robbed plants. Key words: Bombus occidentalis; hummingbird-pollinator behavior; Ipomopsis aggregata; male function; nectar robbing, effect on plant reproductive success; Rocky Mountain Biological Laboratory (Colorado, USA); siring success.

Genetic variation in photosynthetic traits within populations provides the potential for evolution, but few studies relate phenotypic variation in these traits to variation in fitness. We tested the prediction that a lower photosynthetic rate reduces fecundity and survivorship by comparing wild-type (WT) Amaranthus hybridus family lines to those having a single-gene mutation that confers resistance to atrazine (R) and lowers the rate of photosynthetic carbon assimilation. Wild-type and R family lines with nearly uniform nuclear genomes were used to minimize the confounding effects of other loci. We established experimental populations in agricultural and one-year-old field plots and measured chlorophyll fluorescence, gas exchange, and the fecundity and survivorship of WT and R genotypes for two generations. The R genotype had a lower efficiency of electron transport through photosystem II, which translated into a 20-30% decrease in photosynthetic rate at light levels above 400 (mu)mol*m(super -2)*s(super -1). Compared to the WT, the R genotype also had lower water-use efficiency, higher specific leaf area, and greater leaf nitrogen concentration on a mass, but not area, basis. In five of six replicate populations, the R genotype had lower fecundity than the WT in the first generation. Survivorship of seed over winter was similar for the two genotypes, but survivorship of R seedlings during early establishment was lower than the WT in the agricultural field. The consistent pattern of selection against the R genotype during vegetative growth stages suggests that a lower photosynthetic rate reduces fitness. This selection, paired with heritable variation for photosynthetic traits within populations, provides a more complete scenario for the evolution of photosynthetic traits. Key words: Amaranthus hybridus; atrazine resistance; genotype frequency; selection for photosynthesis; water-use efficiency.

In this study, we examined how foliar herbivory affected the relative allocation to male and female reproductive traits in a hermaphroditic plant. In two experiments, one in the greenhouse and the other in a growth chamber, leaves of full sibling plants of the annual wild radish, Raphanus raphanistrum, received one of four damage treatments to leaves (0, 25% [two types], or 50% leaf area removal). We controlled natural damage by Pieris rapae larvae through the use of clip cages. Measurements of both male and female reproductive characters were made on a series of flowers produced by each plant over its lifetime. We measured pollen production, pollen size, ovule number, ovule size, petal size, nectar production, and sugar content. Effects of herbivory on plant phenology were also assessed. At the end of the experiment, we quantified total flower production, fruit production, number of seeds and fruit, and seed and vegetative biomass. In general, male reproductive characters were more affected than female characters by herbivory. Leaf herbivory reduced pollen number and pollen size, whereas ovule size and number were not affected. Petal size, which we also classify more as a male trait, decreased with herbivory. Plants were generally able to completely compensate for herbivory in the total number of flowers and fruit they produced; however, fewer fruits were found in the heaviest damage treatment in the growth chamber experiment. We discuss alternative explanations for this pattern based on developmental constraints and adaptive responses. In summary, male fitness traits were much more strongly affected by herbivory than were female traits; full compensation in seed production after herbivory may come at the cost of male reproductive success in these hermaphroditic plants. Key words: allocation to male vs. female function; compensation; floral traits; foliar herbivory; male plant fitness; Pieris rapae; Raphanus raphanistrum; reproductive traits; wild radish., INTRODUCTION Foliar herbivory can affect plant fitness directly through changes in plant traits, and indirectly by altering pollinator visitation patterns to damaged plants (Strauss et al. 1996, Strauss 1997, Lehtila [...]

We studied resource allocation between sexual reproduction and clonal propagation in a perennial stoloniferous clonal plant, Potentilla anserina, an obligate outcrosser. We manipulated reproductive effort of Potentilla anserina either by hand-pollinating all flowers or by preventing pollination. To test the effect of resource-limiting conditions on resource allocation and reproductive output, we used a control and two levels of heavy metals (copper and nickel) to limit plant growth. The experiment was conducted as a 2 x 3 factorial design to reveal possible interactions between reproductive manipulation and resource limitation. Heavy metals decreased the total biomass of the plants and number of flowers and ramets produced. Only 50% of the plants grown with the higher level of heavy metals produced flowers. Pollination treatment interacted significantly with the heavy-metal treatment. In the metal control (no metals added) and lower heavy-metal treatment, there were no significant differences in total vegetative biomass between the two pollination treatments. Costs of reproduction in terms of subsequent flowering in the later season appeared to be clear, because the number of flowers per whole plant was lower if the plants were hand-pollinated and because the proportion of flowering ramets decreased due to hand-pollination. However, flowering may also be partly hormonally controlled. In contrast, hand-pollinated plants exposed to high concentrations of heavy metals tended to have greater biomass of vegetative plant structures and higher number of flowers compared to nonpollinated plants. Together, these results suggest that environmental factors may alter costs of reproduction but do not support the idea that these costs are more acute in resource (photosynthate)-limiting conditions. Key words: allocation; clonal plants; clonal propagation; compensatory flowering; costs and manipulation of reproduction; hand-pollination; heavy metals; Potentilla anserina; vegetative growth., INTRODUCTION One of the central concepts in life history theory is the principle of allocation of limited resources among competing activities (e.g., Cody 1966, Williams 1966, Reznick 1985). This competition [...]

This paper reports the results of a two-year field experiment on the determinants of fitness responses in Pinguicula vallisneriifolia (Lentibulariaceae), an endemic carnivorous plant of southeastern Spain. For the first time, in this experiment, we have considered irradiance as a factor, in combination with animal prey, for an array of natural field conditions. The goal was to determine how carnivory translates to fitness within different radiation regimes. For this, it was necessary to quantify an array of plant responses, such as survival, growth, reproduction (sexual and vegetative reproduction), as well as responses related to carnivory investment (leaf shape and mucilage secretion). Both irradiance and animal food supply proved to be important limiting factors for P. vallisneriifolia under field conditions. Plants clearly decreased in performance from the sunny habitat to the deep-shade one, with plants growing in the least irradiance registering the lowest values for all variables analyzed. The clearest response to prey was the production of axillary buds. Most vegetative and reproductive responses depended heavily on the initial biomass of the plant before the experiments, the largest plants bearing the most leaves, flowers, stolons, and axillary buds. A gradient from less to more mucilage secretion appeared from deep-shade to sunny habitat, and within each habitat from prey exclusion to prey addition levels. Trapped prey stimulated digestive secretions in a positive feedback (the more prey, the more mucilage secretion) under all irradiance conditions. The curled, more secretory leaves of the sunny plants, in comparison with the nearly flattened, less secretory leaves of the deep-shade plants, illustrate the constraints imposed by differing scenarios on the capture of both prey and photons. With nonlimiting water availability, more irradiance and prey results in more survival, growth, and sexual and vegetative reproduction. Nevertheless, it is not usual to find an optimum combination of resources (i.e., irradiance, prey, and water) available in the same microhabitat during the Mediterranean summer. The spatial uncoupling of limiting resources progressively increased towards the extremes of the irradiance gradient (sunny and deep-shade habitats, respectively), and therefore the dual photosynthetic and carnivore functions of P. vallisneriifolia leaves were not equally efficient in all habitats. These opposing resource gradients determined all vegetative and reproductive plant responses. The perennial character of this endemic plant, together with its vegetative form of propagation, allows the possibility of resisting extinction even in the absence of seedling recruitment or when vegetative growth is strongly limited. Key words: carnivorous plants; ecological heterogeneity; field experiment; growth and survival; irradiance; leaf shape; Lentibulariaceae; Mediterranean ecosystems; mucilage secretion; Pinguicula vallisneriifolia; prey capture; sexual and vegetative reproduction., INTRODUCTION Carnivorous plants use entrapped animal tissues for nutrition, reversing the normal animal-plant interaction and representing a great anomaly in the usual trophic order of life (Thompson 1981). These plants [...]

In many plant species with unisexual flowers, the female flowers lack pollinator rewards and are pollinated by visitors who mistake the female flowers for rewarding male flowers. Here we present the results of observations and experiments designed to investigate pollinator discrimination and its consequences for female reproductive success in two monoecious Begonia species pollinated by deceit. Male and female flowers of B. urophylla are similar in shape, but female flowers are smaller than male flowers, while in B. tonduzii, male and female flowers have strikingly different shapes, but their petaloid tepals are similar in total area. Flower visitors displayed marked discrimination against the rewardless female flowers of both species. Seven bee species were observed on B. urophylla, and for all species taken together the relative visitation rate to male flowers was 8.6 times that to female flowers. The bee Trigona fulviventris accounted for 95% of all visits to B. tonduzii and visited male flowers 15.4 times as often as female flowers. The flower visitors of B. urophylla varied significantly in their degree of discrimination against female flowers, with visitation rates to male flowers 3.7-21.8 times those to female flowers. The two most abundant flower visitors displayed the greatest discrimination against female flowers and, although they comprised nearly 54% of all visits, were responsible for only 15% of all visits to female flowers. Thus, the relative abundance of different flower visitors on B. urophylla is a poor predictor of pollinator importance. In both species the inflorescence sex ratio was male-biased early in the season and female-biased later. Hand-pollination experiments revealed that fruit set in B. tonduzii was pollinator limited both during peak and late flowering, with supplemental pollination having its greatest effect late in the season when male flowers were rare. In contrast, fruit set was only weakly affected by supplemental hand-pollination in B. urophylla. The significant pollinator limitation observed in B. tonduzii and the lack of it in B. urophylla are consistent with our observation that the visitation rate to female flowers was much higher in B. urophylla than in B. tonduzii, which was due in part to the lower level of pollinator discrimination against female flowers of B. urophylla. Our finding that female reproductive success is limited by pollinator visitation in B. tonduzii indicates an opportunity for the evolution of increased resemblance between male and female flowers in this species. Key words: Begonia; deceit pollination; floral display; fruit set; mimicry: monoecy; pollinator limitation; pollinator preference., INTRODUCTION Resemblance between male and female flowers has been observed for a number of plant species with unisexual flowers (Baker 1976, Bawa et al. 1982, Armstrong and Drummond 1986, Barfod [...]

We examined the influence of the leaf spot pathogen Cercospora kalmiae on reproductive success in self-fertilized vs. outcrossed progeny in the mountain laurel (Kalmia latifolia). To explore the interaction between disease incidence and mating system within natural populations of K. latifolia, we ascertained the correlation between disease intensity and two measures of plant reproductive success (proportional fruit set and seedling survival) and partitioned the correlation by pollination treatment (i.e., naturally pollinated, hand pollinated with outcrossed pollen, or self-pollinated). For outcrossed flowers, neither the number of fruits produced nor the performance of seedlings from these fruits was affected by the intensity of disease damage. However, for selfed flowers, both the number of fruits and the performance of seedlings were negatively correlated with disease damage on the maternal parent. Consequently, fungal infection on a maternal plant was associated with an increased proportion of outcrossed progeny produced by the host plant. Motivated by the observed correlation between disease and selfing, we performed a manipulative field experiment to examine the mechanism by which disease could influence the fitness consequences of different mating strategies. We hypothesized that intense C. kalmiae infection reduced K. latifolia resources by reducing photosynthetic leaf area and, consequently, enhanced abortion of self-fertilized fruits more than outcrossed fruits. We found that artificial leaf damage reduced outcrossed fruit set by 17% compared to controls. However, artificial leaf damage reduced selfed fruit set by 31% compared to controls. Similarly, artificial damage did not significantly affect infructescence abortion in outcrossed pollinations. However, for self-pollinated inflorescences, those subtended by damaged leaves experienced a 44% increase in infructescence abortion compared to controls. We conclude that resource limitation, here by leaf damage, can lower the production of self-fertilized seeds relative to outcrossed seeds, thus modifying a plant's mating system. Key words: hermaphrodite; Cercospora kalmiae; Kalmia latifolia; leaf spot pathogen; mating systems; mountain laurel; outcrossing; photosynthetic resources; seed production; self-fertilization., INTRODUCTION Fruit maturation is a dynamic process and may be influenced by many factors (Casper and Niesenbaum 1993). Among these, the source of pollen can affect which seeds will draw [...]

We carried out phenotypic selection analyses to assess the relative importance of several floral traits, as well as microhabitat, in determining both male and female pollination success in a nonrewarding orchid (Cypripedium acaule Ait.) in two Nova Scotia populations. Fruit production in these one-flowered, cosexual plants was strongly pollen limited within a season: 100% of individuals set fruit following hand-pollination, but only 5% and 13% did so under natural circumstances in the two populations. Male and female reproductive success were highly correlated, with >90% of plants that received a pollinium also dispersing one. Despite very low visitation rates, the chance of a removed pollinium fertilizing another ovary was 36% and 51%, respectively, in the two populations. There was no evidence for selection acting differently, either through the two sexual functions or in the two populations. In contrast, the strength of directional selection on several traits differed among microhabitats within each population. Univariate and multivariate regressions indicated statistically significant directional selection acting on flower-opening date through male, female, and total (combined) pollination success in one population. For female, male, and total success in this population, we also detected correlational selection acting on opening date and flower height. There was also negative correlational selection acting on flower height and labellum length through female success. Microhabitats within the populations had a larger effect on both female and male success than did floral traits. In particular, the presence of ericaceous shrubs and an open canopy appeared to be closely linked with higher pollination rates. Key words: Bateman's rule; correlational selection; Cypripedium: deceit pollination; flower size; natural selection; Orchidaceae; phenology; plant height; pollen limitation; selection coefficient., INTRODUCTION The intuition that natural selection favors those floral dimensions and characteristics that result in precise pollen transfer is supported by at least two general patterns. First, most floral traits [...]

For organisms with indeterminate growth, life history theory predicts that in environments where organisms experience high survival rates or gain fecundity with age or size, natural selection favors delayed maturity. In semelparous perennial plants the onset of reproduction is regulated by a threshold size for flowering. We tested this prediction by comparing sand dune populations of the facultative biennial herb Cynoglossum officinale. We collected data on flowering probability, survival, and growth rate in relation to plant size in two habitat types, open areas and poplar thickets, in Meijendel, The Netherlands, and in Holkham, England. Survival of established rosettes was highest in Holkham and lowest in open areas in Meijendel. Relative growth rates in Holkham were about three times as high as those in Meijendel. These findings agreed with the differences in threshold sizes found among the sites: the Holkham field population harbored higher threshold sizes than the Meijendel sites. We used the field data to compare optimal threshold sizes for flowering predicted by three existing models. All three models gave the same rank order for threshold sizes as found in the field: Meijendel open < Meijendel poplar thicket < Holkham. One model, which maximized population growth rate Λ, predicted optimal threshold sizes that agreed very well with threshold sizes found in the field. The predictions of the two other models, both maximizing [R.sub.0], were consistently lower than the threshold sizes observed in Meijendel, while their predictions for Holkham were too high. Key words: Cynoglossum officinale life history; facultative biennial; genetic variation; growth rate; hound's-tongue; optimization models; plant size and fitness; semelparous; survival; threshold size for flowering., INTRODUCTION The onset of reproduction is a key parameter in the life history of most organisms, and age and size at maturity are life history traits with a high impact [...]

This study investigates the role played by showy structures in the reproduction of Lavandula stoechas L. (Lamiaceae), a self-compatible Mediterranean shrub with compact, head-like inflorescences that are terminated by a tuft of conspicuous pink bracts. Pollinator visitation rates to plants with bracts correlated positively with that of paired, treated shrubs with all their bracts removed and, overall, bractless and control plants had equivalent visitation rates. In experimentally split shrubs, however, a strong effect on pollinator choice was evident, and distant visitor approaches were most often to that side keeping the attractive organs. Fecundity was not depressed in plants with reduced visual displays, and neither was the amount of pollen dispersed per flower or the size of stigmatic loads. There were, however, more pollen tubes, on average, at the lowermost part of the style in controls (10.8 tubes/style) than in bractless shrubs (8.3 tubes/style). Small, nonsignificant declines in average seed size, germinability, and seedling mass were detected following bract removal. Data support the notion that bracts are not essential to seed production in mature, relatively dense Lavandula populations where bee pollinators are often in good supply, but may be all-important if plant density is extremely low, or during population establishment. The role of an amplified display in increasing average pollination distance and progeny quality in this species is discussed. Key words: fecundity; floral display; Lavandula; Lamiaceae; Mediterranean; outcrossing; pollinator attraction., INTRODUCTION Many aspects of animal-pollinated plant reproduction are influenced by the size of the floral display, including visitation rates and fecundity (e.g., Willson and Price 1977, Lack 1982, Gori 1983, [...]

INTRODUCTION One reproductive pathway that seems to have occurred repeatedly within the angiosperms is the evolutionary breakdown of hermaphroditism (cosexuality) towards gender dimorphisms such as dioecy and gynodioecy (de Arroyo [...]

Although mutualisms are often viewed as fragile constructs, subject to invasion by 'cheaters' that gain from the mutualism without providing compensating benefits, few studies have explored whether or not apparent cheating behavior by one player actually denies benefits to the other species. Panicled bluebells (Mertensia paniculata) experience high rates of nectar robbery by bumble bees (Bombus mixtus and B. frigidus) in the Wrangell Mountains of southcentral Alaska. Nevertheless, experimental prevention of nectar larceny in two seasons did not enhance components of female reproductive success (nutlet initiation, seed-set, and seed mass) or male success (pollen removal). Observational data show that the absence of a negative impact of nectar robbery is a consequence of bumble bee behaviors; the two bee species that rob mature flowers buzz-pollinate the same flowers at an earlier stage in floral development. The shift in bee behavior is driven by the presentation of different rewards at different times in the flowering period; young flowers contain pollen and older flowers produce nectar. Although flowers typically receive and donate pollen before most nectar is produced, the mature flowers producing a robbed nectar reward may nonetheless contribute to the reproductive success of bluebell plants. Flowers sequestered from pollinators in the early stage were still capable of initiating nutlets, demonstrating that older flowers can enhance female reproduction if pollinators are rare (when many early-stage flowers may go unvisited). Moreover, the removal of mature flowers reduced the visitation rate to early-stage flowers on the same plant. Because individual bumble bees switch frequently between nectar robbing and pollen collection, the nectar reward in mature flowers may act as a key enticement to pollinators, which then enhance plant reproduction by legitimately visiting early-stage flowers. Rather than representing a case of cheating behavior, nectar robbery may be an integral part of this plant-pollinator mutualism. Key words: bluebells; Bombus spp.; bumble bees; cheating behavior; floral color change; hermaphroditic plants; Mertensia paniculata; nectar robbery; plant-pollinator mutualism; pollen removal; reproductive success., INTRODUCTION Many ecologists have decried the scanty attention paid to mutualistic interactions (Risch and Boucher 1976, Boucher et al. 1982, Vandermeer 1984, Keddy 1990, Bronstein 1991). Past history seems to [...]

The form of angiosperm flowers is thought to have evolved in part via selection that excludes ineffective pollinators and increases the efficiency of pollen transfer by effective ones. In previous studies with the montane plant Ipomopsis aggregata, we documented pollinator-mediated selection on several aspects of floral form, including flower width. This character varies continuously within natural populations of I. aggregata, and individuals with wider flowers export more pollen per flower to surrounding plants. We previously showed that this component of phenotypic selection is due primarily to the per-visit effectiveness of hummingbirds, the most important pollinator in our study populations. Here we investigate mechanisms of differential visit effectiveness in greater detail. First, counts of pollen in unvisited flowers showed that pollen production itself increases with width. In aviary experiments, hummingbirds also removed a greater proportion of available pollen as width increased. We next videotaped visits to flowers that varied in width either naturally, or by experimental treatment, and found that hummingbirds inserted their bills more deeply into wider flowers. Finally, we directly manipulated how deeply birds could insert their bills, and found that more pollen was removed after deep insertion. Thus, several mechanisms appear to underlie selection on corolla width via visit effectiveness in pollen export. One involves a phenotypic correlation with pollen production; this underscores the value of experiments for untangling indirect from direct selection. Another mechanism involves direct selection due to the depth of insertion of a hummingbird's bill; this is rare evidence for one form of selection involving the 'fit' between pollinator and flower. Key words: flower shape and size; hummingbird; Ipomopsis; morphology; phenotypic correlation; pollen export and production; pollinator effectiveness; selection., INTRODUCTION Angiosperm flowers exhibit remarkable diversity in form. Floral morphology often serves to distinguish species (Grant 1949), and appears to correlate well in many cases with the morphology of prominent [...]

It is widely believed that desert annual plants maintain between-year seed banks, yet few field studies actually have measured the proportion of the viable seed bank that remains dormant through a season. Dormancy and germination fractions were quantified for a guild of winter annuals on a creosote flat in the Sonoran Desert for three years. Predictions from two types of theoretical models applicable to temporally variable environments were examined: (1) the evolution of life history traits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The density of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and their reproductive success. Long-term data collected from permanent plots over a 10-yr period were used to calculate temporal variation in reproductive success for each species. Species with higher temporal variation in reproductive success had lower germination fractions and smaller seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet-hedging strategies. The data also suggested that this system possesses traits that are necessary for temporal variation to promote coexistence. First, between-year seed banks, necessary to buffer populations in unfavorable years, were documented for 17 species. Second, there was a strong tendency for year-to-year variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We discuss how a correlation between germination and reproductive success enhances the role of temporal variance in success hierarchies in promoting species coexistence. Key words: desert annual plants; dormancy; field experiment; life history traits; predictive germination; reproductive success; seed banks; Sonoran Desert; spatial variation; species coexistence; temporal variation., INTRODUCTION It is widely assumed that desert annual plants have seed banks. However, close inspection of the literature (e.g., Nelson and Chew 1977, Freas and Kemp 1983, Reichman 1984, Price [...]

Reproduction is closely tied to environmental conditions and the availability of resources, and thus typically varies with season. Consequently, perennial organisms that reproduce continuously are generally restricted to tropical regions with relatively aseasonal climates. The temperate marine alga Macrocystis pyrifera is a rare exception in this regard, as most individuals reproduce throughout the year in a seasonally variable habitat. Here we measure reproductive responses of the giant kelp Macrocystis during a period in which resources and environmental conditions fluctuated greatly and contrast these responses with those of the palm kelp, Pterygophora californica, a sympatric species that exhibits strictly seasonal reproduction. The quantity and quality of spore production tracked resource availability within and among years for Macrocystis, but not for Pterygophora. Reproductive allocation and spore standing stock in Macrocystis were negatively correlated with seawater temperature and positively correlated with the nitrogen content of adult plants. Macrocystis generally displayed two seasonally distinct peaks in spore production per year (winter and spring). The only disruption of this pattern coincided with a warmwater El Nino event. Although seawater temperature and the nitrogen content of adults were inversely related in Pterygophora, neither variable was significantly correlated with the quantity or quality of spore production in this species. Unlike Macrocystis, Pterygophora exhibited a well-defined reproductive season in which plants displayed a single broad peak in spore production that varied little in timing and magnitude among years, even during El Nino conditions. Spore C/N ratios remained relatively constant over time in both species, despite large seasonal fluctuations in C/N ratios of vegetative tissue of adults plants. Nonetheless, spore C/N ratios were positively correlated with seawater temperature in Macrocystis, but not in Pterygophora. Spore viability (swimming and germination) varied considerably, and often unpredictably, over time for both species. Our results support the general idea that environmental conditions and resources exert a much greater influence on the quantity and quality of reproduction in species that reproduce continuously than on the majority of species that are strictly seasonal in onset of reproduction. The differential responses of Macrocystis and Pterygophora may reflect their different morphologies and life-spans. Macrocystis is relatively short lived and may 'hedge its bets' by reproducing continuously rather than risk delaying reproduction. Conversely, since Pterygophora lives much longer, plants can afford to release spores only during times when the chances for reproductive success are predictably greatest because these plants are likely to reproduce again in subsequent years. Key words: algae; C/N ratio; El Nino; germination; kelp; Macrocystis pyrifera; Pterygophora californica, reproductive allocation; resource availability; seasonality; spore production; viability., INTRODUCTION Growth and reproduction generally reflect physical conditions of the environment and the availability of resources (Follett 1978, Bronson 1985, Munro et al. 1990, Adiyodi and Adiyodi 1993). Consequently, seasonal [...]

Little is known about the reproductive ecology of aquatic angiosperms, despite their wide distribution and ecological importance. Surfgrass (Phyllospadix torreyi) is a clonal marine angiosperm that lives on turbulent rocky shores. Surfgrass exhibits a strongly female-biased sex ratio in flowering shoots throughout its distribution along the Pacific coast of North America. The ecological causes for this biased sex ratio were examined over 3 yr in a surfgrass population off Catalina Island in southern California, USA. Abundances, biomass, and C:N content of male and female flowering ramets were measured, and the reproductive phenology, reproductive losses to abortions, herbivore damage to fruits and seeds, and clonal survival and growth were quantified. Each year, a quarter of the vegetative ramets reproduced sexually and seed production in the population was prolific. The population sex ratio was female biased across the entire depth distribution of surfgrass, but males were more abundant deeper in the bed. Despite male rarity, pollen was abundant in the population, and virtually all ovules were fertilized, with few losses to abortions or herbivory before seed release. Potential female fitness (seeds produced) was highest at shallow depths where light availability was greatest and abortions were reduced. The spatial segregation of the sexes was not due to environmental sex lability but to greater ramification of female vs. male clones in shallow depths. Female flowering shoots had significantly higher biomass than males across water depth, despite no obvious cost in terms of vegetative growth, or advantage in terms of plant size. If current theory on sex allocation that the sex ratio should be biased toward the gender with the lower reproductive cost is true for surfgrass, then surfgrass provides an example of how a subtle resource allocation difference can result in a dramatic population bias. An alternative explanation for the female bias in surfgrass populations is greater male mortality due to significantly weaker attachment to the substratum. This study demonstrates the potential role hydrodynamics can play in aquatic plant reproduction and in structuring their populations., Key words: biomechanics; Phyllospadix; seagrass reproduction; sex ratio; surfgrass. INTRODUCTION Dioecious plants with sex chromosomes are predicted to have primary sex ratios (i.e., in seeds at the end of parental [...]

We conducted three experiments in which we applied additional hand pollination, flower removal, and leaf removal treatments in various combinations to Primula veris, a perennial spring-flowering rosette species. The purpose of the study was to determine whether the seed set of Primula veris was limited by pollen availability or by other resources, and whether there were measurable costs of reproduction. Hand pollination in the beginning of experiments significantly increased current seed set in only one of the three experiments. It also increased the next-year fruiting probability in that first experiment. In the second experiment, hand pollination did not significantly affect current seed set, but we nevertheless observed enhanced leaf growth in the treatment year and the two following years, and increased flowering frequency, fruiting frequency, and survival in the two following years. In the third experiment, after hand pollination we observed a higher net photosynthetic capacity of the leaves and, again, increased leaf growth in the treatment year and a higher flowering probability in the following year. The positive effect of hand pollination was even clearer when the leaves of the plants were removed at the beginning of the experiment. However, the treatment in which all the flowers were removed had effects on subsequent performance similar to those of the hand-pollination treatment, suggesting trade-offs. Thus, we did not observe any costs associated with reproduction after supplemental hand pollination; on the contrary, hand pollination resulted in increased survival, size, and reproduction of the plants in the subsequent years. Key words: compensation; cost of reproduction; Europe; perennial herb; plant reproduction; plant survival; pollination; Primula veris., INTRODUCTION In some outcrossing perennial plants, seed set may be controlled by pollination success (Bierzychudek 1981, Ayre and Whelan 1989, Lubbers and Lechowicz 1989, Jennersten 1991, Agren and Willson 1994). [...]

Females of dioecious plant species are expected to show greater trade-offs with other life history traits than males because of their high investment in reproduction. We investigated life history traits and resource allocation patterns for both males and females of the dioecious plant Silene latifolia, to document allocation both prior to and after reproductive investment. In addition, the impact of fruit production on subsequent allocation to reproduction and growth was investigated by varying the number of flowers pollinated on females. The mean date of first reproduction was the same for males and females. Early-flowering plants of both sexes invested less in leaf biomass than plants that delayed flowering. The percentage of flowers pollinated had a significant effect on how resources were allocated, and on the amount and timing of further flower production. Sexual dimorphism was observed in resource allocation patterns after, but not prior to, investment in reproduction. Overall, pollinated females were found to have more leaf biomass and invest nearly twice as much in reproduction as males, in spite of an apparent trade-off between leaf production and reproduction within both females and males. We conclude that sexual dimorphism in life history traits that affects resource acquisition causes the resource pool that is allocated to be variable, rather than static, thereby masking the underlying tradeoffs. Key words: dioecious; life history traits; sexual dimorphism; Silene latifolia; trade-offs., INTRODUCTION Traits affecting reproduction and survival will be bound together by physiological trade-offs, because allocation of limited resources to any one activity will occur at the expense of others (Cody [...]

Little is known about how inbreeding depression and maternal effects change from the seed stage through offspring maturation or how these patterns differ for plants germinated under natural vs. artificial conditions. I measured these effects in perennial Aquilegia caerulea (Ranunculaceae) for both field-sown plants and greenhouse-sown progeny that were subsequently transplanted to the field. Overall, field-sown plants and transplants showed similar cumulative inbreeding depression, averaging 0.54 and 0.52, respectively. Selfed individuals had a lower success than outcrossed individuals in all fitness components measured from the number of seeds produced by the cross to size at year 3, and most effects were significant. Inbreeding depression in survival was worse for field-sown progeny in which mortality was much higher. Furthermore, effects of inbreeding on plant size increased each year for greenhouse-sown transplants, but not for field-sown plants, suggesting inbreeding depression is expressed later when survival is artificially high. The effect of seed mass was investigated in greenhouse-sown individuals. Although selfed seeds weighed significantly less than outcrossed seeds, seed mass was not correlated with emergence or survival. It did, however, correlate with leaf expansion rate, which, in turn, correlated with year 3 size in both transplants and field-sown plants. Maternal effects explained a much larger proportion of variance in seed mass than did level of inbreeding. In the greenhouse, effects of seed parent were strongest on early characters, becoming small and nonsignificant by year 3 when inbreeding effects again became significant. In contrast, maternal effects on juvenile traits of field-sown progeny were negligible., INTRODUCTION Inbreeding depression, defined as the lower success of seeds produced from self- compared to outcross-pollination, has long been recognized as a potentially important evolutionary force (Darwin 1876). However, early [...]

The reproductive success of an individual plant is in part a function of its capacity to assimilate resources, and this in turn is partially a result of plant morphology. Simmondsia chinensis, jojoba, a dioecious desert shrub, is dimorphic in secondary sexual characters in some desert populations, where females on average have larger leaves and more open canopies than males. The function of this sexual dimorphism was examined by investigating the importance of shoot morphology to reproductive components of plant fitness. The reproductive behavior of individuals tended to be associated with different morphological attributes for both males and females. Males with more 'male-type' morphology tended to have more inflorescences and a greater likelihood of flowering than did males structured more like females, though they also had smaller inflorescences. Females with more ''female-type' construction produced more flower buds in one of the study years and also produced heavier seeds than females with more male-like structure. Females from four different morphological classes allocated 1.5-4.5 times as much to reproduction as did males, but showed wide variability in whether or not they set fruit. Therefore females either had a much higher or a much smaller reproductive allocation than most males. Fruit set in the population studied is not limited by pollen availability, but is presumably constrained by abiotic resource availability. The asymmetry between sexes in the morphology of reproductively more successful individuals points to a potential for a resource-based evolutionary origin of sexual dimorphism in jojoba, arising from different morphological optima for each sex., INTRODUCTION In a dioecious species, females and males may be subject to divergent directional selection due to differences in each sex's requirements for successful reproduction (Price 1984, Slatkin 1984, Willson [...]

Abstract. To explore how plants may influence dispersal of their own seeds by manipulating the behavior and physiology of their dispersers, we studied the effect of a soluble chemical (or chemicals) in the fruits of Witheringia solanacea (Solanaceae), a Costa Rican cloud forest shrub, on passage of its seeds through the guts of one of its major dispersers, the Black-faced Solitaire, Myadestes melanops (Muscicapidae: Turdinae). Using artificial fruits containing natural seeds, we found that the presence of a crude pulp extract reduced the median seed retention time by nearly 50%. Estimation of seed dispersal distance as a function of retention time suggested that more rapid seed passage results in shorter average dispersal distances, especially for seeds retained

Abstract. The aim of this study was (1) to evaluate the importance of dormant buds for the bud demography of the mountain birch Betula pubescens ssp. tortuosa near the tree line and (2) to study whether sexual reproduction leads to costs for bud production rates. A bud population census was taken in two consecutive years for six branches of each of 90 mountain birch trees. The trees were growing in a common garden and belonged to 10 different progenies originating from different parts of Finnish Lapland. The data were analyzed with matrix population models. The most important transformations of the bud populations were between vegetative short and long shoots. However, if most apparently dead buds are actually latent dormants, they make an even more important contribution to the bud population growth rate than vegetative long and short shoots. Dormant buds may have considerable importance especially after events such as herbivore outbreaks, in which short and long shoots are damaged. Generative long shoots (with male catkins) and short shoots (with female catkins) had approximately the same bud production rate as the corresponding vegetative shoots, i.e., bud populations did not show any major costs due to sexual reproduction. Meristem costs, i.e., a decrease in the number of buds due to sexual reproduction, may be relatively low in mountain birch, because new axillary buds develop and compensate for lost shoot apices. This compensation capacity may be especially well developed under suboptimal conditions, where canopy expansion is limited by the harsh environment rather than by the availability of meristems. The resource cost of reproduction (e.g., in terms of carbon or mineral nutrients) may also be partly compensated especially when flowering intensity is sufficiently low.

To test the prediction that a mutation causing greater reproductive effort will result in reduced vegetative vigor, we compared the seed production, growth, and carbohydrate status of normal and pistillate genotypes of the grass Tripsacum dactyloides differing in seed production by up to fourfold. We evaluated the costs of reproduction by two methods: experimental manipulation of reproductive effort and comparison of high- and low-yielding genotypes. Despite the large difference in seed production, normal and pistillate (high-yielding) genotypes did not differ in growth rate over a 1-yr period. Contrary to predictions, carbohydrate reserves in the high-yielding genotype were significantly higher at the peak of the seed ripening period. Stalk defoliation and complete removal of seed stalks reduced plant growth rate, total aboveground biomass, and biomass of vegetative storage organs, especially when plants were also under stress from biweekly defoliation. However, stalk removal caused increased rates of growth in the year after experimental manipulations. Reproductive tillers were costly to the plant in terms of lost meristems and therefore future plant size, but seed costs were contained within reproductive tillers of both genotypes, explaining the lack of a trade-off in the high-yielding, pistillate genotype. Although experimental reduction of reproductive effort revealed a cost of reproduction, this result could not be used to correctly predict the consequences of a gene for greater seed production.

Intraspecifically, plant seed production is a function of both seed size (mean mass per seed) and plant size. In this paper we examined the interspecific relationship between the size of seeds and plants and the mean long-term annual seed production per tree. For canopy trees, we show that seed production is highly (inversely) correlated with the mean seed mass as a power law argument. Tree size (basal area or leaf mass) is directly proportional to seed production over a limited range. Analysis of seed production for herbaceous plants indicates a relationship similar to that for trees. As the exponent relating seed size to seed production is > -1.0, it follows that large-seeded plants produce more total annual crop mass than do small-seeded species. However, this is balanced by the greater investment in ancillary reproductive tissue by smaller seeded species. The results obtained here ought to be of theoretical and applied interest in, for example, stand-level simulations of population dynamics or for planning the optimal size and shape of clear-cuts intended to be regenerated naturally.

Although numerous studies have addressed plant compensatory responses to removal of vegetative tissue by herbivores, only a few have determined the effect of floral removal on subsequent seed production in natural populations. In Sanicula arctopoides, a perennial monocarp, removal of inflorescences (umbels), both naturally by deer (Odocoileus hemionus) and by artificial clipping early in the flowering season, led to no loss of maternal reproduction as measured by seed number and seed mass. Full compensation occurred when umbels bearing up to one-third of a plant's flowers were removed at the stage when plants are normally grazed. However, there are thresholds both in timing and severity of removal beyond which plants were unable to compensate fully. Compared to controls, artificial removal of developing fruits 20 d later than the normal stage resulted in a 42% decrease in seed production. Successive clippings that removed approximately equal to 55% of each plant's flowers resulted in a 52% decrease in seed production relative to controls; however, a single clipping of the same intensity resulted in full compensation. Differential reactions to clipping of different umbel ranks provided information about the mechanism of the response. Removal of secondary umbels resulted in decreased seed abortion rates in later developing umbels, while removal of tertiary umbels resulted in decreased abortion rates in both earlier and later formed umbels. These responses indicated that delayed abortion of developing seeds may be common in secondary umbels, that removal of secondary umbels by herbivores may relieve plants of some costs of abortion, and that S. arctopoides uses flexible allocation to achieve full compensation at typical levels of floral herbivory.

In a fragmented, dry subtropical forest in northwestern Argentina, we compared pollination levels, fruit set, and seed set among small (2 ha) fragments, and continuous forest in 16 plant species representing a wide range of pollination systems, breeding systems, and growth forms. For three species, Prosopis nigra (Mimosoideae), Cercidium australe (Caesalpinoideae), and Atamisquea emarginata (Capparaceae), the three treatments were replicated across four sites; we achieved less replication for other species. Because comparisons between forest and fragment populations for different species took place in different sites, however, by treating all 16 species as a unit we lessened the potential bias of confounding site effects and could evaluate the overall impact of fragmentation. Significant or marginal (P < .10) fragmentation-related declines in number of pollen tubes per style, fruit set, and seed set occurred in 9 of 16, 5 of 15, and 3 of 14 species, respectively. Overall, significant or nonsignificant declines occurred in 81% (pollen tubes), 73% (fruit set), and 79% (seed set) of the species. In all cases these proportions were greater (P is less than or equal to .06) than the null binomial expectation of a 1:1 ratio of increases to decreases. Breeding system did not explain sensitivity to fragmentation: the magnitudes of declines in pollen tubes, fruit set, and seed set were virtually indistinguishable between self-compatible and self-incompatible species. At least 4 of the 10 self-incompatible species, however, were heavily visited in small fragments by Africanized honey bees, which may have compensated for a decline in visits by native pollinators. The exact nature of responses varied among plant species. In some, the absolute quantity of pollen grains transferred to stigmas decreased with fragmentation, and sometimes this was reflected in reduced fruit or seed set. In Cercidium, Prosopis, and Atamisquea, the quality of the grains transferred apparently changed: number of pollen tubes produced per pollen grain on the stigma declined with increasing fragmentation, and at least in the latter two species seed production declined as well. Overall, levels of pollination and seed production undoubtedly integrated many idiosyncratic effects of fragmentation on particular plant and animal populations, and indicated that 'community health' of fragments suffered in comparison with that of continuous forest. Median decreases in pollination levels and seed output from forest to fragments approached 20%. The impact of these declines on plant recruitment is less clear, however, because cattle grazing and trampling of seedlings and saplings in fragments may constitute a much more serious short-term conservation problem.

The great variety of sexual systems possessed by flowering plants can be understood in terms of sex allocation theory, which assumes that individuals allocate resources to male vs. female reproductive structures so as to maximize total fitness. The relative shapes of the fitness gain curves for each sex can be used to predict and explain such allocation patterns. Several recent studies have attempted to measure the shapes of these gain curves. Typically, these studies have regressed estimates of male or female fitness against flower number for a set of plants of different size. Here I argue that this method is inappropriate, and that it can lead to incorrect conclusions about the true shapes of the gain curves if some third, causal factor, such as genotype, size, or environmental conditions, affects both allocation and the fitness returns from that allocation. I suggest that experimental manipulation of flower number is likely to be the most useful and easily interpretable technique for estimating the shapes of gain curves in plants.

Seed production is usually assumed to be a positive monotonic function of pollen deposition and/or pollinator visitation. If this assumption were correct, there would be only two outcomes of excess pollen levels: an increase in fruit or seed set, or no increase. However, a substantial minority of the studies reviewed here has found that seed production declines with increased pollen loads, both under experimental and natural conditions. To explain this decrease, we propose the following mechanisms: pollen tube crowding, pollen removal or stigma damage by pollen thieves or pollinators, stigma damage during hand-pollination, application of low-diversity or local pollen, effects of bagging flowers, missed stigma receptivity, and the application of inviable pollen. These mechanisms can be distinguished through more complete and more careful experimental designs and incremental pollen supplementation. Key words: excess pollen; hand-pollination; overpollination; pollen limitation; pollination; reproductive success.