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Experimental evidence suggests that larger plant species generally have a competitive advantage and thus should dominate communities where competition for limiting resources (i.e. water, soil nutrients, light, space and/or mutualists) is intense. Additionally, researchers have postulated that competition can generate negative co‐occurrence patterns. However, neither expectation is strongly supported by empirical evidence.In a relatively undisturbed old‐field plant community, we explored the interdependency of these expectations by examining pairwise species co‐occurrences, and pairwise species maximum height differences (measured here as the maximum potential height of the species), and by estimating competition intensity for sample plots using field transplants grown with and without neighbours. Specifically, we tested whether maximum species height differences were greater for negatively co‐occurring species pairs than for positively co‐occurring pairs found growing together. We also tested whether the maximum species height differences and co‐occurrence status of species pairs were associated with the average competitive environment they tended to be found growing in.Negatively co‐occurring species pairs found growing together differed more in maximum height than in positively co‐occurring species pairs that were more similar in maximum height. Additionally, species maximum height differences between negatively co‐occurring species pairs where higher in less competitively intense plots, whereas species maximum heights were more similar between positively co‐occurring species pairs found growing together in less competitively intense plots.Synthesis. Our results suggest an association between estimates of competition intensity generated using transplant experiments and co‐occurrence patterns. These findings contribute to our understanding of the association between size‐mediated competition and spatial organization in plant communities and provide a framework for future research in this area. [ABSTRACT FROM AUTHOR]

Plant competition experiments commonly suggest that larger species have an advantage, primarily in terms of light acquisition. However, within crowded natural vegetation, where competition evidently impacts fitness, most resident species are relatively small. It remains unclear, therefore, whether the size advantage observed in controlled experiments is normally realized in habitats where competition is most intense. We characterized the light environment and tested for evidence of a size advantage in competition for light in an old‐field plant community composed of perennial herbaceous species. We investigated whether larger species contributed to reduced light penetration (i.e., greater shading), and examined the impact of shade on smaller species by testing whether their abundance and richness were lower in plots with less light penetration. Light penetration in plots ranged from 0.3% to 72.4%. Significant effects were more common when analyses focused on small plants that reached reproduction (i.e., flowering rooted units); focusing on only flowering plants (i.e., excluding nonflowering rooted units) can clarify community patterns. Plots with a greater mean species height had significantly lower light penetration, and plots with lower light penetration had significantly lower flowering abundance and richness of small species. However, the impact of shade on the flowering abundance and richness of small species was relatively small (R2 values between 8% and 15%) and depended on how we defined "small species." Synthesis: Our results confirm that light penetration in herbaceous vegetation can be comparable to levels seen in forests, that plots with taller species cast more shade, and that flowering smaller species are less abundant and diverse in plots where light penetration is low. However, variation in mean plot height explained less than 10% of variation in light penetration, and light penetration explained between 5 and 15% of variation in the flowering abundance and richness of small species. Coupled with the fact that flowering small species were present even within the most heavily shaded plots, our results suggest that any advantage in light competition by large species is limited. One explanation is that at least some small species in these communities are shade‐tolerant. Shade tolerance in predominantly herbaceous communities, particularly among small plant species, requires further research. [ABSTRACT FROM AUTHOR]

A sample of woody angiosperm species was used to test a central prediction of the 'leafing intensity premium' hypothesis: higher leafing intensity (number of leaves produced per unit dry mass of shoot vegetative tissue produced in the same growing season) confers a larger bud bank (i.e. number of axillary meristems per unit shoot tissue) that can be deployed for reproduction, and thus confers generally greater fruit numbers, and hence higher potential fecundity allocation (i.e. fecundity per unit size of the supporting shoot tissue that is produced in the same growing season., Current-year shoots (i.e. bearing leaves) were collected to record: shoot dry mass, total number of leaves, total number of fruits or fruit clusters (if derived from inflorescences), mean individual leaf dry mass and mean individual fruit dry mass. Sampled individuals (shrubs and trees) were also measured for body size (main stem height and circumference)., Species with larger individual fruit (or fruit cluster) mass have generally larger leaves, but they also have a negative trade-off relationship with 'fruiting intensity' - that is the total number of reproductive meristems producing fruits (or fruit clusters) per unit dry mass of shoot vegetative tissue produced in the same growing season. Variation in fruiting intensity, however, is better predicted by a positive relationship with variation in bud bank size., Species with smaller leaf size (dry mass) have generally higher leafing intensity; species with higher leafing intensity in turn have generally higher fruiting intensity; and species with higher fruiting intensity in turn have generally higher potential fecundity allocation (based on the typical species maximum number of seeds per fruit, obtained from published floras). Species with smaller body size have generally higher potential fecundity allocation, but body size had no significant relationships with other measured traits when controlling for phylogeny (using phylogenetically independent contrasts)., Synthesis. Our results indicate that bud bank size is an important functional trait for defining adaptive strategy in woody angiosperms. A larger bud bank is generated by higher leafing intensity, which in turn generates higher fruiting intensity, thus generating greater potential fecundity allocation. These traits will be important for maximizing reproductive economy - that is capacity to produce offspring despite growth or body size limitation (e.g. due to crowding/competition, or because of limited time available for growth, flowering, pollination or fruit/seed maturation). [ABSTRACT FROM AUTHOR]

Alternative metrics exist for representing variation in plant body size, but the vast majority of previous research for herbaceous plants has focused on dry mass. Dry mass provides a reasonably accurate and easily measured estimate for comparing relative capacity to convert solar energy into stored carbon. However, from a 'plant's eye view', its experience of its local biotic environment of immediate neighbors (especially when crowded) may be more accurately represented by measures of 'space occupancy' (S-O) recorded in situ-rather than dry mass measured after storage in a drying oven. This study investigated relationships between dry mass and alternative metrics of S-O body size for resident plants sampled from natural populations of herbaceous species found in Eastern Ontario. Plant height, maximum lateral canopy extent, and estimated canopy area and volume were recorded in situ (in the field)-and both fresh and dry mass were recorded in the laboratory-for 138 species ranging widely in body size and for 20 plants ranging widely in body size within each of 10 focal species. Dry mass and fresh mass were highly correlated ( r2 > .95) and isometric, suggesting that for some studies, between-species (or between-plant) variation in water content may be unimportant and fresh mass can therefore substitute for dry mass. However, several relationships between dry mass and other S-O body size metrics showed allometry-that is, plants with smaller S-O body size had disproportionately less dry mass. In other words, they have higher 'body mass density' (BMD) - more dry mass per unit S-O body size. These results have practical importance for experimental design and methodology as well as implications for the interpretation of 'reproductive economy'-the capacity to produce offspring at small body sizes-because fecundity and dry mass (produced in the same growing season) typically have a positive, isometric relationship. Accordingly, the allometry between dry mass and S-O body size reported here suggests that plants with smaller S-O body size-because of higher BMD-may produce fewer offspring, but less than proportionately so; in other words, they may produce more offspring per unit of body size space occupancy. [ABSTRACT FROM AUTHOR]

Aims Studies of community structure and co-occurrence patterns rely on the premise that community data reflect where species successfully grow and which species they grow with. However, plant censuses generally do not distinguish between species with reproductive individuals and those only represented by non-reproductive individuals. We tested whether inclusion of non-reproductive species, which may not reflect success in that location, significantly impacts evaluations of community structure and co-occurrence. Location Queen's University Biological Station, Ontario, Canada, old-field plant communities. Methods We quantified the impact of non-reproductive species in two plant communities by comparing community structure and co-occurrence patterns when non-reproductive species were included or excluded. Results Including non-reproductive species significantly increased plot-level species richness in both communities (54% and 13% increases), altered species evenness in both communities, significantly impacted beta-diversity among plots in one site, and disproportionately impacted assessments of diversity in species-rich plots. Excluding non-reproductive species resulted in reduced negative co-occurrence patterns in both communities, with a substantially larger impact in one community. In that community, the impact of non-reproductive species was even more pronounced when abundance data were used in analysis, and when pair-wise co-occurrence patterns were assessed. Additionally, including non-reproductive species drastically decreased the number of species pairs with perfect negative co-occurrence across sites, indicating that these species can add 'noise' to co-occurrence patterns. We examined possible explanations for the presence of non-reproductive species. In one community, non-reproductive species were 22 times less abundant (per plot) than reproductive species within plots, although they were not rare overall. Differences in the number of non-reproductive species per plot across our focal communities were not clearly driven by differences in clonality, stress from extreme weather or low N. While these patterns are consistent with the interpretation that non-reproductive species are present due to mass effects, this possibility requires further research. Conclusions Including non-reproductive plant species in censuses can significantly impact assessments of community structure and species co-occurrence. The divergent impact of their inclusion on our two communities highlights the possibility that excluding non-reproductive species from surveys may remove noise from community data and clarify theories of plant species co-existence. [ABSTRACT FROM AUTHOR]

Recent studies have shown that accounting for intraspecific trait variation ( ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies. [ABSTRACT FROM AUTHOR]

Question: Are plant traits more closely correlated with mean annual temperature, orwithmean annual precipitation?. Location: Global. Methods: We quantified the strength of the relationships between temperature and precipitation and 21 plant traits from 447,961 species-site combinations worldwide. We used meta-analysis to provide an overall answer to our question. Results: Mean annual temperature was significantly more strongly correlated with plant traits than was mean annual precipitation. Conclusions: Our study provides support for some of the assumptions of classical vegetation theory, and points to many interesting directions for future research. The relatively low R2 values for precipitation might reflect the weak link betweenmean annual precipitation and the availability of water to plants. [ABSTRACT FROM AUTHOR]

Question Experimental evidence suggests that competition among plant species is generally hierarchical and that relatively large species are at a competitive advantage when competition is predominantly above-ground. However, regional species pools are dominated numerically by relatively small plant species, and small species generally have higher local densities of resident plants within natural communities. One explanation is that larger plant species suffer disproportionately more under effects of intra-specific competition (i.e. greater density dependence). We tested this prediction using ten herbaceous plant species in a competition experiment. Location Kingston, Ontario, Canada, glasshouse. Methods Using a glasshouse experiment, we tested whether relatively large species suffer disproportionately more in monoculture relative to mixtures of all ten herbaceous plant species. We measured the effects of competition on biomass production and survival by monitoring both in monocultures and mixtures of our species. Results Larger plant species suffered more under intra-specific relative to diffuse inter-specific competition in terms of survival; however, the slope of this relationship was not significantly greater than one, indicating that larger species did not suffer disproportionate density-dependent suppression. Conclusions Our results support a role for size in plant competition, but also indicate that this role is reduced because relatively larger species suffer greater density-dependent mortality when competing with other, equally large plants. As such, size-based competitive hierarchies may not function as clearly in natural systems because the increased negative density dependence for larger species contributes to balancing out competition across size hierarchies. [ABSTRACT FROM AUTHOR]

The selection consequences of competition in plants have been traditionally interpreted based on a 'size-advantage' hypothesis - that is, under intense crowding/competition from neighbors, natural selection generally favors capacity for a relatively large plant body size. However, this conflicts with abundant data, showing that resident species body size distributions are usually strongly right-skewed at virtually all scales within vegetation. Using surveys within sample plots and a neighbor-removal experiment, we tested: (1) whether resident species that have a larger maximum potential body size (MAX) generally have more successful local individual recruitment, and thus greater local abundance/density (as predicted by the traditional size-advantage hypothesis); and (2) whether there is a general between-species trade-off relationship between MAX and capacity to produce offspring when body size is severely suppressed by crowding/competition - that is, whether resident species with a larger MAX generally also need to reach a larger minimum reproductive threshold size (MIN) before they can reproduce at all. The results showed that MIN had a positive relationship with MAX across resident species, and local density - as well as local density of just reproductive individuals - was generally greater for species with smaller MIN (and hence smaller MAX). In addition, the cleared neighborhoods of larger target species (which had relatively large MIN) generally had - in the following growing season - a lower ratio of conspecific recruitment within these neighborhoods relative to recruitment of other (i.e., smaller) species (which had generally smaller MIN). These data are consistent with an alternative hypothesis based on a 'reproductive-economy-advantage' - that is, superior fitness under competition in plants generally requires not larger potential body size, but rather superior capacity to recruit offspring that are in turn capable of producing grand-offspring - and hence transmitting genes to future generations - despite intense and persistent (cross-generational) crowding/competition from near neighbors. Selection for the latter is expected to favor relatively small minimum reproductive threshold size and hence - as a tradeoff - relatively small (not large) potential body size. [ABSTRACT FROM AUTHOR]

Theory has produced contrasting predictions related to flowering time overlap among coexisting plant species largely because of the diversity of potential influences on flowering time. In this study, we use a trait-based null modelling approach to test for evidence of deterministic assembly of species according to flowering time in an old-field plant community., Plant species coexisting in one-metre-square plots overlapped in flowering time significantly more than expected. This flowering synchrony was more pronounced when analyses focused on bee-pollinated species. Flowering synchrony was also observed for wind-pollinated species, although for only one of our two null model tests, highlighting the sensitivity of some results to different randomization methods. In general, these patterns suggest that relationships between pollinators and plants can influence community assembly processes., Because our study community is composed of approximately 43% native plant species and 57% exotic species, and because the arrival of new species may complicate plant-pollinator interactions, we tested whether flowering time overlap was altered by introduced species. Flowering synchrony was greater in plots with a higher proportion of introduced species. This pattern held for both null model tests, but was slightly stronger when analyses focused on bee-pollinated species. These results indicate that introduced species alter community flowering distributions and in so doing will inevitably affect pollinator-plant interactions., Finally, we tested whether our results were influenced by variation among study plots in above-ground biomass production, which some theory predicts will be related to the importance of competition. Our results were not influenced by this variation, suggesting that resource variation among our plots did not contribute to observed patterns., Synthesis: Our results provide support for predictions that coexisting species should display flowering synchrony, and provide no support for species coexistence via temporal niche partitioning at this scale in this study community. Our results also indicate that introduced species significantly alter the community assembly process such that flowering synchrony is more pronounced in plots with a greater proportion of introduced plant species. [ABSTRACT FROM AUTHOR]

An ongoing debate in ecology concerns the relative importance of competition in driving community patterns, especially along gradients of disturbance and productivity. We used a null model to address this question by testing for non-random organization of forest species according to maximum height, a trait associated with competitive ability for light. Specifically, we compared the species present in 409 different temperate forest plots with the pool of potentially present species (n=639 species), spanning a 50 000 km2 area of southern Ontario, Canada. In contrast to current theory, coexisting forest species were neither more similar (i.e. convergent), nor more different (i.e. limiting similarity) in maximum height than expected by chance. However, coexisting forest species had larger maximum heights on average, and their maximum heights were more symmetrically distributed than expected by chance, suggesting that competition has reduced the representation of smaller plant species within plots (i.e. higher turn-over of species with smaller maximum heights among forest plots). We explored the possibility that our findings resulted from smaller species having relatively narrower niches; however, a conclusive test of this explanation will require knowledge of fundamental, rather than realized niche breadth. We also tested the prediction that the influence of competition changes along gradients of productivity and disturbance by examining how the effect size of our null model tests changed along these gradients. We observed that species with smaller maximum heights were increasingly under-represented in more productive forest communities, suggesting an increased role for competition in determining species membership in more productive communities. In contrast to theory, however, the effect size of our tests did not significantly change along a gradient of forest disturbance. In summary, we found evidence that maximum species height plays a significant role in driving the non-random organization of plant species among hundreds of mature forest plots, and that this role is more pronounced in more productive forest plots. 1 [ABSTRACT FROM AUTHOR]

Competitive ability in plants has been previously measured almost exclusively in terms of traits related to growth (biomass) or plant size. In this study, however, we used a multi-species competition experiment with six annuals to measure relative competitive ability in terms of reproductive output, i.e. the number of offspring produced for the next generation. Under greenhouse conditions, plants of each species were started in pots from germinating seeds and were grown singly (free of competition) and at high density in both monocultures and in mixtures with all study species. Several traits traditionally regarded as determinants of competitive ability in plants were recorded for each species grown singly, including: seed mass, germination time, early growth rate and potential plant size (biomass and height). Under competition, several traits were recorded as indicators of relative performance in both monocultures and mixtures, including: biomass of survivors, total number of survivors, number of reproductive survivors, and reproductive output (total seed production) of the survivors. As expected, species that grew to a larger biomass in isolation had higher seed production in isolation. However, none of the traditional plant growth/size-related traits, measured either in isolation or under competition, could predict between species variation in reproductive output under competition in either monocultures or mixtures. In mixtures, 97% of this variation in reproductive output could be explained by between-species variation in the number of reproductive survivors. The results indicate that traits measured on plants grown singly may be poor predictors of reproductive output under competition, and that species’ rank order of competitive ability in terms of the biomass of survivors may bear no relationship to their rank order in terms of the number of offspring produced by these survivors. This has important implications for the interpretation of mechanisms of species coexistence and community assembly within vegetation. [ABSTRACT FROM AUTHOR]

1. We investigated patterns in the dispersion (i.e. spread and spacing) of plant species traits that are frequently associated with competitive ability, in an old-field plant community. In contrast with previous studies, we found no evidence for significant over- or under-dispersion of maximum plant height, maximum plant biomass, or seed mass. 2. These findings hold across three plot sizes (10 × 10 cm, 30 × 30 cm, 50 × 50 cm), and when plot size is measured in terms of number of ramets (50 ramets, 250 ramets or 500 ramets) rather than per unit area. Plot size, however, significantly affected the direction of dispersion observed across test statistics, supporting previous studies that have observed that trait dispersion patterns are sensitive to plot size. 3. While no significant dispersion was detected, dispersion direction, measured as the tendency for a two-tailed test to indicate trends for a given trait, was more frequently observed when analyses were weighted by abundance. Abundance weighted analyses had significantly different dispersion directions compared with presence/absence analyses for one of three traits considered. These findings suggest that abundance weighted analyses may yield more consistent trait dispersion patterns. 4. Synthesis. Our results may be interpreted as evidence for limited, if any, functional niche partitioning between co-occurring species via size-mediated differences in rooting depth or physical space niches (i.e. limiting similarity). Alternatively, some species with large differences in competitive traits may avoid competition through niche separation, while others may simply compete asymmetrically, leading to overall patterns of dispersion that cannot be distinguished from random. This interpretation supports recent perspectives that niche and neutral theoretical concepts are not mutually exclusive; both, working together, can be applied to the interpretation of plant community assembly and structure. [ABSTRACT FROM AUTHOR]

Why are many human populations presently ‘imploding’ with below-replacement fertility? Why are more and more young adults in these societies choosing to remain single and/or childless? Based on first principles of evolutionary theory, predictions can be derived for changes over time in the relative frequency distributions of four traits in humans proposed as the most direct determinants of the propensity to mate and reproduce: attractions to sex, legacy, leisure and parenting. In the past, high fitness was most profoundly determined by strong sex drive and strong legacy drive, especially in males. Female fertility was largely controlled by dominant males, who were then free to engage in attractions to both leisure and legacy through ‘memes’ (as well as through genes, or offspring) without any penalty on fitness. Natural selection in the past, therefore, neither strongly favoured nor strongly disfavoured any particular intrinsic female inclinations or preferences that might affect offspring production. The recent, widespread, and continuing rise in the empowerment of women, however, defines a dramatically different contemporary selection regime, where women are now free to indulge in their evolved attractions to leisure and legacy through memes inherited from predecessors, both of which represent compelling distractions from parenthood. The implications for the future survival of marriage and parenthood as cultural institutions look dismal in the short term, but promising in the long term. [ABSTRACT FROM AUTHOR]

1 Using a sample of 24 common deciduous angiosperm trees of the Eastern Deciduous Forest region of North America, we tested the hypothesis that leaf size variation across species can be interpreted in terms of a trade-off between individual leaf mass and the number of leaves produced. 2 The true nature of a resource allocation trade-off is detectable only if variation in the total amount of growth is accounted for. We controlled for this effect by measuring all of the components of annual growth associated with leaf production at the individual terminal shoot level. Hence, number of leaves produced was expressed as ‘leafing intensity’, i.e. the number of leaves produced by newly emerged (current year's) shoots, divided by the total volume of these shoots. 3 Ninety per cent ( r2 = 0.90) of the variation in mean individual leaf mass across species, spanning two orders of magnitude, could be accounted for by proportional variation in mean leafing intensity, i.e. representing an isometric trade-off, with a slope for log-transformed data that did not deviate significantly from −1.0. 4 We suggest that this isometric relationship may represent a generalized trade-off strategy for leaf deployment at the shoot level within temperate deciduous woody species. Following traditional interpretations, adaptation here may involve a fitness benefit associated with a particular leaf size. The present results also suggest an alternative, i.e. selection may instead favour high leafing intensity, with small leaf mass resulting not as a direct adaptation, but simply as a trade-off. 5 According to this ‘leafing intensity premium’ hypothesis, the fitness benefits of higher leafing intensity are associated primarily with the fitness benefits of a larger pool of lateral meristems, because each leaf is usually associated with an axillary bud. This may in turn provide greater facility for wide phenotypic plasticity in the allocation of these meristems to vegetative vs. reproductive functions. This represents a plausible hypothesis, we suggest, in accounting for why most woody deciduous angiosperms, even some of the largest/tallest ones, have relatively small leaves. [ABSTRACT FROM AUTHOR]

1 The prominently right-skewed distribution of species sizes has been the subject of a large literature in animal ecology, but has received comparatively little attention from plant ecologists. It is evident that not all explanations that have been offered for animals are directly applicable to plants. 2 We suggest three hypotheses that require further study in the interpretation of size-dependent species richness in plants. 3 These hypotheses are all based on mechanisms that have generated, for smaller plants, a greater historical opportunity for speciation: (i) large adult plant size confers significant adaptation primarily in habitat types that have been relatively uncommon in space, across evolutionary time; (ii) relatively small species are more widely differentiated from each other in the environmental qualities defining their niches, many of which are made possible by the mere presence of larger species residing in the same habitat; and (iii) compared with large species, smaller species generally have higher fecundity allocation, i.e. they can produce a greater number of offspring per unit plant size per unit time, which generally confers a higher premium under most circumstances of natural selection, thus generating a potentially greater number of descendant individuals, and derived species. 4 We discuss the implications of these hypotheses in addressing an underlying paradox in plant competition/coexistence theory, i.e. that large adult size is assumed to be the principal trait that confers competitive ability yet, even in those habitat types where competition is assumed to reach the highest levels of intensity within vegetation, the vast majority of the resident species are, nevertheless, relatively small. [ABSTRACT FROM AUTHOR]

Telford, Vandvik and Birks (2006) (henceforth TVB) refute our conclusion (Pither & Aarssen 2005) that only a minority of fossil diatom taxa within a diverse regional assemblage are pH specialists. They argue that the null model we used exhibited high Type II error rates for both geographically rare taxa (i.e. few lakes occupied) and for acid-optima taxa, and assert that many more taxa would have been designated specialist had we adequate statistical power, and had more acidic lakes been sampled. However, TVB base their conclusions upon the results of additional analyses, which assume a priori that all taxa are pH specialists. Experimental and observational evidence do not support this assumption. As such, we contend that TVBs criticisms are unjustified. [ABSTRACT FROM AUTHOR]

The article focuses on the causes on low fertility rate in Wealthier countries. Lower fertility in wealthier countries can be explained in evolutionary terms by three key factors: (i) higher fertility in poorer countries--an evolutionary consequence of many generations of intense fertility selection favoring innate behaviors promoting high fertility, especially in males; (ii) the empowerment of women in wealthier countries that serves to reduce fertility directly--an evolutionary consequence of selection favoring an inherent preference for lower fertility in females, combined with release from the evolutionary effects of a long history of male control over female fertility; (iii) and offspring access in wealthy countries to public health care, welfare, and other social services, which combined with inherited wealth for offspring, virtually eliminates competition between families for the resource needs of offspring. The combined consequences of (ii) and (iii) mean that the fertility-selection so prevalent in poor countries is relaxed in wealthy countries, thus allowing random genetic drift to produce an increased relative frequency of innate behaviors promoting low fertility and discontentment with high fertility.

To interpret the unimodal relationship between diatom species richness and lake pH within the context of the evolutionary species pool hypothesis (SPH). We test the following primary prediction arising from the SPH: the size of the potential species pool (PSP) will increase along a gradient representing the historical commonness of different pH environments (pH commonness). To do this we assume that the present-day spatial dominance of near-neutral pH conditions compared with acidic and alkaline conditions reliably mimics the relative spatial availabilities of historical pH conditions among freshwater lakes. We also determine whether local richness represents a constant proportion of PSP size along the pH commonness gradient.Two hundred and thirty-four lakes distributed over a 405,000 km2 region of the north-eastern United States of America.Sediment diatom morphospecies lists and pH data were acquired from the US Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP) website. Using 248 morphospecies that occurred in at least 10 of the 234 lakes, four different measures of PSPs were calculated along the pH gradient. Local species richness was equated with the number of species occurring within the lake. Alpha diversity was equated with the average species richness of lakes with similar pH values. A combination of statistical methods were employed, including correlations, quadratic regression and piecewise regression.PSP size increased significantly with pH commonness for all four measures of PSP size, thus supporting the primary prediction of the evolutionary SPH. Local richness comprised a larger proportion of the PSP within acidic lakes than within circumneutral lakes. Alpha diversity and lake species richness both increased significantly with pH commonness, but the former did so in a two-step fashion. We test and reject several alternative contemporary time-scale explanations for our findings.Our findings are consistent with the hypothesis that diatom taxonomic richness is presently lower within acidic and highly alkaline lakes than in circumneutral lakes owing to the limited opportunity in space and/or time for the evolution of suitably adapted species. Whereas ecological processes can explain why certain species are excluded from particular habitats, e.g. acidic lakes, they cannot account for why so few species are adapted to those habitats in the first place. [ABSTRACT FROM AUTHOR]

Recent research suggests that environmental or habitat specialization among plants may be less common than previously thought. These findings need to be reconciled with interpretations of community-level similarity analyses that emphasize a strong role for specialization. Here we examine specialization within a taxonomic group that, owing to their widespread use as environmental indicators, should provide ample supportive evidence of specialization: freshwater sediment diatoms. Using an ideal, 239-lake survey data set that reliably represents the environmental conditions amongc.9500 north-eastern US lakes (encompassing ac.405 000 km2 region), we show that only 29 and 14% of 401 species (those occurring in at least two lakes) exhibited narrower pH and total phosphorus niche breadths, respectively, than expected from the random occupation of lakes. These rates increase slightly using more stringent species-inclusion criteria. Highly significant correlations between compositional and environmental resemblance matrices are shown to reflect a balance between a strong signal generated by the minority specialists, and noise generated by the majority generalists. [ABSTRACT FROM AUTHOR]

Evidence is reported for size-dependent (allometric) gender allocation in the monoecious, wind-pollinated annual Ambrosia artemissifolia. Consistent with established theory, the pattern of allometry displayed adaptive plasticity, depending on the environmental cause of variation in plant size. Plant size gradients were generated in both field and greenhouse experiments using separate and combined gradients of shading, soil nutrient levels, and neighbor proximity. When plant size constraints involved light limitation from shading (e.g., because of close neighbor proximity), decreasing plant size was generally associated with decreasing maleness and increasing femaleness (based on relative male and female flower production, respectively). This is consistent with the ‘pollen-dispersal’ hypothesis in which the consequences of relatively small plant size (among larger neighbors) imposes less severe limitation for female reproductive success than for male reproductive success (because success as an outcrossing donor of wind-dispersed pollen increases with increasing plant height, especially when neighbors are present). However, when size was constrained by soil nutrient limitation alone (i.e., without shading effects), the results had the converse allometric relationship; i.e., decreasing plant size was generally associated with increasing maleness and decreasing femaleness. This is consistent with the ‘size-advantage’ and ‘time-limitation’ hypotheses in which energetic and time limitations (respectively) associated with relatively small plant size impose a less severe limitation for male reproductive success than for female reproductive success. [ABSTRACT FROM AUTHOR]

Using a habitat templet model, we predict that the productivity (total biomass) of plots within a plant community may be positively, negatively or not at all related to variation in the number of species per plot, depending on successional stage (time since major disturbance) and habitat carrying capacity (reflecting the total resource supplying power of the habitat). For plots of a given size, a positive relationship between productivity and species richness is predicted in recently disturbed habitats because local neighbourhoods here will have been assembled largely stochastically, usually from a pool of available species with a right-skewed size frequency distribution. Hence, in the earliest stages of succession, plots will have relatively high total biomass only if they contain at least some of the relatively uncommon larger species which will, in turn, be more likely in those neighbourhoods that contain more species (the sampling effect). Among these will also be some of the more common smaller species; hence, these high biomass, species-rich plots should have relatively low species evenness, in contrast to what is predicted under effects involving species complementarity. In late succession, the plots with high total biomass will still be those that contain relatively large species but these plots will now contain relatively few species owing to increased competitive exclusion over time (the competitive dominance effect). In intermediate stages of succession, no relationship between plot productivity and species richness is predicted because the opposing sampling and competitive dominance effects cancel each other out. We predict that the intensity of both the sampling and competitive dominance effects on the productivity/species richness relationship will decrease with decreasing habitat carrying capacity (e.g. decreasing substrate fertility) owing to the inherently lower variance in between-plot productivity that is predicted for more... [ABSTRACT FROM AUTHOR]

We examined the relationship between meristem allocation and plant size for four annual plant species: Arabidopsis thaliana, Arenaria serphyllifolia, Brassica rapa, and Chaenorrhinum minus. Gradients of light and nutrient availability were used to obtain a range of plant sizes for each of these species. Relative allocation to reproductive, inactive, and growth meristems were used to measure reproductive effort, apical dominance, and branching intensity, respectively. We measured allocation to each of these three meristem fates at weekly intervals throughout development and at final developmental stage. At all developmental stages reproductive effort and branching intensity tended to increase with increasing plant size (i.e., due to increasing resource availability) and apical dominance tended to decrease with increasing plant size. We interpret these responses as a strategy for plants to maximize fitness across a range of environments. In addition, significant differences in meristem response among species may be important in defining the range of habitats in which a species can exist and may help explain patterns of species competition and coexistence in habitats with variable resource availability. [ABSTRACT FROM AUTHOR]

Focuses on the patterns of declining plant species abundance in regional habitat gradient of community biomass. Intensification of competition; selection of habitat types; Variation in potential vegetation productivity.

Explores the measurement of competitive ability in plants. Failure to account for other sources of variation in the size suppression of plants; Variation in the intensity of concurrent facilitative interactions between competitors; Characteristics of the competitive ability.

Examines the physiological and architectural adaptation and acclimation of temperate deciduous trees in Ontario. Measurement of leaf mass per unit area; Effects of architectural acclimation on branching intensity; Minimality of shade tolerance physiology.

Summary1 The three principal fates of axillary meristems in flowering plants are branching or vegetative growth, reproduction, and inactivity. We examined the relationship between allocation to each of these fates and plant size in five genotypes of Arabidopsis thaliana. Nutrient availability was used to obtain a range of plant size. 2 Allocation to each meristem fate was recorded at weekly intervals throughout development and at final developmental stage. 3 There were significant allometric relationships between meristem allocation and plant size for each meristem fate throughout development in each genotype. Allometry and plasticity tended to be strongest early in development. In addition, although there were differences in allometry between genotypes, meristem allocation through development was generally consistent across genotypes. 4 We interpret the results of this study as an adaptive response to nutrient availability. Increased allocation to growth meristems in high nutrient treatments early in development should maximize both size and the total number of meristems that can be committed to reproduction later in development. This suggests that the pattern of meristem allocation is a component of an adaptive strategy through development in Arabidopsis thaliana. [ABSTRACT FROM AUTHOR]

Presents three experiments which tested the reserve meristem hypothesis for the occurrence of branching in large inflorescences of Verbascum thapsus. Experimental procedures; Results and discussion.

Presents a study which investigated the increase in seed or biomass production in Verbascum thapsus and its seed predator, the weevil Gynmnetron tetrum Fab. Methodology; Results and discussion.

(1) The nature and extent of diversity within single natural populations of Holcus lanatus, Lolium perenne and Trifolium repens, collected from four different-aged adjacent pastures in the lower Fraser Valley of British Columbia, Canada, was investigated. (2) One hundred clones of each species were collected from each pasture and planted in a common garden. Fifteen vegetative and floral characteristics were measured on each sampled clone during one growing season. (3) The variability within each population was large, but all three species showed significant differences in the quantity of variation present among the different aged pastures. Many characters, especially for Lolium perenne and Trifolium repens, showed a decline in mean values and variance with increasing pasture age. (4) The overall high diversity may have been maintained by a heterogeneous biotic environment and the general decline in diversity may be attributable to continuous elimination of the less·fit genotypes by competition and grazing. The ecological and evolutionary implications of this are considered. [ABSTRACT FROM AUTHOR]

Pollen extracts from five locally-common grassland species in eastern Ontario, Canada were tested for in vitro allelopathic effect on the pollen germination of 17 sympatric target species in 1987. The flowering phenologies of the test and target species were recorded to determine if any phenologically-divergent species showed evidence of in vitro pollen-allelopathic interactions. Phleum pratense L. demonstrated pollen-allelopathic effects on all targets except Linaria vulgaris Hill. P. pratense L. also exhibited significant phenological divergence with seven of the target species. Pollen extracts from Agrostis stolonifera L., Melilotus alba Desr., and Vicia cracca L. inhibited pollen germination in some of the target species, but the results suggest that these effects were pH-mediated. The pollen extract of Erigeron annuus (L.) Pers. did not cause inhibition of pollen germination in any of the target species. Further tests with pollen extract of P. pratense and another 23 sympatric target species were performed in 1988. Pollen germination was significantly inhibited in all but two of these species: A. stolonifera and P. pratense. Hence, the pollen of P. pratense was not auto-allelopathic. The breeding system, relatively tall growth habit and relatively large quantity of pollen produced all support the in vitro evidence of P. pratense as a Pollen-allelopathic species. [ABSTRACT FROM AUTHOR]