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ABSTRACT. As climate warms, abiotic factors, as well as plant community and biodiversity structure, may constrain or promote the movement of ecotone boundaries. Our study sought to examine how plant [...]

INTRODUCTION Biotic interactions are vital to ecosystem functioning. Interactions among individuals lie at the core of population and community dynamics, and therefore play a central role in the existence and [...]

While ecologists have studied succession for well over 100 y, there has been little characterization of diversity patterns in nonplant organisms or their interactions across successional gradients. In this study we examined herbaceous vertical vegetation structure and diversity in plants, arthropods, and arbuscular mycorrhizal fungi (AMF) in three Great Lakes sand dune chronosequences. Plant species richness increased linearly across the primary successional gradients at the three sampling sites, while plant vertical structure remained constant. Total arthropod abundance and species richness were positively associated with plant vertical cover, while AMF spore abundance and morpho-type richness were positively associated with plant species richness. Carnivore and herbivore functional groups of arthropods responded differently to plant vertical cover and species richness. Diversity across early primary successional gradients does not consistently increase among different trophic levels, and the vertical structure of vegetation can be important in explaining richness and abundance in other trophic levels across a successional gradient., INTRODUCTION Succession, or the directional, predictable change in biodiversity over time, has captured the interest of ecologists for over a century (Huston and Smith, 1987). In a now classic study [...]

ABSTRACT. High-latitude (61.9°-62.8° N) graminoid plant communities located near the northern limit of boreal forest occurrence in the Carmacks-Pelly Crossing area of Yukon were sampled and classified, and four sociations [...]

In experimental plant communities, relationships between biodiversity and ecosystem functioning have been found to strengthen over time (1,2), a fact often attributed to increased resource complementtarity between species in mixtures (3) and negative plant-soil feedbacks in monocultures (4). Here we show that selection for niche differentiation between species can drive this increasing biodiversity effect. Growing 12 grassland species in test monocultures and mixtures, we found character displacement between species and increased biodiversity effects when plants had been selected over 8 years in species mixtures rather than in monocultures. When grown in mixtures, relative differences in height and specific leaf area between plant species selected in mixtures (mixture types) were greater than between species selected in monocultures (monoculture types). Furthermore, net biodiversity and complementarity effects (1,2) were greater in mixtures of mixture types than in mixtures of monoculture types. Our study demonstrates a novel mechanism for the increase in biodiversity effects: selection for increased niche differentiation through character displacement. Selection in diverse mixtures may therefore increase species coexistence and ecosystem functioning in natural communities and may also allow increased mixture yields in agriculture or forestry. However, loss of biodiversity and prolonged selection of crops in monoculture may compromise this potential for selection in the longer term., Higher biodiversity promotes stability and productivity, with an increasing effect over time (1,2). These positive biodiversity effects on stability and productivity can arise from complementarity between species in resource use, [...]

Elk (Cervus canadensis) can significantly alter plant community composition and reduce plant biodiversity, mass, seedling numbers and sapling growth. Yet, few studies have examined the interactions of reintroduced elk with woody plant communities in eastern North America. To test the hypothesis that elk herbivory would reduce woody plant diversity and recruitment and also change woody plant community composition when compared to deer, we evaluated herbivory effects of a captive elk herd and a free-roaming white-tailed deer (Odocoileus virginianus) population in Land Between the Lakes National Recreation Area in western Kentucky. Elk and deer herbivory did not differ in their effects upon overall woody plant diversity. However, elk browsing significantly altered woody plant community composition ([F.sub.1,15] = 2.27, P = 0.005), reduced stem heights of Quercus and Cornus genera (Ps < 0.036) and reduced frequencies of Quercus, Nyssa, and Sassafras (Ps < 0.045) when compared to deer. Our results suggest elk herbivory pressure on Quercus, as well as other tree and shrub species, will affect eastern forest regeneration and thus managing the growth and distribution of reintroduced elk populations will be important for the viability of eastern and midwestern deciduous forests within restoration zones., INTRODUCTION Reintroductions of formerly extirpated species have gained increased public support, government funding, and media coverage in recent years, often with a goal of restoring historical and healthy ecosystems (e.g., [...]

ABSTRACT. Owned by the Red-tail Land Conservancy, Munsee Woods Nature Preserve (MWNP) is an 18.4 ha (~45.5 acres) woodland located 9.5 km southeast of downtown Muncie, Indiana, and just west [...]

We analyzed the vegetation of Alejandro Selkirk Island using the Zurich-Montpellier approach for taking releves and subsequent classification by a multivariate approach and manual refinement. The resulting vegetation table demonstrates patterns of dominance and variation and the resulting vegetation units that were mapped onto aerial photographs to produce a vegetation map. Additional observations of several inaccessible sectors were gained from photos taken during a boat trip around the island. These results are combined in a colored map that shows the following vegetation units: (1) Dicksonia externa Tree Fern Community (upper montane forest); (2) Lophosoria quadripinnata Fern Community; (3) Fern-Grassland Mosaic; (4) Myrceugenia schulzei Forest (lower montane forest); (5) Anthoxanthum-Nassella Grassland; (6) Coastal Grassland with Juncus procerus; (7) Open Grassland (including Coastal Herb Communities); (8) Rocks, Erosional Zones; and (9) Cultivated and Escaped Plants Near the Settlement. In some cases these units consist of several communities together, often forming mosaic patterns where detailed resolution is not practicable. Unit 7, Open Grassland, has been applied to all areas with a plant cover below 40%, and unit 8, Rocks, Erosional Zones, indicates no or scarce vegetation (cover notably below 10%). Some plant assemblages cannot be shown on the map: (a) the small clusters of Drimys confertifolia; (b) the mostly linearly or patchily arranged Gunnera masafuerae community; (c) several plant assemblages found in the canyons; and (d) the Histiopteris incisa clusters between the tree ferns and tall ferns. We discuss composition of the observed plant communities, especially regarding alien impact, and compare our findings with those on Robinson Crusoe, the largest island of the archipelago., Vegetation on remote oceanic islands is determined to a high degree by the unique fate of its floristic elements (Mueller-Dombois and Fosberg 1998). The level of floral diversity on such [...]

The macrofaunal colonization of isolated habitats is affected by many factors, ranging from distance to the nearest source population to the dispersal mechanism of the species. We investigated the initial epifaunal colonization at two sites, one situated in the Northern Gulf of Mexico and the other in the Northern Baltic Sea. At each site, artificial seagrass units were placed at 10- and 20-m distances from a continuous seagrass meadow, as well as inside the meadow over a 5-day colonization time. With the exception of amphipods in Gulf of Mexico, patch isolation had a negative effect on colonization for the other faunal species, irrespective of the sites. This inverse colonization pattern of amphipods suggests that they are not equally sensitive to patch isolation in different regions. Our results indicate that increasing habitat isolation can have serious consequences for the community composition of seagrass epifauna. Furthermore, we emphasize the need for larger-scale latitudinal comparative studies., Introduction Seagrasses are distributed worldwide (Orth et al. 2006) and form large submerged meadows that sustain and provide important ecosystem functions, goods and services, including primary production, carbon storage, erosion [...]

Abstract We review resilience to natural and anthropogenic disturbance of palm populations and communities in tropical America. Response of palms to disturbance depends on their morphological traits, their reproductive strategies [...]

Soils are extremely rich in biodiversity, and soil organisms play pivotal roles in supporting terrestrial life, but the role that individual plants and plant communities play in influencing the diversity and functioning of soil food webs remains highly debated. Plants, as primary producers and providers of resources to the soil food web, are of vital importance for the composition, structure, and functioning of soil communities. However, whether natural soil food webs that are completely open to immigration and emigration differ underneath individual plants remains unknown. In a biodiversity restoration experiment we first compared the soil nematode communities of 228 individual plants belonging to eight herbaceous species. We included grass, leguminous, and non-leguminous species. Each individual plant grew intermingled with other species, but all plant species had a different nematode community. Moreover, nematode communities were more similar when plant individuals were growing in the same as compared to different plant communities, and these effects were most apparent for the groups of bacterivorous, carnivorous, and omnivorous nematodes. Subsequently, we analyzed the composition, structure, and functioning of the complete soil food webs of 58 individual plants, belonging to two of the plant species, Lotus corniculatus (Fabaceae) and Plantago lanceolata (Plantaginaceae). We isolated and identified more than 150 taxa/groups of soil organisms. The soil community composition and structure of the entire food webs were influenced both by the species identity of the plant individual and the surrounding plant community. Unexpectedly, plant identity had the strongest effects on decomposing soil organisms, widely believed to be generalist feeders. In contrast, quantitative food web modeling showed that the composition of the plant community influenced nitrogen mineralization under individual plants, but that plant species identity did not affect nitrogen or carbon mineralization or food web stability. Hence, the composition and structure of entire soil food webs vary at the scale of individual plants and are strongly influenced by the species identity of the plant. However, the ecosystem functions these food webs provide are determined by the identity of the entire plant community. Key words: biodiversity; ecosystem function; food web composition; food web model; Lotus corniculatus; macrofauna; mesofauna; microfauna: Plantago lanceolata; soil biota.

The causes of local diversity and composition remain a central question in community ecology. Numerous studies have attempted to understand community assembly, both within and across trophic levels. However, little is known about how community assembly aboveground influences soil microbial communities belowground. We hypothesized that plant establishment order can affect the community of arbuscular mycorrhizal fungi (AMF) in roots, with the strength of this effect dependent on both host plant identity and neighboring plant identity. Such priority effects of plants on AMF may act through host-specific filters of the initial species pool that limit the available pool for plants that established second. In a greenhouse experiment with four plant hosts, we found that the strength of the priority effect on AMF communities reflected both host plant characteristics and interactions between host and neighbor plant species, consistent with differential host specificity among plants. These patterns were independent of plant biomass and root colonization. Functional studies of AMF associated with a wide array of host plants will be required to further understand this potential driver of community dynamics. Key words: arbuscular mycorrhizal fungi (AMF); Avena barbata; Bromus hordeaceus; community assembly; grassland; Nassella pulchra; phenology; priority effects; T-RFLP; Vulpia microstachys.

Many studies have examined invasion resistance in plant communities, but few have explored the mechanisms of invasion and how subsequent community reassembly affects community functioning. Using natural dispersal and deliberate seed addition into grassland communities with different compositional and richness histories, we show that invaders establish in a nonrandom manner due to negative effects of resident functional groups on invading species from the same functional group. Invaders hence complement communities with originally low richness levels. Consequently, communities converge toward similar levels of species richness, high functional richness, and evenness, but not always maximum productivity. Invasion processes are faster but qualitatively similar when the effect of chance, in the form of dispersal stochasticity, is reduced by seed addition. Thus, dispersal limitation may influence community assembly, but it does not override functionally predictable assembly mechanisms. Some of the most productive communities prior to invasion are unstable in the face of invasion, leading to decreased productivity following invasion. We suggest that invasion into such communities occurs possibly because a pathogen-free niche is available rather than a resource niche. Thus, pathogens in addition to resource niches may be important biological drivers of community assembly. Key words: biodiversity-productivity' relationship; community, stability; dispersal limitation,' ecosystem functioning; invasion resistance; invasiveness; negative feedback; neutral theory; nonrandom invasion; species richness.

Epiphytes are an important component of many forested ecosystems, yet our understanding of epiphyte communities lags far behind that of terrestrial-based plant communities. This discrepancy is exacerbated by the lack of a theoretical context to assess patterns in epiphyte community structure. We attempt to fill this gap by developing an analytical framework to investigate epiphyte assemblages, which we then apply to a data set on epiphyte distributions in a Panamanian rain forest. On a coarse scale, interactions between epiphyte species and host tree species can be viewed as bipartite networks, similar to pollination and seed dispersal networks. On a finer scale, epiphyte communities on individual host trees can be viewed as meta-communities, or suites of local epiphyte communities connected by dispersal. Similar analytical tools are typically employed to investigate species interaction networks and meta-communities, thus providing a unified analytical framework to investigate coarse-scale (network) and fine-scale (meta-community) patterns in epiphyte distributions. Coarse-scale analysis of the Panamanian data set showed that most epiphyte species interacted with fewer host species than expected by chance. Fine-scale analyses showed that.epiphyte species richness on individual trees was lower than null model expectations. Therefore, epiphyte distributions were clumped at both scales, perhaps as a result of dispersal limitations. Scale-dependent patterns in epiphyte species composition were observed. Epiphyte-host networks showed evidence of negative co-occurrence patterns, which could arise from adaptations among epiphyte species to avoid competition for host species, while most epiphyte meta-communities were distributed at random. Application of our 'metanetwork' analytical framework in other locales may help to identify general patterns in the structure of epiphyte assemblages and their variation in space and time. Key words: checkerboard distribution; co-occurrence; epiphyte; host trees; meta-community; nestedness; network; null model; Panamanian rain forest; plant assemblages; scale-dependent patterns; spatiotemporal variation.

Impacts of biological invasions on recipient plant communities vary enormously, depending on both the traits of the introduced species and those of its new ecosystem. The trajectories of many introduced plant species in invaded ecosystems seem very similar to those taken by native plants as part of secondary succession. It is likely that studying them in the framework of modern succession ecology would be fruitful. Other introduced species of plants, animals and pathogens cause massive, rapid and often irreversible changes in the recipient communities, of a magnitude rarely produced by native species. Understanding such invasions, and the great range of impacts caused by introduced species, will probably require at least some approaches different from those of other sciences of vegetation change., Although invasion biology has been somewhat dissociated from other research on vegetation change, at least a partial rapprochement is underway, and, in any event, it is not evident that understanding [...]

Disturbances may add or remove resources from communities and can facilitate the spread of some invaders. Differential successes of exotic plants to spread into different communities suggest that some habitats are more invasible than others. Hesperis matronalis (dame's rocket) is widespread across the continent and has potential to become a major problem in natural communities owing to its ability to exploit excess resources and its negative effect on native species. We grew H. matronalis seedlings in a Colorado foothill community at various levels of nitrogen and water with and without neighbors. We expected H. matronalis to maximize its growth under high resource conditions especially without neighbors because ruderal species are able to take advantage of excess resources. Our results reveal that presence or absence of neighbors was the main decisive factor for H. matronalis (aboveground biomass and relative growth rate) success, though seedlings particularly suffered under low resource conditions. Early determination of the potential invasiveness of a widespread exotic species such as IL matronalis in natural communities may promote effective management., INTRODUCTION Differential successes of exotic plants in spreading into various communities suggest that some habitats are more invasible than others (Lonsdsale, 1999; Stohlgren et al., 1999; Von Holle, 2005). Disturbance, [...]

While goldenrod species are often found to be allelopathic in laboratory settings, its importance in controlling plant community dynamics has been much more difficult to assess. We designed a study to determine whether allelopathy is related to the success of goldenrods in abandoned agricultural land. To accomplish this, we conducted laboratory bioassays for six co-occurring goldenrod species and compared these results to the cover and impacts of these species in the field. We determined the germination responses of two target species to a gradient of leaf extract concentrations to assess the allelopathic potential of these goldenrods. We also used long-term successional data to determine the influence of each goldenrod species on community turnover. Germination percentages in the lab were reduced by leaf extracts for most goldenrod species and varied dramatically among species. In the field, influences of goldenrods on their associated communities were weak and opposite expected allelopathic effects, as the number of associated species generally increased with goldenrod cover. The relative strength of allelopathy among goldenrods was not related to the abundance achieved during succession. In this system, we documented the potential for goldenrods to exhibit allelopathic interactions in a controlled situation. However, these effects were not strong enough to alter community structure and turnover., INTRODUCTION Goldenrods (largely Solidago spp.) are herbaceous perennials commonly found in meadows, prairies, roadsides and abandoned agricultural land in eastern North America. They are self-incompatible, produce numerous wind-dispersed seeds and [...]

Is Allotetraploid Evolution in Terns a Species Pump? Gary Greer, Biology Department, Grand Valley State University Ferns including Psilotopsida, Equisetopsida, Marratiopsida, and Polypodiopsida are currently the second most species rich [...]

Community assembly theory asserts that the contemporary composition of ecological communities may depend critically on events that occur during the formation of the community; a phenomenon termed 'historical contingence.' We tested key aspects of this theory using plant communities in over 200 experimentally created vernal pools at a field site in central California, USA. The experiment was initiated in 1999 with construction of vernal pool basins into which different seeding treatments were imposed to evaluate the effects of dispersal limitation, order of colonization ('priority effects'), and frequency of colonization on plant community composition. We tracked the abundance and distribution of five focal species for seven years following seeding and observed strong but transient effects of seeding, as well as order and frequency of colonization. All five species occurred with higher frequency in seeded pools vs. unseeded control pools, demonstrating dispersal limitation. Three of four species exerted strong priority effects, with much higher abundance in pools in which they were seeded first compared to pools in which they were seeded in the second year of the study, one year after other species were seeded. We tested for effects of frequency of colonization using one species, the endangered Lasthenia conjugens, and observed much higher abundance in frequently vs. infrequently seeded pools for the first four years following seeding. Finally, we observed that the strength of priority effects varied significantly with water depth for one of the species groups, which demonstrates that abiotic context can strongly influence species interactions. We conclude that several aspects of historical contingence play key roles in the early formation of vernal pool plant communities. But we also observed reversals in community trajectories, suggesting that in this system historical effects may be lost within a decade. Key words: California, USA, community assembly; endangered species; grasslands; Lasthenia conjugens; priority effects; restoration; vernal pools.

Introduced plants may compete for pollination with native species, leading to increased pollinator limitation for one or both species. In this study, 1 test the hypothesis that the introduced plant Euphorbia esula (Euphorbiaceae, leafy spurge) competes for pollination with the native prairie perennial Sisyrinchium campestre (Iridaceae, blue-eyed grass). A breeding system study revealed that Sisyrinchium is self-incompatible, potentially increasing its vulnerability to competition for pollination. Interspecific competition for pollinator visits occurred, as visit rates were lower for Sisyrinchium near Euphorbia than for Sisyrinchium more than 10 m from Euphorbia. However, supplemental hand pollinations of Sisyrinchium did not increase fruit or seed set either near to or tar from Euphorbia, indicating that visits were not limiting. More than one- third of Sisyrinchium stigmas received Euphorbia pollen, but hand-pollination experiments detected no effect of Euphorbia pollen receipt on fruit or seed set, whether Euphorbia pollen was applied immediately or 2 h before application of Sisyrinchium pollen. Overall, this study suggests that Euphorbia does not reduce Sisyrinchium's pollination success despite competing for pollinator visits and being a source of heterospecific pollen on Sisyrinchium stigmas., INTRODUCTION Interspecific competition for pollination occurs when one species' fecundity is diminished by pollinator-mediated interactions with a simultaneously flowering species (Rathcke, 1983). Competition for pollination may be caused by competition [...]

Landscape fragmentation and exotic species invasions are two modern-day forces that have strong and largely irreversible effects on native diversity worldwide. The spatial arrangement of habitat fragments is critical in affecting movement of individuals through a landscape, but little is known about how invasive species respond to landscape configuration relative to native species. This information is crucial for managing the global threat of invasive species spread. Using network analysis and partial Mantel tests to control for covarying environmental conditions, we show that forest plant communities in a fragmented landscape have spatial structure that is best captured by a network representation of landscape connectivity. This spatial structure is less pronounced in invasive species and exotic species dispersed by animals. Our research suggests that invasive species can spread more easily in fragmented landscapes than native species, which may make communities more homogeneous over time. Key word: beta diversity; graph theory; habitat fragmentation: invasion: seed dispersal; spatial pattern.

Plant species composition and diversity is often influenced by early life history stages; thus, global change could dramatically affect plant community structure by altering seed production. Unfortunately, plant reproductive responses to global change are rarely studied in field settings, making it difficult to assess this possibility. To address this issue, we quantified the effects of elevated C[O.sub.2], nitrogen deposition, and declining diversity on inflorescence production and inflorescence mass of 11 perennial grassland species in central Minnesota, USA. We analyzed these data to ask whether (1) global change differentially affects seed production of co-occurring species; (2) seed production responses to global change are similar for species within the same functional group (defined by ecophysiology and growth form); and (3) seed production responses to global change match productivity responses. We found that, on average, allocation to seed production decreased under elevated C[O.sub.2], although individual species responses were rarely significant due to low power (C[O.sub.2] treatment df = 2). The effects of nitrogen deposition on seed production were similar within functional groups: Ca grasses tended to increase while [C.sub.3] grasses tended to decrease allocation to seed production. Responses to nitrogen deposition were negatively correlated to productivity responses, suggesting a trade-off. Allocation to seed production of some species responded to a diversity gradient, but responses were uncorrelated to productivity responses and not similar within functional groups. Presumably, species richness has complex effects on the biotic and abiotic variables that influence seed production. In total, our results suggest that seed production of co-occurring species will be altered by global change, which may affect plant communities in unpredictable ways. Although functional groups could be used to generalize seed production responses to nitrogen deposition in Minnesota prairies, we caution against relying on them for predictive purposes without a mechanistic understanding of how resource availability and biotic interactions affect seed production. Key words: diversity; elevated C[O.sub.2]; fecundity; global change, grasslands; nitrogen deposition: prairies; recruitment; seed production.

As ecosystem engineers, prairie dogs (Cynomys spp.) physically alter their environment, but the mechanism by which these alterations affect associated faunal composition is not well known. We examined how rodent and vegetation communities responded to prairie dog colonies and landcover at the Cimarron National Grassland in southwest Kansas, USA. We trapped rodents and measured vegetation structure on and off colonies in 2000 and 2003. We plotted two separate ordinations of trapping grids: one based on rodent counts and a second based on vegetation variables. We regressed three factors on each ordination: (1) colony (on-colony and off-colony), (2) cover (shortgrass and sandsage), and (3) habitat (factorial cross of colony x cover). Rodent communities differed by colony but not cover. Vegetation differed across both gradients. Rodent responses to habitat reflected those of colony and cover, but vegetation was found to differ across cover only in the sandsage prairie. This interaction suggested that rodent composition responded to prairie dog colonies, but independently of vegetation differences. We conclude that burrowing and soil disturbance are more important than vegetation cropping in structuring rodent communities. Key words: black-tailed prairie dogs; community composition; Cynomys spp.; ecosystem engineer: nonmetric multidimensional scaling; ordination: rodenis: vegetation structure.

Herbaceous plant species are important components of forest ecosystems, and their persistence in forests may be affected by invasive plant species that reduce mycorrhizal colonization of plant roots. I examined the effect of the invasive plant Alliaria petiolata on arbuscular mycorrhizal fungi (AMF) colonizing the roots of three forest plant species. AMF root colonization and community structure was examined from plants that were growing either in the absence or presence of Alliaria under natural forest conditions. AMF root colonization varied among the plant species but was not significantly affected by Alliaria. With molecular methods, ~ 12 different taxa of AMF could be distinguished among the root samples, and these taxa belonged to the genera Acaulospora and Glomus, with Glomus dominating AMF communities. There were significant differences between the community of AMF colonizing roots of Maianthemum racemosum and Trillium grandiflorum, but only AMF communities of Maianthemum roots were significantly affected by Alliaria. Indicator species analysis found that an Acaulospora species type was a significant indicator of Maianthemum plants grown in the absence of Alliaria. These results suggest invasive plants like Alliaria may selectively suppress AMF fungi, and this suppression can affect AMF communities colonizing the roots of some native plant species. Key words: Alliaria petiolata; arbuscular mycorrhizal fungi; Arisaema triphyllum; diversity; Maianthemum racemosum; terminal restriction fragment length polymorphisms; TRFLP; Trillium grandiflorum.

Plants can alter soil properties in ways that feed back to affect plant performance. The extent that plant soil feedback affects co-occurring plant species differentially will determine its impact on plant community structure. Whether feedback operates consistently across similar plant communities is little studied. Here, the same grasses from two eastern U.S. serpentine grasslands and two midwestern tallgrass prairie remnants were examined for plant soil feedback in parallel greenhouse experiments. Native soils were homogenized and cultured (trained) for a year with each of the four grasses. Feedback was evaluated by examining biomass variation in a second generation of (tester) plants grown in the trained soils. Biomass was lower in soils trained by conspecifics compared to soils trained by heterospecifics in seven of 15 possible cases; biomass was greater in conspecific soils in one other. Sorghastrum nutans exhibited lower biomass in conspecific soils for all four grasslands, so feedback may be characteristic of this species. Three cases from the Hayden prairie site were explained by trainer species having similar effects across all tester species so the relative performance of the different species was little affected; plants were generally larger in soils trained by Andropogon gerardii and smaller in soils trained by S. nutans. Differences among sites in the incidence of feedback were independent of serpentine or prairie soils. To explore the causes of the feedback, several soil factors were measured as a function of trainer species: nutrients and pH, arbuscular mycorrhizal (AM) spore communities, root colonization by AM fungi and putative pathogens, and functional diversity in bacterial communities as indicated by carbon substrate utilization. Only variation in nutrients was consistent with any patterns of feedback, and this could explain the greater biomass in soils trained by A. gerardii at Hayden. Feedback at Nottingham (one of the serpentine sites) differed, most notably for A. gerardii, from that of similar past studies that used different experimental protocols. To understand the consequences of feedback for plant community structure, it is important to consider how multiple species respond to the same plant-induced soil variation as well as differences in the feedback detected between greenhouse and field settings. Key words: Andropogon gerardii; arbuseular mycorrhizae,' root pathogens; Schizachyrium scoparium; serpentine grassland; Sorghastrum nutans; Sporobolus heterolepis; tallgrass prairie.

Abstract: We studied the process of change in plant species composition in a pine-spruce forest influenced by endozoochorous seed dispersal by the European bison Bison bonasus L. The intensity of [...]

Biological invasions can have strong ecological effects on native communities by altering ecosystem functions, species interactions, and community composition. Even though these ecological effects frequently impact the population dynamics and fitness of native species, the evolutionary consequences of biological invasions have received relatively little attention. Here, I show that invasions impose novel selective pressures on a native plant species. By experimentally manipulating community composition, I found that the exotic plant Medicago polymorpha and the exotic herbivore Hypera brunneipennis alter the strength and, in some instances, the direction of natural selection on the competitive ability and anti-herbivore defenses of the native plant Lotus wrangelianus. Furthermore, the community composition of exotics influenced which traits were favored. For example, high densities of the exotic herbivore Hypera selected for increased resistance to herbivores in the native Lotus; however, when Medicago also was present, selection on this defense was eliminated. In contrast, selection on tolerance, another plant defense trait, was highest when both Hypera and Medicago were present at high densities. Thus, multiple exotic species may interact to influence the evolutionary trajectories of native plant populations, and patterns of selection may change as additional exotic species invade the community. Key words: biological invasion; competition; diffuse selection; herbivory; Hypera brunneipennis; Lotus wrangelianus; Medicago polymorpha; natural selection; plant defense; plant-insect interaction.

Birds and vegetation were surveyed in a 9 ha plot in spring 1976 in a 73 y-old jack pine (Pinus banksiana)--black spruce (Picea mariana) forest in northeastern Minnesota. A 1368 ha wildfire burned across the area that autumn. The plot was resurveyed in 1977 and periodically through 2006. Before the fire, birds with the highest importance values were Blackburuian Warbler (Dendroicafusca), Ovenbird (Seiurus aurocapillus), Red-eyed Vireo (Vireo olivaceus) and Bay-breasted Warbler (D. castanea). Within 7 y following the fire, canopy tree cover decreased to near zero as fire-damaged trees died. Afterwards, the canopy began increasing, reaching 53% cover by 30 y. Shrub cover, 8% before the fire, peaked at over 70% two decades after fire, primarily as a result of dense jack pine and black spruce regeneration, and then decreased to 58% 30 y after fire. The total number of bird species using the area doubled the first year following the fire while the number of bird species with discernable territories decreased 40%. Thereafter, territorial species began increasing and 30 y after the fire the number exceeded the pre-fire richness by 60%. Overall, density of bird territories decreased nearly three-fold the first 3 y after the fire, but by year 30, was over 56% greater than in the pre-burn mature pine forest. Loss of canopy was related to a reduction in warbler and vireo diversity while increases in woody debris and near-ground vegetation were related to an increase in ground-brush foragers such as White-throated Sparrow (Zonotrichia albicollis) and Chipping Sparrow (Spizella passerina). Brown Creeper (Certhia americana) populations increased briefly as trees died, and for five years following fire there was an increase in woodpeckers and secondary cavity nesting species. At 7 to 10 y after fire, White-throated Sparrow, Magnolia Warbler (Dendroica magnolia), Chestnut-sided Warbler (D. pensylvanica), Nashville Warbler (Vermivora ruficapilla) and Mourning Warbler (Oporornis philadelphia) dominated. White-throated Sparrow continued to be the most important bird species through the first two decades, followed by Magnolia Warbler and Red-eyed Vireo. Thirty years after fire, the dominant birds were Nashville Warbler and Ovenbird, followed distantly by Veery (Catharus fuscescens) Swainson's Thrush (Catharus ustulatus), Least Flycatcher (Empidonax minimus) and Black-and-white Warbler (Mniotilta varia). Overall, bird species using the area after 30 y remained over 70% higher than in the mature forest before the fire., INTRODUCTION In Jun. 1976, as part of a long-term study of post-fire bird community development, we established a permanent plot in a 73 y-old jack pine (Pinus banksiana)--black spruce (Picea [...]

Light availability in the understory of deciduous forests changes drastically within the growing season due to the foliage dynamics of canopy trees. Because flowering phenology, photosynthetic characteristics, and fruiting success respond to such strong seasonality in light availability, we hypothesized that understory plants in such ecosystems should describe distinct phenological groups or syndromes where 'syndrome' is defined only as a set of characteristics that co-occur. To identify these phenological syndromes, we studied the flowering phenology, fruit or seed set, and photosynthetic characteristics for 18 perennial understory herbaceous species that differed in reproductive strategy over eight years in a deciduous forest in northern Japan. Three phenological groups emerged from this study: (1) spring bloomers, flowering and fruiting before the completion of canopy closure; (2) early-summer bloomers, flowering during the progress of canopy closure and fruiting after canopy closure; and (3) late-summer bloomers, flowering and fruiting after canopy closure. The spring bloomers had high photosynthetic rates and high fruiting abilities, but the flowering time varied considerably among years due to yearly fluctuations of snowmelt date. Bumble bee-pollinated species of spring bloomers showed variable seed-set success, while fly-pollinated species showed relatively stable seed sets over the years. The early-summer bloomers showed low fruiting abilities irrespective of pollination success, reflecting severe resource limitation with decelerating light availability during fruit development. Although the late-summer bloomers showed low photosynthetic rates under low-light conditions, high fruit-set success was attained if pollination was sufficient. These results support our hypothesis that phenological syndromes may be found in deciduous forest understory plants. Given that reproductive success of bee-pollinated spring bloomers is highly susceptible to seasonal fluctuation, climate change may have its strongest impacts on this group. Key words: decidous forest; flowering phenology; fruit set; light availability; photosynthetic rate; pollination; seasonal dynamics; understory herbs.

Although rare in Wisconsin, seven examples of the cedar glade community type were included John T. Curtis's study of the vegetation conducted in the 1950s. Comparison of the contemporary vegetation to historical records revealed major successional changes. Juniperus virginiana (L.) remains the dominant woody plant, though it has since formed closed canopies. Canopy closure led to the replacement of understory savanna and prairie plants by shade-tolerant species, and the stands have become more similar to one another in composition. Many of the taxa that dominated the 2004 understory were present in the 1950s, though at lower abundances. The number of exotic taxa and the abundance of exotic individuals increased, although species native to Wisconsin still accounted for ~90% of plants present in the understory.

Studies comparing historical data with modern surveys can provide important insights into avian population trends. In 1993-1995 I repeated a breeding bird survey completed by G. W. Salt in 1952 and 1954 in Grand Teton National Park, Wyoming. The composition of breeding birds on all transects were comparable during the four decades, with most species showing no discernable difference in relative abundance (60.7%; n = 54), although 45.6% (n = 41) showed strong annual variation in the 1990s. Long-distance migrants represented the greatest number of species present (52.8%; n = 47) and the largest proportion showing a decline (25.5%; n = 12). Resident species had the largest proportion showing an increase (31.8%; n = 7). One resident species (Clark's nutcracker, Nucifraga columbiana) and one short distance migrant (red-naped sapsucker, Sphyrapicus nuchalis) showed a declining trend. American bitterns (Botaurus lentiginosus) may have disappeared from the sites surveyed in this study. Willow flycatcher (Empidonax traillii) abundance, while highly variable in the 1990s, also showed a significant decline. Close to half of the species occurring in spruce-fir forest declined (42.2%; n = 11), while species occurring in lodgepole pine (Pinus contorta) forest increased (35.3%; n = 12). Large national parks, such as Grand Teton National Park, could act as avian refugium, fundamental in preserving species that are experiencing declines in surrounding areas or on wintering grounds. The value of relatively undisturbed landscapes, such as national parks, to protect and conserve species numbers and diversity is increasingly vital, as the landscape and habitat in the surrounding areas continue to change.

Invasive species richness often is negatively correlated with native species richness at the small spatial scale of sampling plots, but positively correlated in larger areas. The pattern at small scales has been interpreted as evidence that native plants can competitively exclude invasive species. Large-scale patterns have been understood to result from environmental heterogeneity, among other causes. We investigated species richness patterns among submerged and floating-leaved aquatic plants (87 native species and eight invasives) in 103 temperate lakes in Connecticut (northeastern USA) and found neither a consistently negative relationship at small (3-[m.sup.2]) scales, nor a positive relationship at large scales. Native species richness at sampling locations was uncorrelated with invasive species richness in 37 of the 60 lakes where invasive plants occurred; richness was negatively correlated in 16 lakes and positively correlated in seven. No correlation between native and invasive species richness was found at larger spatial scales (whole lakes and counties). Increases in richness with area were uncorrelated with abiotic heterogeneity. Logistic regression showed that the probability of occurrence of five invasive species increased in sampling locations (3 [m.sup.2], n = 2980 samples) where native plants occurred, indicating that native plant species richness provided no resistance against invasion. However, the probability of three invasive species' occurrence declined as native plant density increased, indicating that density, if not species richness, provided some resistance with these species. Density had no effect on occurrence of three other invasive species. Based on these results, native species may resist invasion at small spatial scales only in communities where density is high (i.e., in communities where competition among individuals contributes to community structure). Most hydrophyte communities, however, appear to be maintained in a nonequilibrial condition by stress and/or disturbance. Therefore, most aquatic plant communities in temperate lakes are likely to be vulnerable to invasion. Key words: beta diversity; biodiversity; biological invasion; density; exotic plants; invasion resistance; macrophytes; species richness.

The term 'cedar glades' has been applied to several different types of plant communities that occur on rocky calcareous soils in eastern North America. A previous paper by the first two authors reviewed in considerable detail the use of this term with regard to type of vegetation on rocky limestone soils in the Nashville (Central) Basin of Tennessee. The present paper reviews use of 'cedar glades' as a descriptive term for vegetation in other physiographic regions of eastern North America. In the Nashville Basin, and in some other physiographic regions, the term has been applied to true open cedar glades ('glades') plus the surrounding redcedar/redcedar-hardwood forest, or (more recently) to the natural rocky treeless openings only. However, outside the Nashville Basin, it has also been used to describe several other disparate vegetation types, such as xeric limestone prairies (limestone glades, prairie barrens), redcedar-little bluestem savanna, dense redcedar forest, redcedar-hardwood forest, and even an open stand of redcedar-pine-sweetgum with warm-season perennial prairie grasses in the understory. In the Great Lakes region of North America and in the Baltic region of Europe, some alvars are similar to cedar glades sensu stricto of southeastern United States. Since many of the rocky calcareous openings in eastern North America can be classified as either cedar glades or xeric limestone prairies, a suite of abiotic, biotic, and anthropogenic-related factors is presented for use in distinguishing between these two vegetation types., Abstract Introduction Vegetation on Rocky Calcareous Soils Southeastern United States Midwest and Ozark Regions Texas Alvars of the Great Lakes Region (Canada/USA) and the Baltic Region (Europe) Open Cedar Glades [...]

Coevolutionary theory proposes that the diversity of chemical structures found in plants is, in large part, the result of selection by herbivores. Because herbivores often feed on chemically similar plants, they should impose selective pressures on plants to diverge chemically or bias community assembly toward chemical divergence. Using a coevolved interaction between a group of chrysomelid beetles and their host plants, I tested whether coexisting plants of the Mexican tropical dry forest tend to be chemically more dissimilar than random. Results show that some of the communities are chemically overdispersed and that overdispersion is related to the tightness of the interaction between plants and herbivores and the spatial scale at which communities are measured. As coevolutionary specialization increases and spatial scale decreases, communities tend to be more chemically dissimilar. At fairly local scales and where herbivores have tight, one-to-one interactions with plants, communities have a strong pattern of chemical disparity. herbivore specialization | insect-herbivore interaction | plant chemical diversity

Small isolated wetlands in the southern Appalachian mountain region contain a disproportionate amount of the region's rare and uncommon herbaceous species. Many of these herbaceous communities may be threatened by invading trees and shrubs. We investigated changes in vegetative communities between 1994 and 2001 and the impact of woody encroachment upon herbaceous wetland communities in a southern Appalachian wetland complex in western North Carolina. We conducted a quantitative investigation of herbaceous, shrub and tree vegetation in forested and unforested fen and floodplain wetland sites. Additionally, soil chemistry, water level and precipitation data were collected. We found that woody encroachment was the strongest predictor of diminished herbaceous wetland communities, while water levels had the strongest positive relationship with herbaceous coverage.

Microbial communities are in close contact with the wetland soil microenvironment and can therefore function effectively as monitors of soil pollution. The objective of this study was to determine changes in the functional responses of microbial communities as a result of an external input of nutrients, while controlling for vegetation. A controlled experiment was performed at the mesocosm scale, consisting of two 1 m by 13 m raceways containing an organic peat soil, each planted with Cladium sp. and Typha sp. communities. One of the mesocosms was loaded with N (2 g N [m.sup.-2][yr.sup.-1]) and P (1 g P [m.sup.-2] [yr.sup.-1]) for 18 mo. Nutrient loading resulted in increases in the soil and detritus labile nutrient pools, however, insufficient N and P where added to significantly alter their total levels. Over the experimental period, the extracellular enzyme acid phosphatase showed a significant decrease in activity across both plant communities (P < 0.01) in contrast to [beta]-glucosidase activity, which varied primarily by plant community. Other microbial response variables such as the microbial activities (C[O.sub.2] and C[H.sub.4] production, P = 0.0016 and 0.0213, respectively), microbial biomass (P = 0.0018) also varied primarily by vegetation type, with Typha sp. dominated areas exhibiting the highest level of activities. The nutrient dosing experiment indicated that the most immediate microbial response measures to nutrient enrichment are those directly associated to specific nutrients, such as P or N, while other measures showed a more complex response involving C source (e.g., vegetation type).

How expected increases in climate variability will affect species diversity depends on the role of such variability in regulating the coexistence of competing species. Despite theory linking temporal environmental fluctuations with the maintenance of diversity, the importance of climate variability for stabilizing coexistence remains unknown because of a lack of appropriate long-term observations. Here, we analyze three decades of demographic data from a Kansas prairie to demonstrate that interannual climate variability promotes the coexistence of three common grass species. Specifically, we show that (i) the dynamics of the three species satisfy all requirements of 'storage effect' theory based on recruitment variability with overlapping generations, (ii) climate variables are correlated with interannual variation in species performance, and (iii) temporal variability increases low-density growth rates, buffering these species against competitive exclusion. Given that environmental fluctuations are ubiquitous in natural systems, our results suggest that coexistence based on the storage effect may be underappreciated and could provide an important alternative to recent neutral theories of diversity. Field evidence for positive effects of variability on coexistence also emphasizes the need to consider changes in both climate means and variances when forecasting the effects of global change on species diversity. climate change | competition | grassland | plant community | population dynamics

Abstract. -- Historic land survey data were used to test a previously proposed nineteenth century landscape model of southeastern Edwards Plateau (Balcones Canyonlands or Texas Hill Country) counties; the model [...]

Herbivore-driven ecosystem, which changes frequently representing predictable transitions from one vegetation state to another, is described. The concept of alternative stable states to arctic tundra is applied as a framework to capture predictable stepwise vegetation transitions in which the productivity and hence, herbivore-carrying capacity increases with grazing pressure in order to explore herbivore-driven habitat degradation.

Consideration of the scale at which communities are defined both taxonomically and spatially can reconcile apparently contradictory results on the extent to which plants show phylogenetic niche conservatism. In plant communities in north central Florida, we collected species abundances in 55 0.1-ha plots in several state parks. When communities were defined narrowly to include a single phylogenetic lineage, such as Quercus, Pinus, or Ilex, neighbors tended to be less related than expected (phylogenetic overdispersion)or there was no pattern. If the same communities were defined more broadly, such as when all seed plants were included, neighbors tended to be more related than expected (phylogenetic clustering). These results provide evidence that species interactions among close relatives influence community structure, but they also show that niche conservatism is increasingly evident as communities are defined to include greater phylogenetic diversity. We also found that, as the spatial scale is increased to encompass greater environmental heterogeneity, niche conservatism emerges as the dominant pattern. We then examined patterns of trait evolution in relation to trait similarity within communities for 11 functional traits for a single phylogenetic lineage (Quercus) and for all woody plants. Among the oaks, convergent evolution of traits important for environmental filtering contributes to the observed pattern of phylogenetic overdispersion. At the broader taxonomic scale, traits tend to be conserved, giving rise to phylogenetic clustering. The shift from overdispersion to clustering can be explained by the increasing conservatism of traits at broader phylogenetic scales. Key words: environmental heterogeneity; Florida; Ilex; niche conservatism; overdispersion; phylogenetic structure of communities; Pinus; Quercus; taxonomic scale; trait convergence.

R.H. Whittaker's idea that plant diversity can be divided into a hierarchy of spatial components from [alpha] at the within-habitat scale through [beta] for the turnover of species between habitats to [gamma] along regional gradients implies the underlying existence of [alpha], [beta], and [gamma]] niches. We explore the hypothesis that the evolution of [alpha], [beta], and [gamma] niches is also hierarchical, with traits that define the [alpha] niche being labile, while those defining [beta] and [gamma], niches are conservative. At the a level we find support for the hypothesis in the lack of close significant phylogenetic relationship between meadow species that have similar [alpha] niches. In a second test, [alpha] niche overlap based on a variety of traits is compared between congeners and noncongeners in several communities; here, too, there is no evidence of a correlation between [alpha] niche and phylogeny. To test whether [beta] and [gamma], niches evolve conservatively, we reconstructed the evolution of relevant traits on evolutionary trees for 14 different clades. Tests against null models revealed a number of instances, including some in island radiations, in which habitat ([beta] niche) and elevational maximum (an aspect of the [gamma] niche) showed evolutionary conservatism. Key words: coexistence; community assembly; diversity; evolutionary lability; geographical range; habitat; hydrology; niche overlap; plant community; plant phylogeny.