Long‐term effects of plant diversity and composition on plant stoichiometry

Plant elemental composition can indicate resource limitation, and changes in key elemental ratios (e.g. plant C:N ratios) can influence rates including herbivory, nutrient recycling, and pathogen infection. Although plant stoichiometry can influence ecosystem‐level processes, very few studies have addressed whether and how plant C:N stoichiometry changes with plant diversity and composition. Here, using two long‐term experimental manipulations of plant diversity (Jena and Cedar Creek), we test whether plant richness (species and functional groups) or composition (functional group proportions) affects temporal trends and variability of community‐wide C:N stoichiometry. Site fertility determined the initial community‐scale C:N ratio. Communities growing on N‐poor soil (Cedar Creek) began with higher C:N ratios than communities growing on N‐rich soil (Jena). However, site‐level plant C:N ratios converged through time, most rapidly in high diversity plots. In Jena, plant community C:N ratios increased. This temporal trend was stronger with increasing richness. However, temporal variability of C:N decreased as plant richness increased. In contrast, C:N decreased over time at Cedar Creek, most strongly at high species and functional richness, whereas the temporal variability of C:N increased with both measures of diversity at this site. Thus, temporal trends in the mean and variability of C:N were underlain by concordant changes among sites in functional group proportions. In particular, the convergence of community‐scale C:N over time at these very different sites was mainly due to increasing proportions of forbs at both sites, replacing high mean C:N (C4 grasses, Cedar Creek) or low C:N (legumes, Jena) species. Diversity amplified this convergence; although temporal trends differed in sign between the sites, these trends increased in magnitude with increasing species richness. Our results suggest a predictive mechanistic link between trends in plant diversity and functional group composition and trends in the many ecosystem rates that depend on aboveground community C:N. Synthesis We compared the effect of plant diversity on the temporal dynamics of community stoichiometry in two long‐term grassland diversity experiments: the Cedar Creek and Jena Experiments. Changes in community C:N ratios were accelerated by increasing diversity at both sites, but in opposite directions depending on soil fertility. Stoichiometry changes were driven by shifts of functional group composition differing in their elemental compositions, the identity of the functional groups depending on the site. Thus, we highlighted that community turnover constrained the effect of diversity on plant stoichiometry at both sites     

Adjacency arrangement effects on plant diversity and composition in woodland patches

To understand "adjacency arrangement" (configuration of a patch and its adjoining elements) as a basic unit of landscape pattern, we studied 30 woodland patches ( Populus tremuloides) in Alberta, Canada, with adjacent vegetation varying from 0% shrubland (100% grassland) to 100% shrubland (0% grassland). We evaluated (1) how important adjacency effects are relative to resource availability and disturbance in affecting plant species richness and composition in the woodland understory, and (2) which species and species groups change in abundance and presence with increasing percent shrubland in the adjacency arrangement. We found that, in addition to topography and previous grazing, adjacency arrangement is a major significant variable affecting the vegetation composition of woodland patches. Along a gradient from 0% to 100% adjacent shrubland, the species composition in woodland patches changed from an abundance of weedy and introduced plants to an abundance of moist-environment plants, the proportion of native species and perennials increased linearly, the number of "shrubland species" increased linearly, "grassland species" decreased linearly, and total species richness remained constant. Woodland vegetation did not differ significantly where adjacent shrubland was to the east or to the west of the wooded patch, nor between patches which did or did not experience a change in the adjacent shrubland in the recent past. We conclude that adjacency arrangement is an easily measured, highly promising concept for ecological understanding, as well as for land planning, design, conservation and management.     

Oat plant effects on net nitrogen mineralization

Living plants have been reported to stimulate, inhibit, or have no effect on net nitrogen mineralization in soil. A series of experiments were conducted to evaluate the influence of living oat plants Avena sativa on net N mineralization. Oat plants were grown in plastic cylinders containing soil, and net N mineralization was assessed by determining the N balance in these microcosms. Measured N inputs included N contained in the oat seeds and N₂ fixation. N losses by NH₃ volatilization and denitrification were also measured. We observed that in some soils net N mineralization was stimulated by as much as 81%, but in other soils there was no effect of living oat plants on net N mineralization. N mineralization responses are related to past cropping histories of the soils.     

Evidence for the indirect effects of pesticides on farmland birds

Indirect effects of pesticides, operating through the food chain, have been proposed as a possible causal factor in the decline of farmland bird species. To demonstrate such a link, evidence is needed of (1) an effect of food abundance on breeding performance or survival; (2) an effect of breeding performance or survival on population change; and (3) pesticide effects on food resources, sufficient to reduce breeding performance or survival, and hence to affect the rate of population change. Evidence under all three categories is only available for one species, the Grey Partridge Perdix perdix , although data showing effects of pesticides on food resources and relationships between food resources and breeding performance are also available for some other species. This paper reports on recent work investigating the effects of pesticides on Yellowhammer Emberiza citrinella and Skylark Alauda arvensis during the breeding season. The probability of brood reduction in Yellowhammer was affected by the proportion of the foraging area around the nest which was sprayed with insecticide. No significant effects of pesticides were recorded on Skylark chick condition or growth rate, but sample sizes were small. Invertebrate food abundance affected chick condition (Skylark) and the number of chicks fledging (Yellowhammer and Corn Bunting Miliaria calandra ; relationship for the latter derived from re-analysis of data from an earlier study). Other recent work is briefly reviewed and the current evidence for the indirect effects of pesticides is summarized. Significant knowledge gaps are identified and some of the issues involved in resolving these are discussed.     

Shifting grassland plant community structure drives positive interactive effects of warming and diversity on aboveground net primary productivity

Ecosystems worldwide are increasingly impacted by multiple drivers of environmental change, including climate warming and loss of biodiversity. We show, using a long‐term factorial experiment, that plant diversity loss alters the effects of warming on productivity. Aboveground primary productivity was increased by both high plant diversity and warming, and, in concert, warming (≈1.5 °C average above and belowground warming over the growing season) and diversity caused a greater than additive increase in aboveground productivity. The aboveground warming effects increased over time, particularly at higher levels of diversity, perhaps because of warming‐induced increases in legume and C4 bunch grass abundances, and facilitative feedbacks of these species on productivity. Moreover, higher plant diversity was associated with the amelioration of warming‐induced environmental conditions. This led to cooler temperatures, decreased vapor pressure deficit, and increased surface soil moisture in higher diversity communities. Root biomass (0–30 cm) was likewise consistently greater at higher plant diversity and was greater with warming in monocultures and at intermediate diversity, but at high diversity warming had no detectable effect. This may be because warming increased the abundance of legumes, which have lower root : shoot ratios than the other types of plants. In addition, legumes increase soil nitrogen (N) supply, which could make N less limiting to other species and potentially decrease their investment in roots. The negative warming × diversity interaction on root mass led to an overall negative interactive effect of these two global change factors on the sum of above and belowground biomass, and thus likely on total plant carbon stores. In total, plant diversity increased the effect of warming on aboveground net productivity and moderated the effect on root mass. These divergent effects suggest that warming and changes in plant diversity are likely to have both interactive and divergent impacts on various aspects of ecosystem functioning.     

Interactive effects between plant functional types and soil factors on tundra species diversity and community composition

Plant communities are coupled with abiotic factors, as species diversity and community composition both respond to and influence climate and soil characteristics. Interactions between vegetation and abiotic factors depend on plant functional types (PFT) as different growth forms will have differential responses to and effects on site characteristics. However, despite the importance of different PFT for community assembly and ecosystem functioning, research has mainly focused on vascular plants. Here, we established a set of observational plots in two contrasting habitats in northeastern Siberia in order to assess the relationship between species diversity and community composition with soil variables, as well as the relationship between vegetation cover and species diversity for two PFT (nonvascular and vascular). We found that nonvascular species diversity decreased with soil acidity and moisture and, to a lesser extent, with soil temperature and active layer thickness. In contrast, no such correlation was found for vascular species diversity. Differences in community composition were found mainly along soil acidity and moisture gradients. However, the proportion of variation in composition explained by the measured soil variables was much lower for nonvascular than for vascular species when considering the PFT separately. We also found different relationships between vegetation cover and species diversity according the PFT and habitat. In support of niche differentiation theory, species diversity and community composition were related to edaphic factors. The distinct relationships found for nonvascular and vascular species suggest the importance of considering multiple PFT when assessing species diversity and composition and their interaction with edaphic factors. Synthesis: Identifying vegetation responses to edaphic factors is a first step toward a better understanding of vegetation–soil feedbacks under climate change. Our results suggest that incorporating differential responses of PFT is important for predicting vegetation shifts, primary productivity, and in turn, ecosystem functioning in a changing climate.     

Effects of above-ground plant species composition and diversity on the diversity of soil-borne microorganisms

A coupling of above-ground plant diversity and below-ground microbial diversity has been implied in studies dedicated to assessing the role of macrophyte diversity on the stability, resilience, and functioning of ecosystems. Indeed, above-ground plant communities have long been assumed to drive below-ground microbial diversity, but to date very little is known as to how plant species composition and diversity influence the community composition of micro-organisms in the soil. We examined this relationship in fields subjected to different above-ground biodiversity treatments and in field experiments designed to examine the influence of plant species on soil-borne microbial communities. Culture-independent strategies were applied to examine the role of wild or native plant species composition on bacterial diversity and community structure in bulk soil and in the rhizosphere. In comparing the influence of Cynoglossum officinale (hound's tongue) and Cirsium vulgare (spear thistle) on soil-borne bacterial communities, detectable differences in microbial community structure were confined to the rhizosphere. The colonisation of the rhizosphere of both plants was highly reproducible, and maintained throughout the growing season. In a separate experiment, effects of plant diversity on bacterial community profiles were also only observed for the rhizosphere. Rhizosphere soil from experimental plots with lower macrophyte diversity showed lower diversity, and bacterial diversity was generally lower in the rhizosphere than in bulk soil. These results demonstrate that the level of coupling between above-ground macrophyte communities and below-ground microbial communities is related to the tightness of the interactions involved. Although plant species composition and community structure appear to have little discernible effect on microbial communities inhabiting bulk soil, clear and reproducible changes in microbial community structure and diversity are observed in the rhizosphere. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effects of sod cutting and additional liming on potential net nitrification in heathland soils

The effects of sod cutting, a common restoration measure to remove excess nutrients from grass-dominated heathlands, on nitrification were studied in dry and wet Dutch heathlands and in incubation experiments. In the field, soil ammonium and nitrate concentrations were measured after treatment by sod cutting, with or without additional liming. Potential net nitrification was measured by incubating soil samples of all treatments with extra ammonium in a climate chamber at pH 6. Potential net nitrification of heaths dominated by Molinia caerulea was significantly higher than that of dwarf-shrub dominated heaths. Sod cutting of the former areas significantly decreased potential net nitrification, whereas in the latter areas no differences were found. Liming of sod-cut soils greatly increased potential net nitrification and the accumulation of ammonium in the soil up to toxic concentrations could be prevented. Our results show that the combination of sod cutting and liming would create suitable soil conditions for the germination and establishment of endangered plant species of dry and wet heathlands. The success of restoration projects of these areas can thus be increased.     

Roadside habitats: effects on diversity and composition of plant, arthropod, and small mammal communities

Road edge effects cover extensive areas and exert a wide range of ecological influences on nearby plants and animals. Most studies have focused on individual and population level effects of the road edge; less is known about how communities and their functionality are altered in proximity to roads. Here, we studied the effect of road edges on species richness, rarity, endemism, composition, and functional (trophic) classification of communities of plants, ground-dwelling arthropods (beetles, spiders, scorpions, diplopods), and small mammals. The study, conducted in a Mediterranean ecosystem in central Israel, included sampling of these taxa in 10 plots adjacent to a regional road, and in 12 nearby control plots located in a typical shrubland habitat. We found a variety of community level road edge effects on the structure, composition, and function of the studied communities. The extent of effects varied among taxa, but they were generally positive or neutral. For the species-rich taxa (plants, beetles, and spiders), distinct road edge communities characterized by higher richness and altered species composition were found. Rarity and endemism were lower, and the proportion of disturbance-associated plant species was higher at the road edge. Among the species-poor taxa, scorpions and small mammals were more abundant along road edges than in control plots, while diplopods, the only negatively affected taxon, showed decreased abundance along the road. No ecologically meaningful changes in richness or composition were detected for the species-poor taxa along the road edge. Road edges profoundly affect floral and faunal communities, with possible implications for biodiversity conservation.     

干扰对植物群落物种组成及多样性的影响

在介绍了干扰的概念及其性质和干扰对物种多样性影响的有关假说基础上,以森林干扰为主要对象,探讨了干扰对植物群落物种组成的影响,并从干扰类型、干扰强度和干扰频率等几个方面阐述了干扰对植物群落物种多样性的影响;另外,还分析了重要的小尺度干扰——林隙在该方面的影响。分析总结出干扰对植物群落的影响差异主要与干扰特征、植物群落特征及植物的生物学特性和受干扰地点的资源条件有关。最后,总结分析了国内外在该方面研究存在的问题,并对今后研究提出一些建议,为相关工作提供参考。     

Direct and indirect effects of plant genetic variation on enemy impact

Abstract.

• 1 The Tritrophic and Enemy Impact concepts predict that natural enemy impact varies: (a) among plant genotypes and (b) may depend on the abundance of heterospecific herbivores, respectively. I tested these predictions using three herbivore species on potted, cloned genotypes of Salik sericea Marshall in a common garden experiment.
• 2 Densities of the leaf miner (Phyllonorycter salicifoliella (Chambers)) and two leaf galling sawflies (Phyllocolpa nigrita (Marlatt) and Phyllocolpa eleanorae Smith and Fritz) varied significantly among willow clones, indicating genetic variation in resistance.
• 3 Survival and natural enemy impact caused by egg and larval parasitoids and/or unknown predators differed significantly among willow clones for each of the three herbivore species, indicating genetic variation in survival and enemy impact.
• 4 Survival of Phyllonorycter was negatively density-dependent among clones.
• 5 Survival of Phyllonorycter and Phyllocolpa eleanorae were positively correlated with densities of heterospecific herbivores among clones and parasitism of these species were negatively correlated with densities of the same heterospecific herbivores among clones.
• 6 At least for Phyllonorycter this positive correlation may suggest either facilitation of survival between herbivore species, which do not share natural enemies, or an apparent interaction caused by host plant genetic variation.
• 7 Among clones, egg parasitism of Phyllocolpa eleanorae was weakly positively correlated with density of Phyllocolpa nigrita. Since these species share the same Trichogramma egg parasitoid, this interaction could support the hypothesis of apparent competition.

     

Mutualistic rhizobia reduce plant diversity and alter community composition

Mutualistic interactions can be just as important to community dynamics as antagonistic species interactions like competition and predation. Because of their large effects on both abiotic and biotic environmental variables, resource mutualisms, in particular, have the potential to influence plant communities. Moreover, the effects of resource mutualists such as nitrogen-fixing rhizobia on diversity and community composition may be more pronounced in nutrient-limited environments. I experimentally manipulated the presence of rhizobia across a nitrogen gradient in early assembling mesocosm communities with identical starting species composition to test how the classic mutualism between nitrogen-fixing rhizobia and their legume host influence diversity and community composition. After harvest, I assessed changes in α-diversity, community composition, β-diversity, and ecosystem properties such as inorganic nitrogen availability and productivity as a result of rhizobia and nitrogen availability. The presence of rhizobia decreased plant community diversity, increased community convergence (reduced β-diversity), altered plant community composition, and increased total community productivity. These community-level effects resulted from rhizobia increasing the competitive dominance of their legume host Chamaecrista fasciculata. Moreover, different non-leguminous species responded both negatively and positively to the presence of rhizobia, indicating that rhizobia are driving both inhibitory and potentially facilitative effects in communities. These findings expand our understanding of plant communities by incorporating the effects of positive symbiotic interactions on plant diversity and composition. In particular, rhizobia that specialize on dominant plants may serve as keystone mutualists in terrestrial plant communities, reducing diversity by more than 40 %.     

Plant community diversity and composition affect individual plant performance

Effects of plant community diversity on ecosystem processes have recently received major attention. In contrast, effects of species richness and functional richness on individual plant performance, and their magnitude relative to effects of community composition, have been largely neglected. Therefore, we examined height, aboveground biomass, and inflorescence production of individual plants of all species present in 82 large plots of the Jena Experiment, a large grassland biodiversity experiment in Germany. These plots differed in species richness (1–60), functional richness (1–4), and community composition. On average, in more species-rich communities, plant individuals grew taller, but weighed less, were less likely to flower, and had fewer inflorescences. In plots containing legumes, non-legumes were higher and weighed more than in plots without legumes. In plots containing grasses, non-grasses were less likely to flower than in plots without grasses. This indicates that legumes positively and grasses negatively affected the performance of other species. Species richness and functional richness effects differed systematically between functional groups. The magnitude of the increase in plant height with increasing species richness was greatest in grasses and was progressively smaller in legumes, small herbs, and tall herbs. Individual aboveground biomass responses to increasing species richness also differed among functional groups and were positive for legumes, less pronouncedly positive for grasses, negative for small herbs, and more pronouncedly negative for tall herbs. Moreover, these effects of species richness differed strongly between species within these functional groups. We conclude that individual plant performance largely depends on the diversity of the surrounding community, and that the direction and magnitude of the effects of species richness and functional richness differs largely between species. Our study suggests that diversity of the surrounding community needs to be taken into account when interpreting drivers of the performance of individual plants.     

景观组成、结构和梯度格局对植物多样性的影响

植物多样性作为生物多样性研究的主要内容,一直以来受到广泛关注。近20年来,随着景观生态学的兴起和地理信息技术的发展,景观生态学与岛屿生物地理学、异质种群理论相结合,在植物多样性的保护和利用研究中得到运用。本文就在这3个理论的基础上,简述了景观组成(斑块、廊道、基质)、景观结构(斑块面积、边缘、隔离程度)和梯度格局(海拔、演替、土壤养分、干扰)对植物多样性的影响,强调了地理信息技术应用的重要性和景观层次上进行植物多样性研究的必要性。因此,在多个尺度上共同研究多个影响因子对植物多样性的复合作用,利于进一步揭示植物多样性的变化过程及其机制,有利于植物多样性的保护和利用。     

Plot shape effects on plant species diversity measurements

Abstract. Question: Do rectangular sample plots record more plant species than square plots as suggested by both empirical and theoretical studies? Location: Grasslands, shrublands and forests in the Mediterranean‐climate region of California, USA. Methods: We compared three 0.1‐ha sampling designs that differed in the shape and dispersion of 1‐m² and 100‐m² nested subplots. We duplicated an earlier study that compared the Whittaker sample design, which had square clustered subplots, with the modified Whittaker design, which had dispersed rectangular subplots. To sort out effects of dispersion from shape we used a third design that overlaid square subplots on the modified Whittaker design. Also, using data from published studies we extracted species richness values for 400‐m² subplots that were either square or 1:4 rectangles partially overlaid on each other from desert scrub in high and low rainfall years, chaparral, sage scrub, oak savanna and coniferous forests with and without fire. Results: We found that earlier empirical reports of more than 30% greater richness with rectangles were due to the confusion of shape effects with spatial effects, coupled with the use of cumulative number of species as the metric for comparison. Average species richness was not significantly different between square and 1:4 rectangular sample plots at either 1‐ or 100‐m². Pairwise comparisons showed no significant difference between square and rectangular samples in all but one vegetation type, and that one exhibited significantly greater richness with squares. Our three intensive study sites appear to exhibit some level of self‐similarity at the scale of 400 m², but, contrary to theoretical expectations, we could not detect plot shape effects on species richness at this scale. Conclusions: At the 0.1‐ha scale or lower there is no evidence that plot shape has predictable effects on number of species recorded from sample plots. We hypothesize that for the mediterranean‐climate vegetation types studied here, the primary reason that 1:4 rectangles do not sample greater species richness than squares is because species turnover varies along complex environmental gradients that are both parallel and perpendicular to the long axis of rectangular plots. Reports in the literature of much greater species richness recorded for highly elongated rectangular strips than for squares of the same area are not likely to be fair comparisons because of the dramatically different periphery/area ratio, which includes a much greater proportion of species that are using both above and below‐ground niche space outside the sample area.     

Direct and indirect effects of environmental factors,spatial constraints,and functional traits on shaping the plant diversity of montane forests

Understanding the relative importance of the factors driving the patterns of biodiversity is a key research topic in community ecology and biogeography. However, the main drivers of plant species diversity in montane forests are still not clear. In addition, most existing studies make no distinction between direct and indirect effects of environmental factors and spatial constraints on plant biodiversity. Using data from 107 montane forest plots in Sichuan Giant Panda habitat, China, we quantified the direct and indirect effects of abiotic environmental factors, spatial constraints, and plant functional traits on plant community diversity. Our results showed significant correlations between abiotic environmental factors and trees (r = .10, p value = .001), shrubs (r = .19, p value = .001), or overall plant diversity (r = .18, p value = .001) in montane forests. Spatial constraints also showed significant correlations with trees and shrubs. However, no significant correlations were found between functional traits and plant community diversity. Moreover, the diversity (richness and abundance) of shrubs, trees, and plant communities was directly affected by precipitation, latitude, and altitude. Mean annual temperature (MAT) had no direct effect on the richness of tree and plant communities. Further, MAT and precipitation indirectly affected plant communities via the tree canopy. The results revealed a stronger direct effect on montane plant diversity than indirect effect, suggesting that single‐species models may be adequate for forecasting the impacts of climate factors in these communities. The shifting of tree canopy coverage might be a potential indicator for trends of plant diversity under climate change.     

Independent effects of arbuscular mycorrhiza and earthworms on plant diversity and newcomer plant establishment

Questions: How do arbuscular mycorrhiza and earthworms affect the structure and diversity of a ruderal plant community? Is the establishment success of newcomer plants enhanced by these soil organisms and their interactions? Methods: We grew a native ruderal plant community composed of different functional groups (grasses, legumes and forbs) in the presence and absence of arbuscular mycorrhizal fungi (AMF) and endogeic earthworms in mesocosms. We introduced seeds of five, mainly exotic, plant species from the same functional groups after a disturbance simulating mowing. The effects of the soil organisms on the native ruderal plant community and seedling establishment of the newcomer plants were assessed. Results: After disturbance, the total above‐ground regrowth of the native plant community was not affected by the soil organisms. However, AMF increased plant diversity and shoot biomass of forbs, but decreased shoot biomass of grasses of the native plant community. Earthworms led to a reduction in total root biomass. Establishment of the introduced newcomer plants increased in the presence of AMF and earthworms. Especially, seedling establishment of the introduced non‐native legume Lupinus polyphyllus and the native forb Plantago lanceolata was promoted in the presence of AMF and earthworms, respectively. The endogeic earthworms gained more weight in the presence of AMF and led to increased extraradical AMF hyphal length in soil. However, earthworms did not seem to modify the effect of AMF on the plant community. Conclusion: The present study shows the importance of mutualistic soil organisms in mediating the establishment success of newcomer plants in a native plant community. Mutualistic soil organisms lead to changes in the structure and diversity of the native plant community and might promote newcomer plants, including exotic species.