Biodiversity Effects on Plant Stoichiometry

In the course of the biodiversity-ecosystem functioning debate, the issue of multifunctionality of species communities has recently become a major focus. Elemental stoichiometry is related to a variety of processes reflecting multiple plant responses to the biotic and abiotic environment. It can thus be expected that the diversity of a plant assemblage alters community level plant tissue chemistry. We explored elemental stoichiometry in aboveground plant tissue (ratios of carbon, nitrogen, phosphorus, and potassium) and its relationship to plant diversity in a 5-year study in a large grassland biodiversity experiment (Jena Experiment). Species richness and functional group richness affected community stoichiometry, especially by increasing C:P and N:P ratios. The primacy of either species or functional group richness effects depended on the sequence of testing these terms, indicating that both aspects of richness were congruent and complementary to expected strong effects of legume presence and grass presence on plant chemical composition. Legumes and grasses had antagonistic effects on C:N (−27.7% in the presence of legumes, +32.7% in the presence of grasses). In addition to diversity effects on mean ratios, higher species richness consistently decreased the variance of chemical composition for all elemental ratios. The diversity effects on plant stoichiometry has several non-exclusive explanations: The reduction in variance can reflect a statistical averaging effect of species with different chemical composition or a optimization of nutrient uptake at high diversity, leading to converging ratios at high diversity. The shifts in mean ratios potentially reflect higher allocation to stem tissue as plants grew taller at higher richness. By showing a first link between plant diversity and stoichiometry in a multiyear experiment, our results indicate that losing plant species from grassland ecosystems will lead to less reliable chemical composition of forage for herbivorous consumers and belowground litter input.     

Temperature mean and variance alter phytoplankton biomass and biodiversity in a long‐term microcosm experiment

Global predictions of increasing mean temperature and its variance result in concerns about the fate of biodiversity under future climatic conditions. On the local scale of interacting species, the response of diversity to warming will depend much on the change in interspecific interactions such as competition or predation. Using a phytoplankton model community, we conducted a 2 × 2 × 2 factorial long‐term microcosm experiment (16 months) with an underlying seasonal temperature profile. We manipulated the mean temperature (T_mean), the temperature variance (T_var), and the presence of consumers (ciliates) and monitored treatment effects on algal biomass, species composition and species richness. Temperature effects on algal biomass depended on the seasonal development and consumer presence. Algal biomass decreased with increasing T_mean at consumer absence, but increased at consumer presence. Overall, algal biomass decreased with increasing T_var. Consumer presence reduced algal biomass from summer to winter, but then ciliates and the consumer effect disappeared. Almost all treatment combinations collapsed to monocultures after 16 months, but extinction occurred faster at higher T_mean (especially at consumer absence) and slower at consumer presence (especially at higher T_mean). Contrasting our predictions, increasing T_var reduced richness and increased extinction rate. The treatment effects on biomass and richness were not independent, as algal species richness and biomass were positively correlated. Moreover, accelerated loss of species was consistently correlated to higher temporal variability in biomass. In conclusion, altered temperature regimes strongly affected algal biomass and diversity by interdependently altering competitive and consumer interactions.     

Resource stoichiometry and consumers control the biodiversity-productivity relationship in pelagic metacommunities

Recent theory suggests that both biodiversity and productivity are constrained by resource supply rates and ratios and that resource stoichiometry is the key to understanding the relationship between biodiversity and productivity. We experimentally tested this theory using pelagic metacommunities. We amended existing predictions by explicitly considering evenness as an aspect of biodiversity and including control of algal biomass by consumption in addition to competition. The metacommunities received a different phosphorus (P) supply and the three patches within each metacommunity differed in their nitrogen (N) supply, which created different N∶P ratios (2, 16, and 128). All patches were inoculated with a phytoplankton assemblage consisting of five species, and half of the metacommunities received two ciliate species as consumers. At the level of the entire metacommunity, algal biomass increased with increasing P supply, whereas species richness and evenness decreased with increasing P supply. Without consumers, resource use efficiency (RUE; realized biomass per unit of P) increased with increasing richness and evenness. Consumer presence reduced overall biomass and richness and precluded a correlation between RUE and biodiversity. At the patch level, local evenness correlated with higher RUE at both imbalanced N∶P ratios (2 and 128) but not at a balanced N∶P ratio. In conclusion, overall P supply constrained realized biomass and altered diversity, whereas resource stoichiometry shaped the relationship between biodiversity and RUE.     

Tree species identity determines wood decomposition via microclimatic effects

Empirical evidence suggests that the rich set of ecosystem functions and nature's contributions to people provided by forests depends on tree diversity. Biodiversity–ecosystem functioning research revealed that not only species richness per se but also other facets of tree diversity, such as tree identity, have to be considered to understand the underlying mechanisms. One important ecosystem function in forests is the decomposition of deadwood that plays a vital role in carbon and nutrient cycling and is assumed to be determined by above‐ and belowground interactions. However, the actual influence of tree diversity on wood decay in forests remains inconclusive. Recent studies suggest an important role of microclimate and advocate a systematical consideration of small‐scale environmental conditions. We studied the influence of tree species richness, tree species identity, and microclimatic conditions on wood decomposition in a 12‐year‐old tree diversity experiment in Germany, containing six native species within a tree species richness gradient. We assessed wood mass loss, soil microbial properties, and soil surface temperature in high temporal resolution. Our study shows a significant influence of tree species identity on all three variables. The presence of Scots pine strongly increased wood mass loss, while the presence of Norway spruce decreased it. This could be attributed to structural differences in the litter layer that were modifying the capability of plots to hold the soil surface temperature at night, consequently leading to enhanced decomposition rates in plots with higher nighttime surface temperatures. Therefore, our study confirmed the critical role of microclimate for wood decomposition in forests and showed that soil microbial properties alone were not sufficient to predict wood decay. We conclude that tree diversity effects on ecosystem functions may include different biodiversity facets, such as tree identity, tree traits, and functional and structural diversity, in influencing the abiotic and biotic soil properties.     

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     

Multiple nutrients and herbivores interact to govern diversity,productivity, composition,and infection in a successional grassland

In spite of increasing awareness that interactions between herbivory and the supply rates of multiple nutrients control biodiversity, ecosystem functions and ecosystem services in ecological communities, few experimental studies have concurrently examined the independent and joint effects of multiple nutrients and mammalian consumers on these responses in natural systems. Here we quantify the independent and interactive effects of multiple concurrent changes to resources and consumers in an invaded annual grassland community in California. In a two‐year study using thirty‐seven 400‐m² plots, we examine interactions among four nutrient treatments (N, P, K and micronutrients) and a keystone herbivore (pocket gopher Thomomys bottae) on four plant community outcomes: 1) plant diversity, 2) functional group composition, 3) net biomass production, an important ecosystem function, and 4) infection risk by a group of viral pathogens shared by crop and non‐crop grasses (barley and cereal yellow dwarf viruses), an important regulating ecosystem service. We found that grassland biodiversity and infection risk were controlled by nutrient identity and supply ratio whereas nutrients interacted strongly with consumers to control grassland composition and net primary productivity. The most important insights arising from this multi‐factor experiment are that net biomass production increased with phosphorus or nitrogen supply; however, when gophers were present, nitrogen caused no net effect on biomass production. In addition, infection risk was driven by phosphorus, nitrogen and micronutrient supply. Infection in a sentinel host increased strongly with the addition of micronutrients or phosphorus; however, infection declined with increasing N/P supply ratio, indicating stoichiometric control of infection risk. Finally, in spite of manipulating multiple factors, plant species richness declined with nitrogen, alone. The importance of higher‐order interactions demonstrates that a multi‐factor approach is critical for effective predictions in a world in which anthropogenic activities are simultaneously changing herbivore abundance and the relative supply of many nutrients.     

Co-limitation of plant primary productivity by nitrogen and phosphorus in a species-rich wooded meadow on calcareous soils

High small-scale species richness of calcareous grasslands is generally thought to result from evening of species competitive potentials by limited N availability, because of relatively low herb N/P ratios in these communities. However, P mobility is low in alkaline soils as well. We studied soil chemistry and productivity of herb and moss layers in a very diverse calcareous meadow (up to 76 vascular plant species per m²) to test the hypotheses of a co-limitation of herb productivity by both soil N and P availabilities and moss productivity primarily by P availability. The effect of nutrient supply on productivity was investigated using both a natural productivity gradient as well as fertilization experiments. We observed strong positive correlations of soil P availability and total soil N with the above-ground productivity of herb layer. A long-term fertilization experiment demonstrated that P alone and N and P together increased productivity of vascular species, and that the productivity continuously declined after cessation of fertilization with the effect of previous fertilization occasionally visible even 14 years after treatment termination. A short-term fertilization experiment further demonstrated that N and P when supplied alone increase productivity of vascular plants, suggesting that both elements were limiting. Furthermore, there was a significant interaction between N and P on productivity, indicating that simultaneous N and P supply increased productivity more than separate nutrient additions. Moss productivity was negatively associated with vascular plant productivity. In particular, N addition decreased moss productivity, but moss productivity did not decline in P addition treatments. P requirements of mosses were larger than those of vascular plants. Our data indicate co-limitation of herb productivity by both soil N and P in this highly diverse grassland, while limitation of moss productivity mainly by P. We suggest that N and P co-limitations are common in calcareous diverse grasslands, and may partly explain the extreme small-scale species diversity in these communities.     

Gastropod grazers and nutrients,but not light,interact in determining periphytic algal diversity

The potential interactions of grazing, nutrients and light in influencing autotroph species diversity have not previously been considered. Earlier studies have shown that grazing and nutrients interact in determining autotroph species diversity, since grazing decreases species diversity when nutrients (i.e. N or P) limit autotroph growth, but increases it when nutrients are replete. We hypothesized that increased light intensities would intensify the interactions between grazing and nutrients on algal species diversity, resulting in even stronger reductions in algal species diversity through grazing under nutrient–poor conditions, and to even stronger increases of algal species diversity through grazing under nutrient-rich conditions. We studied the effects of grazing (absent, present), nutrients (ambient, N + P enriched) and light (low light, high light) on benthic algal diversity and periphyton C:nutrient ratios (which can indicate algal nutrient limitation) in a factorial laboratory experiment, using the gastropod grazer Viviparus viviparus. Grazing decreased algal biomass and algal diversity, but increased C:P and N:P ratios of periphyton. Grazing also affected periphyton species composition, by decreasing the proportion of Spirogyra sp. and increasing the proportion of species in the Chaetophorales. Grazing effects on diversity as well as on periphyton N:P ratios were weakened when nutrients were added (interaction between grazing and nutrients). Chlorophyll a (Chl a) per area increased with nutrient addition and decreased with high light intensities. Light did not increase the strength of the interaction between grazing and nutrients on periphytic algal diversity. This study shows that nutrient addition substantially reduced the negative effects of grazing on periphytic algal diversity, whereas light did not interact with grazing or nutrient enrichment in determining periphytic algal diversity.     

Is Tree Species Diversity or Species Identity the More Important Driver of Soil Carbon Stocks,C/N Ratio,and pH?

We explored tree species diversity effects on soil C stock, C/N ratio, and pH as compared with effects of tree species identity. We sampled forest floors and mineral soil (0–40 cm) in a diversity gradient of 1–5 tree species composed of conifers and broadleaves in Bia?owie?a Forest, Poland. Diversity was a weaker driver than identity of soil C stocks, C/N ratio, and pH in the soil profile. However, there were significant non-additive effects of diversity and significant effects of identity on C stock and C/N ratio within different parts of the soil profile. More diverse forests had higher C stocks and C/N ratios in the 20–40 cm layer, whereas identity in terms of conifer proportion increased C stocks and C/N ratios only in forest floors. A positive relationship between C stocks and root biomass in the 30–40 cm layer suggested that belowground niche complementarity could be a driving mechanism for higher root carbon input and in turn a deeper distribution of C in diverse forests. Diversity and identity affected soil pH in topsoil with positive and negative impacts, respectively. More diverse forests would lead to higher soil nutrient status as reflected by higher topsoil pH, but there was a slight negative effect on N status as indicated by higher C/N ratios in the deeper layers. We conclude that tree species diversity increases soil C stocks and nutrient status to some extent, but tree species identity is a stronger driver of the studied soil properties, particularly in the topsoil.     

Competition between native Antarctic vascular plants and invasive <Emphasis Type="Italic">Poa annua</Emphasis> changes with temperature and soil nitrogen availability

Over the last decades human have introduced non-native organisms to Antarctica, including the grass species Poa annua. This non-native grass under constant growth temperatures has been shown negatively affect the growth of the only two native Antarctic vascular plants, Deschampsia antarctica and Colobanthus quitensis, under constant growth temperatures. However, whether there are changes in the interaction between these species under warmer conditions is an important question. In cold ecosystems, soil nutrient status directly affects plant responses to increases in temperature and Antarctic soils are highly variable in nutrient supply. Thus, in this study we experimentally assessed the interaction between the non-native Poa with the two native Antarctic vascular plant species at two different temperatures and levels of nutrient availability. Individual mats of the study species were collected in King George Island, and then transported to Concepcion where we conducted competition experiments. In the first experiment we used soil similar to that of Antarctica and plants in competition were grown at two temperatures: 5°/2° and 11°/5 °C (day/night temperature). In a second experiment plants were grown in these two temperature regimes, but we varied nitrogen (N) availability by irrigating plants with Hoagland solutions that contained 8000 or 300 µM of N. Overall, Poa exerted a competitive effect on Deschampsia but only at the higher temperature and higher N availability. At 5°/11 °C the competitive response of Deschampsia to Poa was of similar magnitude to the competitive effect of P. Deschampsia, and the competitive effect was greater with at low N. The competitive effect of Poa was similar to the competitive response of Colobanthus to Poa at both temperatures and N levels. Thus, at low temperatures and N soil content the native Antarctic species might withstand Poa invasion, but this might change with climate warming.     

Reciprocal causality between marine macroalgal diversity and productivity in an upwelling area

The relationship between biodiversity and ecosystem functioning has become a prominent topic in the ecological literature. However, the contemporary approach that species diversity controls primary productivity contrasts with the historical perspective that species diversity responds to productivity. Moreover, previous experimental results have not been consistent with the patterns observed in nature. To resolve these questions, the multivariate productivity–diversity (MPD) hypothesis proposes a bidirectional relationship between diversity and productivity. It predicts that the resource supply, expressed in terms of resource availability and imbalance, establishes the number of species that can locally coexist. Simultaneously, the resource supply also indirectly affects biomass production, determining the form and cause of the effects of species richness on resource use and biomass. To test the MPD hypothesis, we conducted three field experiments with a subtidal marine macroalgal community using a seasonal upwelling process as a driver of distinct levels of nutrient supply. Seasonally, macroalgal species richness and biomass were assessed and experimental manipulations conducted to investigate the relative importance of species richness and identity effects on biomass production and the mechanisms underlying these. Changes in macroalgal biomass and species richness were observed in response to the nutrient supply. Stronger effects of species identity were detected for all periods investigated, although species richness effects also occurred to some extent. The magnitudes of the net biodiversity and of the complementarity effects were a unimodal function of nutrient supply, whereas a concave‐up curve was observed for selection effects. The nutrient supply directly affected the number of species that dominated the local community and, consequently, determined the efficiency with which resources were exploited and converted to biomass. Our results provide evidence consistent with the MPD hypothesis and aids in explaining the discrepancies between experimental results and natural patterns through the merging of two contrasting perspectives in ecology.     

Productivity, herbivory, and species traits rather than diversity influence invasibility of experimental phytoplankton communities

Biological invasions are a major threat to natural biodiversity; hence, understanding the mechanisms underlying invasibility (i.e., the susceptibility of a community to invasions by new species) is crucial. Invasibility of a resident community may be affected by a complex but hitherto hardly understood interplay of (1) productivity of the habitat, (2) diversity, (3) herbivory, and (4) the characteristics of both invasive and resident species. Using experimental phytoplankton microcosms, we investigated the effect of nutrient supply and species diversity on the invasibility of resident communities for two functionally different invaders in the presence or absence of an herbivore. With increasing nutrient supply, increased herbivore abundance indicated enhanced phytoplankton biomass production, and the invasion success of both invaders showed a unimodal pattern. At low nutrient supply (i.e., low influence of herbivory), the invasibility depended mainly on the competitive abilities of the invaders, whereas at high nutrient supply, the susceptibility to herbivory dominated. This resulted in different optimum nutrient levels for invasion success of the two species due to their individual functional traits. To test the effect of diversity on invasibility, a species richness gradient was generated by random selection from a resident species pool at an intermediate nutrient level. Invasibility was not affected by species richness; instead, it was driven by the functional traits of the resident and/or invasive species mediated by herbivore density. Overall, herbivory was the driving factor for invasibility of phytoplankton communities, which implies that other factors affecting the intensity of herbivory (e.g., productivity or edibility of primary producers) indirectly influence invasions.     

Final thermal conditions override the effects of temperature history and dispersal in experimental communities

Predicting the effect of climate change on biodiversity is a multifactorial problem that is complicated by potentially interactive effects with habitat properties and altered species interactions. In a microcosm experiment with communities of microalgae, we analysed whether the effect of rising temperature on diversity depended on the initial or the final temperature of the habitat, on the rate of change, on dispersal and on landscape heterogeneity. We also tested whether the response of species to temperature measured in monoculture allowed prediction of the composition of communities under rising temperature. We found that the final temperature of the habitat was the primary driver of diversity in our experimental communities. Species richness declined faster at higher temperatures. The negative effect of warming was not alleviated by a slower rate of warming or by dispersal among habitats and did not depend on the initial temperature. The response of evenness, however, did depend on the rate of change and on the initial temperature. Community composition was not predictable from monoculture assays, but higher fitness inequality (as seen by larger variance in growth rate among species in monoculture at higher temperatures) explained the faster loss of biodiversity with rising temperature.     

Co-limitation of plant primary productivity by nitrogen and phosphorus in a species-rich wooded meadow on calcareous soils

High small-scale species richness of calcareous grasslands is generally thought to result from evening of species competitive potentials by limited N availability, because of relatively low herb N/P ratios in these communities. However, P mobility is low in alkaline soils as well. We studied soil chemistry and productivity of herb and moss layers in a very diverse calcareous meadow (up to 76 vascular plant species per m²) to test the hypotheses of a co-limitation of herb productivity by both soil N and P availabilities and moss productivity primarily by P availability. The effect of nutrient supply on productivity was investigated using both a natural productivity gradient as well as fertilization experiments. We observed strong positive correlations of soil P availability and total soil N with the above-ground productivity of herb layer. A long-term fertilization experiment demonstrated that P alone and N and P together increased productivity of vascular species, and that the productivity continuously declined after cessation of fertilization with the effect of previous fertilization occasionally visible even 14 years after treatment termination. A short-term fertilization experiment further demonstrated that N and P when supplied alone increase productivity of vascular plants, suggesting that both elements were limiting. Furthermore, there was a significant interaction between N and P on productivity, indicating that simultaneous N and P supply increased productivity more than separate nutrient additions. Moss productivity was negatively associated with vascular plant productivity. In particular, N addition decreased moss productivity, but moss productivity did not decline in P addition treatments. P requirements of mosses were larger than those of vascular plants. Our data indicate co-limitation of herb productivity by both soil N and P in this highly diverse grassland, while limitation of moss productivity mainly by P. We suggest that N and P co-limitations are common in calcareous diverse grasslands, and may partly explain the extreme small-scale species diversity in these communities.     

The roles of biotic resistance and nitrogen deposition in regulating non-native understory plant diversity

Understanding the mechanisms that allow for plant invasions is important for both ecologists and land managers, due to both the environmental and economic impacts of native biodiversity losses. We conducted an observational field study in 2008 to examine the relationship between native and non-native forest understory plant species and to investigate the influence of soil nitrogen (N) on plant community richness and diversity. In 2009, we conducted a companion fertilization experiment to investigate how various forms of N deposition (inorganic and organic) influenced native and non-native species richness and diversity. We found that native species richness and diversity were negatively correlated with 1) non-native species richness and diversity and 2) higher total soil inorganic N. In the deposition experiment, adding organic N fertilizers decreased native richness and diversity compared to inorganic N fertilizers. Together, these results indicate that increasing soil N can be detrimental to native species; however, native species richness and diversity may counteract the N-stimulation of non-native species. Furthermore, the negative effects of organic N deposition on native plants may be just as strong, if not stronger, than the effects of inorganic N deposition.     

Interactive effect of warming,nitrogen and phosphorus limitation on phytoplankton cell size

Cell size is one of the ecologically most important traits of phytoplankton. The cell size variation is frequently related to temperature and nutrient limitation. In order to disentangle the role of both factors, an experiment was conducted to determine the possible interactions of these factors. Baltic Sea water containing the natural plankton community was used. We performed a factorial combined experiment of temperature, type of nutrient limitation (N vs. P), and strength of nutrient limitation. The type of nutrient limitation was manipulated by altering the N:P ratio of the medium (balanced, N and P limitation) and strength by the dilution rate (0% and 50%) of the semicontinuous cultures. The negative effect of temperature on cell size was strongest under N limitation, intermediate under P limitation, and weakest when N and P were supplied at balanced ratios. However, temperature also influenced the intensity of nutrient imitation, because at higher temperature there was a tendency for dissolved nutrient concentrations to be lower, while the C:N or C:P ratio being higher…higher at identical dilution rates and medium composition. Analyzing the response of cell size to C:N ratios (as index of N limitation) and C:P ratios (as index of P limitation) indicated a clear dominance of the nutrient effect over the direct temperature effect, although the temperature effect was also significant.     

Species Diversity Across Nutrient Gradients: An Analysis of Resource Competition in Model Ecosystems

The capture and efficient use of limiting resources influence the competitive success of individual plant species as well as species diversity across resource gradients. In simulations, efficient nutrient acquisition or nutrient retention by species were key predictors of success when nutrients were limiting. Increased nutrient supply favored species with characteristics that improved light interception or light use. Ecological theory suggests that low diversity on fertile sites may be a consequence of competitive exclusion by one or a few species with superior light-interception characteristics. On infertile sites, competitive exclusion may be a function of superior nutrient-acquisition characteristics in species. At intermediate fertility, a shift from single-resource specialization to a balanced effort in the acquisition of multiple resources should allow for greater species diversity. Thus, a unimodal relationship between diversity and nutrient supply, vegetation biomass, or productivity is predicted. However, simulations demonstrated alternate relationships depending on the ecosystem characteristic to which diversity was compared. Diversity was greatest at intermediate total biomass but increased monotonically with net primary production and nitrogen (N) supply. The highest diversity occurred midrange on a scale of community-level leaf area to fine-root length ratios, which in the context of the model indicates that the vegetation as a whole was simultaneously limited by both N and light and that effort toward the acquisition of both resources is distributed in such a way that both resources are equally exploited. Diversity was lowered by the presence of species with a superior ability to sequester resources.     

Light and nitrogen competition limit Lolium perenne in experimental grasslands of increasing plant diversity

Positive species richness effects on aboveground community productivity in experimental grasslands have been reported to correlate with variable responses of individual species. So far, it is largely unknown whether more complete use of resources at the community level correlates with resource limitation of particular species and may explain their decreasing performance with increasing plant diversity. Using the subordinate grass species Lolium perenne L. as a model, we monitored populations in 82 experimental grasslands of different plant diversity (Jena Experiment) from year 2 to 6 after establishment, and measured ecophysiological leaf traits related to light and nutrient acquisition and use. Population and plant individual sizes of L. perenne decreased with increasing species richness. A decrease in transmitted light with increasing species richness and legume proportion correlated with increasing specific leaf area (SLA). Despite this morphological adaptation to lower light availability, decreasing foliar δ(13) C signatures with increasing species richness and low variation in leaf gas exchange and chlorophyll concentrations suggested a low capacity of L. perenne for adjustment to canopy shade. Leaf nitrogen concentrations and foliar δ(15) N signatures indicated a better N supply in communities with legumes and a shift in the uptake of different N forms with increasing species richness. Leaf blade nitrate and carbohydrate concentrations as indicators of plants nutritional status supported that light limitation with increasing species richness and legume proportions, combined with a N limitation in communities with increasing proportions of non-legumes, correlated with the decreasing performance of L. perenne in communities of increasing plant diversity.     

The resource balance hypothesis of plant species diversity in grassland

Abstract: We hypothesize that plant species diversity is favoured when actual resource supply ratios are balanced according to the optimum resource supply ratios for the vegetation as a whole. This ‘resource balance hypothesis of plant species diversity’ (RBH) follows from two different mechanisms of plant species coexistence, namely: ‘differential resource limitation’, which allows species to coexist in a competitive equilibrium in a homogeneous environment and ‘micro-habitat differentiation’, which builds on spatial heterogeneity. Both mechanisms require that resource supply ratios are intermediate between the optimum supply ratios of the species present in the species pool. Additional conditions, concerning the resource acquisition and requirement ratios of the species, are easier to meet for the second mechanism than for the first. To test the RBH we measured species diversity parameters in 74 grassland plots, as well as the N, P and K concentrations in the above-ground biomass. We used a new ceiling detection algorithm to examine the relationship between maximum observed diversity and the N/P-, P/K- and K/N-ratios in the biomass. Most of these ceiling relationsips could be described by parabolic curves with significant quadratic terms. This indicates that high diversity does not occur at the extremes of the observed ranges of nutrient ratios. This supports the RBH.     

Temperature and species richness effects in phytoplankton communities

Phytoplankton play an important role as primary producers and thus can affect higher trophic levels. Phytoplankton growth and diversity may, besides other factors, be controlled by seasonal temperature changes and increasing water temperatures. In this study, we investigated the combined effects of temperature and diversity on phytoplankton growth. In a controlled laboratory experiment, monocultures of 15 freshwater phytoplankton taxa (green algae, cyanobacteria, and diatoms) as well as 25 mixed communities of different species richness (2–12 species) and taxa composition were exposed to constant temperatures of 12, 18, and 24 °C. Additionally, they were exposed to short-term daily temperature peaks of +4 °C. Increased species richness had a positive effect on phytoplankton growth rates and phosphorous content at all temperature levels, with maximum values occurring at 18 °C. Overyielding was observed at almost all temperature levels and could mostly be explained by complementary traits. Higher temperatures resulted in higher fractions of cyanobacteria in communities. This negative effect of temperature on phytoplankton diversity following a shift in community composition was most obvious in communities adapted to cooler temperatures, pointing to the assumption that relative temperature changes may be more important than absolute ones.