Patterns of Species Richness and Composition in Re-Created Grassland

The success of many prairie restorations is not well documented. A restoration begun in 1975 at the Fermi National Accelerator Laboratory near Chicago, Illinois allows assessment of restoration efforts as well as changes through time. Data are presented on species richness and composition for 13 restorations planted in successive years between 1975 and 1990 and two remnant prairies. Presence of species was recorded using a stratified random design. Species richness at several scales and non‐metric multidimensional scaling ordination were used to assess trends in the vegetation. Species richness declined through time at all scales examined and was always less in the restored prairies than that found in the remnant prairies. Species composition changed with time but not in the direction of the composition found in the remnants. Our understanding of the maintenance of species richness is not sufficient to allow the re‐creation of patterns of species found in remnant grassland communities.     

Contrasting Soil Ciliate Species Richness and Abundance between Two Tropical Plant Species: A Test of the Plant Effect

We still have a rudimentary understanding about the mechanism by which plant roots may stimulate soil microbial interactions. A biochemical model involving plant-derived biochemical fractions, such as exudates, has been used to explain this “rhizosphere effect” on bacteria. However, the variable response of other soil microbial groups, such as protozoa, to the rhizosphere suggests that other factors could be involved in shaping their communities. Thus, two experiments were designed to: (1) determine whether stimulatory and/or inhibiting factors associated with particular plant species regulate ciliate diversity and abundance and (2) obtain a better understanding about the mechanism by which these plant factors operate in the rhizosphere. Bacterial and chemical slurries were reciprocally exchanged between two plant species known to differ in terms of ciliate species richness and abundance (i.e., Canella winterana and plantation Tectona grandis). Analysis of variance showed that the bacteria plus nutrients and the nutrients only treatment had no significant effect on overall ciliate species richness and abundance when compared to the control treatment. However, the use of only colpodean species increased the taxonomic resolution of treatment effects revealing that bacterial slurries had a significant effect on colpodean ciliate species richness. Thus, for particular rhizosphere ciliates, biological properties, such as bacterial diversity or abundance, may have a strong influence on their diversity and possibly abundance. These results are consistent with a model of soil bacteria-mediated mutualisms between plants and protozoa.     

Seed Density Significantly Affects Species Richness and Composition in Experimental Plant Communities

Studies on the importance of seed arrival for community richness and composition have not considered the number of seeds arriving and its effect on species richness and composition of natural communities is thus unknown. A series of experimental dry grassland communities were established. All communities were composed of the same 44 species in exactly the same proportions on two substrates using three different seed densities.The results showed that seed density had an effect on species richness only at the beginning of the experiment. In contrast, the effects on species composition persisted across the entire study period. The results do not support the prediction that due to higher competition for light in nutrient-rich soil, species richness will be the highest in the treatment with the lowest seed density. However, the prevalence of small plants in the lowest seed density supported the expectation that low seed density guarantees low competition under high soil nutrients. In the nutrient-poor soil, species richness was the highest at the medium seed density, indicating that species richness reflects the balance between competition and limitations caused by the availability of propagules or their ability to establish themselves. This medium seed density treatment also contained the smallest plants.The results demonstrate that future seed addition experiments need to consider the amount of seed added so that it reflects the amount of seed that is naturally found in the field. Differences in seed density, mimicking different intensity of the seed rain may also explain differences in the composition of natural communities that cannot be attributed to habitat conditions. The results also have important implications for studies regarding the consequences of habitat fragmentation suggesting that increasing fragmentation may change species compositions not only due to different dispersal abilities but also due to differential response of plants to overall seed density.     

Species Composition Changes in Conservation Reserve Program (CRP) Grassland When Managed for Biomass Feedstock Production

Grasslands enrolled in the Conservation Reserve Program (CRP) serve as one of the potential national herbaceous resources for use as a dedicated bioenergy feedstock. The goal of this project was to assess the yield potential and suitability of CRP grassland as a bioenergy feedstock source across the USA in regions with significant CRP land resources. In addition to that goal, one major objective of this project was to assess vegetation composition changes that also occurred on these different CRP grasslands over time with different harvest and fertilization management strategies. Three levels of nitrogen fertilization (0, 56, and 112 kg ha^?1) and two harvest timings [peak standing crop (PSC) or end of growing season (EGS)] were evaluated for effects on biomass production and resulting species composition changes. Three sites in regions containing concentrated tracts of CRP grassland and representing variable climatic parameters were analyzed for vegetation composition trends over the course of six growing seasons (2008–2013). Specifically, a mixture of warm-season perennial grasses was evaluated in Kansas (KS), while a cool-season mixture was evaluated in Missouri (MO). North Dakota (ND) contained a mixture of both warm- and cool-season grasses. At the MO and KS sites, nitrogen fertilization significantly altered the grass and legume composition over time by lowering the legume percentage in the stand. In KS and ND, the two sites with warm-season grasses, harvesting in mid-summer at PSC, greatly reduced warm-season grass composition over time in favor of annual cool-season grass invaders or perennial cool-season grasses. Any shift to less desirable or less productive species limits the ability of these lands to provide a sustainable or reliable feedstock for bioenergy production.     

The Rengen Grassland Experiment: relationship between soil and biomass chemical properties, amount of elements applied, and their uptake

The Rengen Grassland Experiment (RGE) was established in the Eifel Mountains (Germany) on a low productive Nardetum in 1941. Since then, the following fertilizer treatments have been applied with a late two-cut system: unfertilized control, Ca, CaN, CaNP, CaNPKCl and CaNPK₂SO₄. We aimed to understand how concentrations of macro (N, P, K, Ca and Mg), micro (Cu, Fe, Mn and Zn) and trace (As, Cd, Cr, Ni and Pb) elements in the plant biomass were affected by long-term fertilizer application, soil chemical properties and biomass production. In 2008, biomass samples from the first cut (early July) and the second cut (mid-October) were collected and analyzed. The simultaneous application of N, P and K decreased nitrogen concentration in the aboveground biomass, but substantially increased biomass production. Late cutting management decreased forage quality in highly productive more than in low productive plant communities. The concentrations of P and K in the plant biomass were positively related to P and K application and, therefore, to plant available P and K concentrations in the soil. The concentrations of some micro (Fe, Mn and Zn) and trace (As, Cd, Cr, Ni and Pb) elements in the plant biomass were negatively correlated with the amount of elements supplied by fertilizers and biomass production, probably because of the dilution effect. Long-term fertilizer application resulted in the accumulation of macro (P, Ca and Mg), micro (Fe and Mn) and trace (As and Cr) elements in the soil, but in many cases this accumulation was not connected with an increase in the concentrations of these elements in the plant biomass. Nutritional status, as indicated by the biomass N:P ratio, was consistent with N or P limitation as indicated by the nitrogen and phosphorus nutrition indices. Furthermore, additional K (co-)limitation was indicated by the N:K and K:P ratios in the biomass from the NP treatment. The results from the RGE indicate that there is no simple positive relationship between the applied elements and their concentrations in the plant biomass.     

TaqMan Real-Time PCR Assays To Assess Arbuscular Mycorrhizal Responses to Field Manipulation of Grassland Biodiversity: Effects of Soil Characteristics,Plant Species Richness,and Functional Traits

Large-scale (temporal and/or spatial) molecular investigations of the diversity and distribution of arbuscular mycorrhizal fungi (AMF) require considerable sampling efforts and high-throughput analysis. To facilitate such efforts, we have developed a TaqMan real-time PCR assay to detect and identify AMF in environmental samples. First, we screened the diversity in clone libraries, generated by nested PCR, of the nuclear ribosomal DNA internal transcribed spacer (ITS) of AMF in environmental samples. We then generated probes and forward primers based on the detected sequences, enabling AMF sequence type-specific detection in TaqMan multiplex real-time PCR assays. In comparisons to conventional clone library screening and Sanger sequencing, the TaqMan assay approach provided similar accuracy but higher sensitivity with cost and time savings. The TaqMan assays were applied to analyze the AMF community composition within plots of a large-scale plant biodiversity manipulation experiment, the Jena Experiment, primarily designed to investigate the interactive effects of plant biodiversity on element cycling and trophic interactions. The results show that environmental variables hierarchically shape AMF communities and that the sequence type spectrum is strongly affected by previous land use and disturbance, which appears to favor disturbance-tolerant members of the genus Glomus. The AMF species richness of disturbance-associated communities can be largely explained by richness of plant species and plant functional groups, while plant productivity and soil parameters appear to have only weak effects on the AMF community.Arbuscular mycorrhizae are mutualistic associations between roots of plants and fungi that have been present for more than 400 million years (54). Approximately 80% of examined land plants (71), and almost all fungi of the phylum Glomeromycota (60), are capable of forming such associations. The main benefit of this relationship for plants is that it facilitates their acquisition of nutrients (especially P and N), while the fungus receives photoassimilates (7, 62). About 200 Glomeromycota species have been described to date, based on spore morphology (http://www.lrz-muenchen.de/∼schuessler/amphylo/amphylogeny.html), but there is increasing molecular evidence of significantly higher diversity in arbuscular mycorrhizal fungi (AMF) (10, 72).Diverse AMF communities have been detected in a wide range of plant communities (inter alia grasslands, boreal forests, and tropical communities; for an overview, see reference 48). Hence, AMF have been considered to be tolerant of wide ranges of ecological conditions and capable of associating with diverse plant partners. Identifying the factors regulating their community assemblages is challenging, but AMF community composition has been shown to be influenced by plant species diversity (e.g., see references 10, 22, and 33), and conversely, significant effects of AMF species and communities on the diversity and productivity of plant communities have been described (25, 68). Soil physicochemical parameters like phosphorus, nitrogen, and carbon availability (e.g., see references 4, 9, and 31); pH (17); moisture content (53); and disturbance (30) also reportedly influence AMF distribution. Hence, there is some support for niche theory, which presumes that two species of the same trophic level cannot coexist in a limited system and, if two species are present in such circumstances, one should become extinct (21). As a corollary, two cooccurring species must occupy niches that differ in some dimensions, e.g., plant hosts and/or soil properties (28). However, there are also indications that neutral ecological processes, as well as niche-defining parameters, may influence AMF diversity and community composition (17, 39). In contrast to niche theory, neutral theory (27) postulates that all individuals of every species at a given trophic level in a food web have ecological equivalence, and thus, all species within trophically defined communities can be regarded as open nonequilibrium assemblages that are solely shaped by dispersal and distinctions in spatiotemporal dimensions. According to the work of Hubbell (27), neutrality is defined at the level of individual organisms with identical probabilities of birth, death, migration, and speciation and not at the species level. In order to explore AMF communities more thoroughly and to test competing hypotheses, such as those raised by the niche and neutral theories, robust methods for high-throughput analyses of the communities are required.Recent investigations of variables that affect the structure of AMF communities have considered relationships between niche-defining dimensions, such as soil types (39) and pH gradients (17), and spatial variations in AMF community structure but not the role of plant diversity or functional traits of host plants. There have been several plant diversity manipulation experiments designed for coanalyzing multiple sets of ecological variables (e.g., the BIODEPTH and Cedar Creek projects) that would have been ideal for detailed examinations of effects of ecological variables on AMF, but previously reported AMF analyses in these experiments have been limited to counts of spores in a single study (11). However, not all AMF species regularly sporulate, and when present, spores poorly reflect AMF diversity (69), since active AMF occur as mycelia in roots and soils (e.g., see references 12 and 26). PCR-based molecular techniques enable much more rigorous characterization of AMF communities in these compartments (e.g., see references 26, 36, and 72), but assessments of broad spatial (42) and/or temporal (52) variations in AMF communities require analysis of large numbers of samples, which is not feasible using conventional PCR amplification followed by cloning and sequencing. This challenge can be potentially met by real-time PCR-based approaches, in which the AMF sequence types present in compartments of interest are first identified and then sequence type-specific probes are used for large-scale screening in real-time PCR TaqMan assays.In the study presented here, we explored AMF diversity in plots used in the Jena Experiment, a grassland plant diversity manipulation of 60 plant species representing four functional groups in 81 plots of 400 m² (56). The overall AMF diversity and community structure were first assessed by PCR amplification, cloning, and sequencing (55) of internal transcribed spacer (ITS) ribosomal DNA (rDNA) gene sequences in soil samples from 23 representative plots. Using the acquired data, we then developed sequence type-specific probes, which were applied in high-throughput real-time PCR TaqMan assays of samples from all 81 experimental plots, and the effects of 15 plant and soil variables on the AMF community assemblage were investigated.     

Influence of Plant Community Composition on Biomass Production in Planted Grasslands

United States energy policy mandates increased use of renewable fuels. Restoring grasslands could contribute to a portion of this requirement through biomass harvest for bioenergy use. We investigated which plant community characteristics are associated with differences in biomass yield from a range of realistic native prairie plantings (n = 11; i.e., conservation planting, restoration, and wildlife cover). Our primary goal was to understand whether patterns in plant community composition and the Floristic Quality Index (FQI) were related to productivity as evidenced by dormant season biomass yield. FQI is an objective measure of how closely a plant community represents that of a pre-European settlement community. Our research was conducted in planted fields of native tallgrass prairie species, and provided a gradient in floristic quality index, species richness, species diversity, and species evenness in south-central Wisconsin during 2008 and 2009. We used a network of 15 randomly located 1 m² plots within each field to characterize the plant community and estimate biomass yield by clipping the plots at the end of each growing season. While plant community composition and diversity varied significantly by planting type, biomass yield did not vary significantly among planting types (ANOVA; P >0.05). Biomass yield was positively correlated with plant community evenness, richness, C4 grass cover, and floristic quality index, but negatively correlated with plant species diversity in our multi-season multiple linear mixed effects models. Concordantly, plots with biomass yield in the lowest quartile (biomass yield < 3500 kh/ha) had 8% lower plant community evenness and 9% lower FQI scores than those in the upper quartile (biomass yield > 5800 kh/ha). Our results suggest that promoting the establishment of fields with high species evenness and floristic quality may increase biomass yield, while simultaneously supporting biodiversity.     

Realistic Variation in Species Composition Affects Grassland Production,Resource Use and Invasion Resistance

We investigated the effects of realistic variation in plant species and functional group composition, with species occurring at realistic abundances, on ecosystem processes in exotic-dominated California grassland communities. Progressive species removals from microcosm communities, designed to mimic nested variation in diversity observed in the field, reduced grassland production, resistance to intentional invasions, and resistance to natural colonization by new species. Three lines of evidence point to the particular importance of intensified competition within a single functional group—late-active forbs—in explaining the observed effects of realistic species loss order on community resistance. First, reduced success of naturally colonizing species in more diverse assemblages was dominated by declining colonization by late-active forbs. Second, increasing late-active forb biomass appeared to reduce the biomass of intentionally introduced yellow starthistle (Centaurea solstitialis, a late-season forb) both within and across diversity levels. Finally, starthistle addition reduced biomass of resident late-season forbs but not of any other functional group. Increasing diversity increased light levels and soil moisture availability in spring and summer, providing a proximate mechanism linking our realistic species loss order to decreased community resistance. Starthistle addition reduced light and soil moisture availability but not N across richness levels, mirroring the apparent effects of the additional late-active forb species present in higher diversity treatments. Species losses that entail the early loss of whole or key functional groups could, through mechanisms like those we explore, have greater ecosystem consequences than those suggested by randomized-loss experiments.     

The Relation Between Unpalatable Species, Nutrients and Plant Species Richness in Swiss Montane Pastures

In agriculturally marginal areas, the control of unpalatable weeds on species rich pastures may become problematic due to agricultural and socio-economic developments. It is unclear how increased dominance of unpalatable species would affect the botanical diversity of these grasslands. We investigated whether there was any relationship between plant species diversity and the abundance of unpalatable species and whether soil conditions affected this relationship. In three species-rich montane pastures in western Switzerland, we related plant species richness to soil attributes, the relative cover of all unpalatable species and the relative cover of the locally dominant, toxic Veratrum album in 25 plots of 4 m². We furthermore determined species richness in small transects through patches of V. album. Species richness was significantly lower in and near (≤ 0.3 m) patches of V. album. At the field scale, plant species richness was best described by total soil N:P ratio (positive relation) in one site and the relative abundance of unpalatable species (negative relation) and soil N:P ratio (positive relation) in a second site. In the third site, species richness was not significantly related to any measured variable. Vegetation diversity (Simpson's D) was negatively related to the relative abundance of unpalatable species in one site and positively related to pH in another site. The results suggest that no single factor can explain plant species richness and diversity in montane pastures. At very high densities unpalatable species can have adverse effects but soil nutrient status appears to be a more general determinant of plant species richness. Conservation efforts should give priority to the prevention of intensification of these pastures.     

Predicting Coral Species Richness: The Effect of Input Variables,Diversity and Scale

Coral reefs are facing a biodiversity crisis due to increasing human impacts, consequently, one third of reef-building corals have an elevated risk of extinction. Logistic challenges prevent broad-scale species-level monitoring of hard corals; hence it has become critical that effective proxy indicators of species richness are established. This study tests how accurately three potential proxy indicators (generic richness on belt transects, generic richness on point-intercept transects and percent live hard coral cover on point-intercept transects) predict coral species richness at three different locations and two analytical scales. Generic richness (measured on a belt transect) was found to be the most effective predictor variable, with significant positive linear relationships across locations and scales. Percent live hard coral cover consistently performed poorly as an indicator of coral species richness. This study advances the practical framework for optimizing coral reef monitoring programs and empirically demonstrates that generic richness offers an effective way to predict coral species richness with a moderate level of precision. While the accuracy of species richness estimates will decrease in communities dominated by species-rich genera (e.g. Acropora), generic richness provides a useful measure of phylogenetic diversity and incorporating this metric into monitoring programs will increase the likelihood that changes in coral species diversity can be detected.     

The Effect of Host-Plant Phylogenetic Isolation on Species Richness,Composition and Specialization of Insect Herbivores: A Comparison between Native and Exotic Hosts

Understanding the drivers of plant-insect interactions is still a key issue in terrestrial ecology. Here, we used 30 well-defined plant-herbivore assemblages to assess the effects of host plant phylogenetic isolation and origin (native vs. exotic) on the species richness, composition and specialization of the insect herbivore fauna on co-occurring plant species. We also tested for differences in such effects between assemblages composed exclusively of exophagous and endophagous herbivores. We found a consistent negative effect of the phylogenetic isolation of host plants on the richness, similarity and specialization of their insect herbivore faunas. Notably, except for Jaccard dissimilarity, the effect of phylogenetic isolation on the insect herbivore faunas did not vary between native and exotic plants. Our findings show that the phylogenetic isolation of host plants is a key factor that influences the richness, composition and specialization of their local herbivore faunas, regardless of the host plant origin.     

Survival, Gap Formation, and Recovery Dynamics in Grassland Ecosystems Exposed to Heat Extremes: The Role of Species Richness

A field experiment was performed in which the richness of perennial grasses (S) was varied in model ecosystems exposed to a simulated heat wave (free air temperature increase and drought). The proportion of individuals that survived the heat wave decreased with S, which could be ascribed to higher water consumption in the species-rich systems. Higher transpiration at high diversity was also observed in other studies using functional groups and could have originated from increased leaf area, less intense stomatal closure, or a combination of both. The increased tiller number per plant that we observed, while leaf area per tiller remained constant, suggests that an enhanced leaf area index was most likely responsible. However, competitive interactions also seemed to play a role in the influence of S on survival. Regrowth of the surviving individuals, expressed as leaf area per living plant after a recovery period following the heat wave, increased with S, most likely due to the dominance of productive species, which was facilitated by the additional space yielded by more intense gap formation at higher S (due to higher plant mortality). Species richness affected both the size and density of the gaps. Mean size increased exponentially with S, while density increased at low S but decreased at higher S when connectance of the gaps occurred. Size distribution of the gaps was not affected. Received 18 January 2000; accepted 31 May 2001.     

Impact of Plant Functional Group, Plant Species, and Sampling Time on the Composition of nirK-Type Denitrifier Communities in Soil

We studied the influence of eight nonleguminous grassland plant species belonging to two functional groups (grasses and forbs) on the composition of soil denitrifier communities in experimental microcosms over two consecutive years. Denitrifier community composition was analyzed by terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. The impact of experimental factors (plant functional group, plant species, sampling time, and interactions between them) on the structure of soil denitrifier communities (i.e., T-RFLP patterns) was analyzed by canonical correspondence analysis. While the functional group of a plant did not affect nirK-type denitrifier communities, plant species identity did influence their composition. This effect changed with sampling time, indicating community changes due to seasonal conditions and a development of the plants in the microcosms. Differences in total soil nitrogen and carbon, soil pH, and root biomass were observed at the end of the experiment. However, statistical analysis revealed that the plants affected the nirK-type denitrifier community composition directly, e.g., through root exudates. Assignment of abundant T-RFs to cloned nirK sequences from the soil and subsequent phylogenetic analysis indicated a dominance of yet-unknown nirK genotypes and of genes related to nirK from denitrifiers of the order Rhizobiales. In conclusion, individual species of nonleguminous plants directly influenced the composition of denitrifier communities in soil, but environmental conditions had additional significant effects.     

Plant Species Composition Effects on Belowground Properties and the Resistance and Resilience of the Soil Microflora to a Drying Disturbance

We hypothesised that plant species composition and richness would affect soil chemical and microbial community properties, and that these in turn would affect soil microbial resistance and resilience to an experimentally imposed drying disturbance. We performed a container experiment that manipulated the composition and species richness of common pasture plant species (Trifolium repens, Lolium perenne, and Plantago lanceolata) by growing them in monoculture, and in all the possible two and three-way combinations, along with an unplanted control soil. Experimental units were harvested at four different times over a 16-month period to determine the effect of plant community development and seasonal changes in temperature and moisture on belowground properties. Results showed that plant species composition influenced soil chemistry, soil microbial community properties and soil microbial resistance and resilience. Soil from planted treatments generally showed reduced soil microbial resistance to drying compared to unplanted control soils. Soils from under T. repens showed a higher resistance and resilience than the soils from under P. lanceolata, and a higher resistance than soils from under L. perenne. We suggest that differences across soils in either resource limitation or soil microbial community structure may be responsible for these results. Plant species richness rarely affected soil microbial community properties or soil microbial resistance and resilience, despite having some significant effects on plant community biomass and soil nitrogen contents in some harvests. The effect that treatments had for most variables differed between harvests, suggesting that results can be altered by the stage of plant community development or by extrinsic environmental factors that varied with harvest timing. These results in combination show that soil microbial resistance and resilience was affected by plant community composition, and the time of measurement, but was largely unrelated to plant species richness.     

Complex Interrelationships Among Aboveground Biomass,Soil Chemical Properties,and Events Caused by Feral Goats and Their Eradication in a Grassland Ecosystem of an Island

This study examined the recovery, via biotic and abiotic pathways, of a grassland ecosystem after eradication of introduced exotic goats. We used path analyses to evaluate the relative strength of relationships among aboveground biomass, soil chemical properties (carbon, nitrogen, and phosphorus content; soil acidity), presence of nesting seabirds after goat eradication, extent of vegetation degraded by goats before their eradication, plant species composition after removal of goats, and topography. Models including the same variables with different paths were constructed using the Bayesian estimation method, and the best-fit models were constructed by comparing deviance information criterion values. Results of the path analyses demonstrated that vegetation degradation and soil erosion prior to goat eradication increased soil exchangeable acidity, which resulted in limitation of aboveground biomass. Seabird nesting after goat eradication increased the quantity of soil nutrients, possibly through inputs of feces, eggshells, and dead chicks or adults. The increase in nutrients was affected indirectly, via seabird nesting, by topography and vegetation type after goat eradication. The direct and indirect relationships demonstrated by our results suggest the existence of complex interrelationships during recovery of ecosystem function after eradication of exotic mammals.     

Effects of Elevated Carbon Dioxide on Plant Biomass Production and Competition in a Simulated Neutral Grassland Community

Using open-top chambers, four prominent species (Lolium perenne,Cynosurus cristatus, Holcus lanatusandAgrostis capillaris) ofIrish neutral grasslands were grown at ambient and elevated(700 µmol mol^-1) atmospheric CO₂for a period of 8 months.The effects of interspecific competition on plant responsesto CO₂enrichment were investigated by growing the species ina four-species mixture. The results indicate that the speciesdiffer in their ability to respond to elevated CO₂. CO₂-enrichmenthad the largest effect on the biomass production ofH. lanatus,but substantial stimulations in biomass production were alsofound for the other three species. The CO₂-stimulation of biomassproduction forH. lanatuswas accompanied by increased tillering.In addition, reductions in specific leaf area were found forall species. Exposure to elevated CO₂increased the communitybiomass of the four-species mixture. This increase can be mainlyattributed to a significant increase in the biomass ofH. lanatusatelevated CO₂. No statistically-significant changes in speciescomposition of community biomass were found. However,H. lanatusdidincrease its share of community biomass at each of the harvests,with the other three species, mainlyL. perenne, suffering lossesin their shares at elevated CO₂. The results show that: (1)the species varied in their response to elevated CO₂; and (2)species composition in natural plant communities is likely tochange at elevated CO₂, but these changes may occur rather slowly.Much longer periods of exposure to elevated atmospheric CO₂maybe required to permit detection of significant changes in speciescomposition.Copyright 1998 Annals of Botany Company Carbon dioxide (CO₂) enrichment, competition, Lolium perenne,Cynosurus cristatus, Holcus lanatus, Agrostis capillaris, biomass, specific leaf area, tillering.     

Effect of Herbivory and Plant Species Replacement on Primary Production

Grazing optimization occurs when herbivory increases primary production at low grazing intensities. In the case of simple plant-herbivore interactions, such an effect can result from recycling of a limiting nutrient. However, in more complex cases, herbivory can also lead to species replacement in plant communities, which in turn alters how primary production is affected by herbivory. Here we explore this issue using a model of a limiting nutrient cycle in an ecosystem with two plant species. We show that two major plant traits determine primary production at equilibrium: plant recycling efficiency (i.e., the fraction of the plant nutrient stock that stays within the ecosystem until it is returned to the nutrient pool in mineral form) and plant ability to deplete the soil mineral nutrient pool through consumption of this resource. In cases where sufficient time has occurred, grazing optimization requires that herbivory improve nutrient conservation in the system sufficiently. This condition sets a minimum threshold for herbivore nutrient recycling efficiency, the fraction of nutrient consumed by herbivores that is recycled within the ecosystem to the mineral nutrient pool. This threshold changes with plant community composition and herbivore preference and is, therefore, strongly affected by plant species replacement. The quantitative effects of these processes on grazing optimization are determined by both the recycling efficiencies and depletion abilities of the plant species. However, grazing optimization remains qualitatively possible even with plant species replacement.     

Impact of Precipitation Patterns on Biomass and Species Richness of Annuals in a Dry Steppe

Annuals are an important component part of plant communities in arid and semiarid grassland ecosystems. Although it is well known that precipitation has a significant impact on productivity and species richness of community or perennials, nevertheless, due to lack of measurements, especially long-term experiment data, there is little information on how quantity and patterns of precipitation affect similar attributes of annuals. This study addresses this knowledge gap by analyzing how quantity and temporal patterns of precipitation affect aboveground biomass, interannual variation aboveground biomass, relative aboveground biomass, and species richness of annuals using a 29-year dataset from a dry steppe site at the Inner Mongolia Grassland Ecosystem Research Station. Results showed that aboveground biomass and relative aboveground biomass of annuals increased with increasing precipitation. The coefficient of variation in aboveground biomass of annuals decreased significantly with increasing annual and growing-season precipitation. Species richness of annuals increased significantly with increasing annual precipitation and growing-season precipitation. Overall, this study highlights the importance of precipitation for aboveground biomass and species richness of annuals.