Experimental Grassland Plots Research Articles

Microhymenoptera in roadside verges and the potential of arthropod‐friendly mowing for their preservation

AbstractThe worldwide decline of insects is one of the major challenges for humankind. One of its main drivers is intensive farming, which reduces habitats and food resources for insect populations and causes direct mortality by pesticides. In addition, mowing of grassland poses another threat to insects, especially when it is done frequently, such as in roadside verges. Roadside verges comprise large areas worldwide and can serve as habitats for animals and plants and as corridors to connect populations when they are maintained ecologically, for example when mowing is done arthropod‐friendly and with low frequency. Microhymenoptera are highly diverse, mostly show a parasitoid lifestyle and occur in large numbers in meadows and grasslands. Although they have an important ecological function in regulating their arthropod host populations, they were mostly neglected in studies on the influence of mowing on insects. Therefore, we assessed which families of microhymenoptera occur in roadside verges and meadows, examined if they are affected by the mowing process with a conventional mulching mower, and studied which groups benefit from arthropod‐friendly mowing, using the mowing head Eco 1200 plus from MULAG, and a flushing bar made of track tarpaulin. In our experimental grassland plots, we found specimens of 18 families from the six microhymenoptera superfamilies Chalcidoidea, Ceraphronoidea, Diaprioidea, Ichneumonoidea, Platygastroidea and Proctotrupoidea. Mowing with a conventional mulching mower caused a significant loss of up to 64% for parasitoid Hymenoptera. The Eco 1200 plus showed an arthropod‐friendly effect only on the number of individuals of Chalcidoidea, saving 38% of individuals compared with the conventional mower. The flushing bar showed a significant effect on total number of individuals with a reduction only on Chalcidoidea and a tendency for Ichneumonoidea with 30% and 47%, respectively. Our study demonstrates the detrimental effect of mowing with a conventional mulching mower on microhymenoptera and shows that this effect can be partly reversed for Chalcidoidea and Ichneumonoidea through the arthropod‐friendly mower and a flushing bar. Our study highlights the importance to consider microhymenoptera in the context of insect decline.

Focusing on individual plants to understand community scale biodiversity effects: the case of root distribution in grasslands

Spatial resource partitioning between species via differences in rooting depth is one of the main explanations for the positive biodiversity–productivity relationship. However, evidence for the importance of this mechanism is limited. This may be due to the community scale at which these interactions are often investigated. Community measures represent net outcomes of species interactions and may obscure the mechanisms underlying belowground interactions. Here, we assess the performance of ~1700 individual plants and their heterospecific neighbours over three growing seasons in experimental grassland plots containing one, four or sixteen different plant species and tested whether their performance in mixtures compared to monocultures was related to their own rooting depth versus the rooting depth of their heterospecific neighbours. Overall, individuals of deep‐rooting species performed better in mixtures and this effect significantly increased when surrounded by more shallow‐rooting species. This effect was not apparent for the shallow rooting species. Together, including both deep and shallow rooting species increased mixture performance. Our results show that taking the perspective of the individual rather than the community can elucidate the interactions between species that contribute to positive biodiversity effects, emphasizing the need for studies at different scales to disentangle the myriad interactions that take place in diverse communities.

Grassland biodiversity restoration increases resistance of carbon fluxes to drought

Abstract Evidence suggests that the restoration of plant diversity in grasslands not only brings benefits for biodiversity conservation, but also the delivery of ecosystem services. While biodiversity‐function experiments show that greater plant diversity increases resistance of plant productivity to climate extremes, it is not known whether real‐world management options for grassland restoration likewise stabilize ecosystem responses to extreme climate events. We used a long‐term (23 year) field experiment in northern England to test the hypothesis that management aimed at biodiversity restoration increases the resistance and recovery of ecosystem carbon (C) fluxes to short‐term summer drought. This was tested by measuring plant, soil and microbial responses to a simulated drought in experimental grassland plots where fertilizer application and seed addition have been managed to enhance plant species diversity. The cessation of fertilizer application brought about small increases in plant species richness. Additionally, cessation of fertilizer application reduced overall plant productivity and promoted hemi‐parasitic plants at the expense of grasses and forbs. Resistance of CO2 fluxes to drought, measured as ecosystem respiration, was greater in non‐fertilized plots, as lower plant biomass reduced water demand, likely aided by proportionally more hemi‐parasitic plants further reducing plant biomass. Additionally, legumes increased under drought, thereby contributing to overall resistance of plant productivity. Recovery of soil microbial C and nitrogen was more rapid after rewetting than soil microbial community composition, irrespective of restoration treatment, suggesting high resilience of soil microbial communities to drought. Synthesis and applications. This study shows that while grassland diversity restoration management increases the resistance of carbon fluxes to drought, it also reduces agricultural yields, revealing a trade‐off for land managers. Furthermore legumes, promoted through long‐term restoration treatments, can help to maintain plant community productivity under drought by increasing their biomass. As such, grassland management strategies not only have consequences for ecosystem processes, but also the capacity to withstand extreme weather events.

Land-Use Intensity Rather Than Plant Functional Identity Shapes Bacterial and Fungal Rhizosphere Communities.

The rhizosphere encompasses the soil surrounding the surface of plants’ fine roots. Accordingly, the microbiome present is influenced by both soil type and plant species. Furthermore, soil microbial communities respond to land-use intensity due to the effects on soil conditions and plant performance. However, there is limited knowledge about the impact of grassland management practices under field conditions on the composition of both bacteria and fungi in the rhizosphere of different plant functional groups. In spring 2014 we planted four phytometer species, two forbs (Plantago lanceolata, Achillea millefolium) and two grasses (Dactylis glomerata, Arrhenatherum elatius) into 13 permanent experimental grassland plots, differing in management. After 6 months, rhizosphere and bulk soil associated with the phytometer plants were sampled, microbial genomic DNA was extracted and bacterial 16S and fungal ITS rDNA were sequenced using Illumina MiSeq. Our study revealed that the rhizosphere microbial community was more diverse than the bulk soil community. There were no differences in microbial community composition between the two plant functional groups, but a clear impact of root traits and edaphic conditions. Land-use intensity strongly affected plant productivity, neighboring plant richness and edaphic conditions, especially soil C/N ratio, which in turn had a strong influence on root traits and thereby explained to large extent microbial community composition. Rhizosphere microbes were mainly affected by abiotic factors, in particular by land-use intensity, while plant functional type had only subordinate effects. Our study provides novel insights into the assembly of rhizosphere bacterial and fungal communities in response to land-use intensity and plant functional groups in managed grassland ecosystems.

Microbial community structure and functions differ between native and novel (exotic-dominated) grassland ecosystems in an 8-year experiment

AimsGrasslands dominated by non-native (exotic) species have replaced purely native-dominated areas in many parts of the world forming ‘novel’ ecosystems. Altered precipitation patterns are predicted to exacerbate this trend. It is still poorly understood how soil microbial communities and their functions differ between high diversity native- and low diversity exotic-dominated sites and how altered precipitation will impact this difference.MethodsWe sampled 64 experimental grassland plots in central Texas with plant species mixtures of either all native or all exotic species; half with summer irrigation. We tested how native vs. exotic plant species mixtures and summer irrigation affected bacterial and fungal community composition and structure, the influence of niche vs. neutral processes for microbial phylotype co-occurrence (C-score analysis), and rates of phosphorus and nitrogen mineralization across an 8-year experiment.ResultsNative and exotic-dominated plots had significantly different fungal community composition and structure, but not diversity, throughout the length of the study, while changes in bacterial communities were limited to certain wet and cool years. Nitrogen and phosphorus mineralization rates were higher under native plant mixtures and correlated with the abundance of particular fungal species. Microbial communities were more structured in exotic than native grassland plots, especially for the fungal community.ConclusionsThe results indicate that conversion of native to exotic C4 dominated grasslands will more strongly impact fungal than bacterial community structure. Furthermore, these impacts can alter ecosystem functioning belowground via changes in nitrogen and phosphorus cycling.

Effect of plant diversity on the diversity of soil organic compounds.

The effect of plant diversity on aboveground organisms and processes was largely studied but there is still a lack of knowledge regarding the link between plant diversity and soil characteristics. Here, we analyzed the effect of plant identity and diversity on the diversity of extractible soil organic compounds (ESOC) using 87 experimental grassland plots with different levels of plant diversity and based on a pool of over 50 plant species. Two pools of low molecular weight organic compounds, LMW1 and LMW2, were characterized by GC-MS and HPLC-DAD, respectively. These pools include specific organic acids, fatty acids and phenolics, with more organic acids in LMW1 and more phenolics in LMW2. Plant effect on the diversity of LMW1 and LMW2 compounds was strong and weak, respectively. LMW1 richness observed for bare soil was lower than that observed for all planted soils; and the richness of these soil compounds increased twofold when dominant plant species richness increased from 1 to 6. Comparing the richness of LMW1 compounds observed for a range of plant mixtures and for plant monocultures of species present in these mixtures, we showed that plant species richness increases the richness of these ESOC mainly through complementarity effects among plant species associated with contrasted spectra of soil compounds. This could explain previously reported effects of plant diversity on the diversity of soil heterotrophic microorganisms.

Effects of diversity and identity of the neighbouring plant community on the abundance of arthropods on individual ragwort (Jacobaea vulgaris) plants

AbstractThe diversity of plant community can greatly affect the abundance and diversity of arthropods associated to that community, but can also influence the composition or abundance of arthropods on individual plants growing in that community. We sampled arthropods and recorded plant size of individual ragwort, Jacobaea vulgaris Gaertner ssp. vulgaris [synonym Senecio jacobaea L. (Asteraceae)], plants transplanted into 70 experimental grassland plots that differed in plant diversity (1–9 species) or that were kept without vegetation. The arthropod fauna was dominated by the specialist aphid Aphis jacobaeae Schrank (Hemiptera: Aphididae). The abundance of aphids on ragwort plants decreased significantly with increasing plant diversity. The abundance of other arthropod species was not affected by the diversity of the surrounding plant community. Plant size was also not affected by the diversity of the surrounding plant community, but varied significantly among monocultures. Ragwort plants were largest in monocultures of legumes. Aphid abundance on ragwort plants, however, was not related to the size of the individual ragwort plants, but was high in monocultures consisting of Tanacetum vulgare L. (Asteraceae) plants. This plant is morphologically similar to ragwort. Even though we observed significant effects of plant diversity, ragwort plants were considerably larger – and the abundance of aphids and other arthropods on ragwort plants substantially higher – in plots without vegetation than in vegetated plots. Our results show that the presence and the diversity of neighbouring plants can provide associational resistance to focal plants growing in that community. We conclude that the surrounding plant community directly affects the abundance of arthropods on focal ragwort plants, and not via the effects of neighbouring plants on the performance of the focal plants.

Complexity of multitrophic interactions in a grassland ecosystem depends on plant species diversity

1. We studied the theoretical prediction that a loss of plant species richness has a strong impact on community interactions among all trophic levels and tested whether decreased plant species diversity results in a less complex structure and reduced interactions in ecological networks. 2. Using plant species-specific biomass and arthropod abundance data from experimental grassland plots (Jena Experiment), we constructed multitrophic functional group interaction webs to compare communities based on 4 and 16 plant species. 427 insect and spider species were classified into 13 functional groups. These functional groups represent the nodes of ecological networks. Direct and indirect interactions among them were assessed using partial Mantel tests. Interaction web complexity was quantified using three measures of network structure: connectance, interaction diversity and interaction strength. 3. Compared with high plant diversity plots, interaction webs based on low plant diversity plots showed reduced complexity in terms of total connectance, interaction diversity and mean interaction strength. Plant diversity effects obviously cascade up the food web and modify interactions across all trophic levels. The strongest effects occurred in interactions between adjacent trophic levels (i.e. predominantly trophic interactions), while significant interactions among plant and carnivore functional groups, as well as horizontal interactions (i.e. interactions between functional groups of the same trophic level), showed rather inconsistent responses and were generally rarer. 4. Reduced interaction diversity has the potential to decrease and destabilize ecosystem processes. Therefore, we conclude that the loss of basal producer species leads to more simple structured, less and more loosely connected species assemblages, which in turn are very likely to decrease ecosystem functioning, community robustness and tolerance to disturbance. Our results suggest that the functioning of the entire ecological community is critically linked to the diversity of its component plants species.

Initial species pattern affects invasion resistance in experimental grassland plots

AbstractQuestionDoes the spatial patterning of plant species affect plant community dynamics in ways that are independent of effects attributable to species richness or abundances? Does the initial species pattern affect subsequent diversity and invasion in a perennial grassland system?LocationField experiment, Iowa State University Horticultural Research Station, Ames, Iowa, USA.MethodsExperimental plots (4 m2) were planted with seedlings of four grassland species arranged into increasingly larger groups (patches) of conspecific individuals while controlling plot‐scale richness and evenness. Available light (photosynthetically active radiation; PAR) at the soil surface was measured each month for three growing seasons after planting. Species’ relative abundances were quantified via point‐intercept sampling within each plot at the end of each growing season.ResultsIn the three growing seasons after planting, planted species richness and evenness (plot scale) did not vary among plots planted with different species patterns. However, early in the second growing season more light reached the soil surface in plots with initially larger conspecific patches. Invader abundance was also consistently higher in plots planted with initially larger conspecific patches.ConclusionsOur findings support the hypothesis that invasion resistance increases as communities become more heterogeneous at fine scales and suggest that within‐plot heterogeneity should be considered as an additional factor when assessing invasion resistance in perennial plant communities.

Identifying population‐ and community‐level mechanisms of diversity–stability relationships in experimental grasslands

Summary1. While positive effects of biodiversity on temporal stability of communities have been demonstrated in theoretical and empirical studies, diversity–stability relationships at the population level remain poorly understood.2. We investigated temporal variability of plant populations in experimental grassland plots of varying species richness (1, 2, 4, 8, 16–60 species), functional group richness and composition (presence/absence of legumes × grasses × small herbs × tall herbs) in a long‐term biodiversity experiment from 2003 to 2009 (‘Jena Experiment’).3. Average population stability, defined as the reciprocal of the coefficient of variation of above‐ground biomass production over time, differed largely between species but was generally higher in grasses and small herbs than in legumes and tall herbs. Furthermore, population stability was positively related to a species’ proportional contribution to community biomass. Thus, an increasing number of subordinate species explained lower average population stabilities at higher diversity levels.4. A negative covariance (CV) across all species‐richness levels suggested negatively correlated species dynamics. Species belonging to different functional groups fluctuated asynchronously, while species dynamics within functional groups were more synchronous. Community‐wide species synchrony decreased with increasing species richness, and temporal stability at the community level increased.5. Synthesis: Our results suggest that diversity–stability relationships are driven by fluctuations in the population biomass of individual species which are less synchronized in more diverse than in less diverse mixtures and monocultures. Dominant plant species tend to be more stabilized than subdominant species, independently of community species richness. However, asynchrony of population dynamics outweighs decreasing population stability with increasing species richness, resulting in higher temporal stability at the plant community level.

Foliar and soil δ15N values reveal increased nitrogen partitioning among species in diverse grassland communities

Plant and soil nitrogen isotope ratios (δ¹⁵N) were studied in experimental grassland plots of varying species richness. We hypothesized that partitioning of different sources of soil nitrogen among four plant functional groups (legumes, grasses, small herbs, tall herbs) should increase with diversity. Four years after sowing, all soils were depleted in ¹⁵N in the top 5 cm whereas in non-legume plots soils were enriched in ¹⁵N at 5-25 cm depth. Decreasing foliar δ¹⁵N and Δδ¹⁵N (= foliar δ¹⁵N-soil δ¹⁵N) values in legumes indicated increasing symbiotic N₂ fixation with increasing diversity. In grasses, foliar Δδ¹⁵N also decreased with increasing diversity suggesting enhanced uptake of N depleted in ¹⁵N. Foliar Δδ¹⁵N values of small and tall herbs were unaffected by diversity. Foliar Δδ¹⁵N values of grasses were also reduced in plots containing legumes, indicating direct use of legume-derived N depleted in ¹⁵N. Increased foliar N concentrations of tall and small herbs in plots containing legumes without reduced foliar δ¹⁵N indicated that these species obtained additional mineral soil N that was not consumed by legumes. These functional group and species specific shifts in the uptake of different N sources with increasing diversity indicate complementary resource use in diverse communities.

Microbial biomass, organic matter mineralization and nitrogen in soils from long‐term experimental grassland plots (Palace Leas meadow hay plots, UK)

The Palace Leas meadow hay plots have received different annual manure and fertilizer applications since 1897 and represent the second oldest permanent grassland experiment in the world. Although the soil properties have diverged as a result of the experimental treatments since 1897, the soil organic carbon (SOC) contents of the plots have remained largely unchanged between the 1980s and 2006. In this study we have determined the effects of contrasting manure and fertilizer treatments on the total N contents of the soil, the recalcitrance of soil organic matter, the contributions made from different depths to the overall C mineralization, and the distribution of soil microbial biomass in the soil profiles. Relative to the control plot, which has received no manure or fertilizer additions since 1897, the two plots that have received manure or P plus K contain significantly more N, which in the case of the P plus K plot is consistent with abundant Trifolium spp. in the sward. The plot that has received farmyard manure (FYM) contained 11% more N than the control. The two plots that received large inputs of (NH4)2SO4 and which have consequently become highly acidic (around pH 4) contain 15–27% less N than the control despite the N addition and the accumulation of SOC in these plots. The plot that has only received P has also apparently lost total N (20%) relative to the control. The rates of C mineralization of the microbial biomass were closely related to total SOC contents but have been positively influenced by the increased N in the P plus K plot. Values for soil N, C mineralization rate and microbial C are reported because such data from sites that have been under known long‐term known land‐use are extremely rare and because there is increasing realization of the potential importance of soil data, especially biological characteristics, for monitoring environmental changes.

Functional identity versus species richness: herbivory resistance in plant communities.

The resistance of a plant community against herbivore attack may depend on plant species richness, with monocultures often much more severely affected than mixtures of plant species. Here, we used a plant–herbivore system to study the effects of selective herbivory on consumption resistance and recovery after herbivory in 81 experimental grassland plots. Communities were established from seed in 2002 and contained 1, 2, 4, 8, 16 or 60 plant species of 1, 2, 3 or 4 functional groups. In 2004, pairs of enclosure cages (1 m tall, 0.5 m diameter) were set up on all 81 plots. One randomly selected cage of each pair was stocked with 10 male and 10 female nymphs of the meadow grasshopper, Chorthippus parallelus. The grasshoppers fed for 2 months, and the vegetation was monitored over 1 year. Consumption resistance and recovery of vegetation were calculated as proportional changes in vegetation biomass. Overall, grasshopper herbivory averaged 6.8%. Herbivory resistance and recovery were influenced by plant functional group identity, but independent of plant species richness and number of functional groups. However, herbivory induced shifts in vegetation composition that depended on plant species richness. Grasshopper herbivory led to increases in herb cover at the expense of grasses. Herb cover increased more strongly in species-rich mixtures. We conclude that selective herbivory changes the functional composition of plant communities and that compositional changes due to selective herbivory depend on plant species richness.

Soil organic carbon contents in long‐term experimental grassland plots in the UK (Palace Leas and Park Grass) have not changed consistently in recent decades

AbstractA recent report of widespread declines in soil organic C (SOC) in the UK over the 10–25 years until the early 2000s has focussed attention on the importance of resampling previously characterized sites to assess long‐term trends in SOC contents and the importance of soils as a potentially volatile and globally significant reservoir of terrestrial C. We have used two sets of long‐term experimental plots which have been under constant and known management for over a century and for which historical data exist that allow comparison over recent decades to determine what, if any, changes in SOC content have occurred. The plots used are the Palace Leas (PL) Meadow Hay Plots in north‐east England (UK) established in 1897, and from the Park Grass (PG) Continuous Hay experiment established in 1856 at Rothamsted in south‐east England. Collectively, these plots represent the only grassland sites in the UK under long‐term management where changes in SOC over several decades can be assessed, and are probably unique in the world. The plots have received different manure and fertilizer treatment and have been under known management for at least 100 years. In 1982, total SOC contents were determined for the 0–27 cm layer of six of the PL plots using measurements of SOC concentrations, bulk density and soil depth. In 2006, the same six PL plots were resampled and SOC contents determined again. Four of the plots showed no net change in SOC content, but two plots showed net loss of SOC of 15% and 17% (amounting to decreases of 18 and 15 t C ha−1) since 1982. However, these differences in total SOC content were in a similar range to the variations in bulk density (6–31%) with changing soil water content. In 1959, the soil masses and SOC concentrations to 23 cm depth were measured on six PG plots with fertilizer and manure treatments corresponding closely with those measured on PL. In 2002, the SOC concentrations on the same plots were measured again. On three of the PG plots, SOC concentrations had declined by 2–10%, but in the other three it had increased by 4–8% between 1959 and 2002. If it is assumed that the soil bulk density had not changed over this period, the losses of SOC from the top soils ranged range from 10 to 3 t C ha−1, while the gains ranged from 4 to 7 t C ha−1. When the differences with time in SOC contents for the six PL and the six PG plots were examined using paired t‐tests, that is, regarding the plots as two sets of six replicate permanent grasslands, there were no significant differences between 1982 and 2006 for the PL plots or between 1959 and 2002 for the PG plots. Thus, these independent observations on similar plots at PL and PG indicate there has been no consistent decrease in SOC stocks in surface soils under old, permanent grassland in England in recent decades, even though meteorological records for both sites indicate significant warming of the soil and air between 1980 and 2000. Because the potential influences of changes in management or land use have been definitively excluded, and measured rather than derived bulk densities have been used to convert from SOC concentrations to SOC amounts, our observations question whether for permanent grassland in England, losses in SOC in recent decades reported elsewhere can be attributed to widespread environmental change.

Aboveground overyielding in grassland mixtures is associated with reduced biomass partitioning to belowground organs

We investigated effects of plant species richness in experimental grassland plots on annual above- and belowground biomass production estimated from repeated harvests and ingrowth cores, respectively. Aboveground and total biomass production increased with increasing plant species richness while belowground production remained constant. Root to shoot biomass production ratios (R/S) in mixtures were lower than expected from monoculture performance of the species present in the mixtures, showing that interactions among species led to reduced biomass partitioning to belowground organs. This change in partitioning to belowground organs was not confined to mixtures with legumes, but also measured in mixtures without legumes, and correlated with aboveground overyielding in mixtures. It is suggested that species-rich communities invest less in belowground biomass than do monocultures to extract soil resources, thus leading to increased investment into aboveground organs and overyielding.

Long‐term effects of plant diversity and composition on soil nematode communities in model grasslands

An important component of plant-soil feedbacks is how plant species identity anddiversity influence soil organism communities. We examine the effects of grassland plant species growing alone and together up to a richness of 12 species on nematode diversity and feeding group composition, eight years after the establishment of experimental grassland plots at the BIODEPTH site in northern Sweden. This is a substantially longer time than most other experimental studies of plant effects on soil fauna. We address the hypotheses that (la) higher species or functional diversity of plants increases nematode diversity, as well as influences nematode community composition. Alternatively, (1b) individual plant species traits are most important for nematode diversity and community composition. (2) Plant effects on soil organisms will decrease with increasing number of trophic links between plants and soil fauna. Plant species identity was often more important than plant diversity for nematode community composition, supporting hypothesis 1b. There was a weak positive relation between plant and nematode richness;which could be attributed to the presence of the legume Trifolium pratense, but also to some other plant species, suggesting a selection or sampling effect. Several plant species in different functional groups affected nematode community composition. For example, we found that legumes increased bacterial-feeding nematodes, most notably r-selected Rhabditida, while fungal-feeding nematodes were enhanced by forbs. Other bacterial feeders and obligate root feeders were positively related to grasses. Plant effects were usually stronger on plant-, bacterial- and fungal-feeding nematodes than on omnivores/predators, which supports hypothesis 2. Our study suggests that plant identity has stronger effects than plant diversity on nematode community composition, but when comparing our results with similar previous studies the effects of particular plant species appear to vary. We also found that more productive plant species affected bacterial-feeding nematodes more than fungal feeders. Moreover, we observed stronger effects the fewer the number of trophic links there were between a nematode feeding group and plants. Although we found clear effects of plants on soil nematodes, these were probably not large enough to result in strong and persistent plant-soil-organism-plant feedback loops.

Can complementarity in water use help to explain diversity–productivity relationships in experimental grassland plots?

Positive diversity-productivity relationships have repeatedly been found in experimental grassland plots, but mechanistic explanations are still under debate. We tested whether complementarity for the exploitation of the soil water resource helps to explain these relationships. In the dry summer of 2003, evapotranspiration (ET) was assessed at the Swedish BIODEPTH site using two different approaches: snapshot measurements of canopy surface temperature and simulation of time-accumulated ET by means of a soil water balance model. More diverse plots were characterized by lower surface temperatures and higher accumulated ET. Transgressive overyielding tests revealed that ET in polycultures was higher than in the best-performing monocultures, but this pattern was reversed at high degrees of water stress. Our results indicate that a more complete exploitation of soil water by more diverse grassland systems is on the one hand likely to be a driver for their increased biomass production, but on the other hand causes the more diverse communities to be affected earlier by drought. Nevertheless, the results also suggest that productivity may (at least partially) be maintained under dry conditions due to the higher likelihood of including drought-tolerant species in the more diverse communities.

Does biodiversity increase spatial stability in plant community biomass?

We tested the hypothesis that biodiversity decreases the spatial variability of biomass production between subplots taken within experimental grassland plots. Our findings supported this hypothesis if functional diversity (weighted Rao's Q) was considered. Further analyses revealed that diversity in rooting depth and clonal growth form were the most important components of functional diversity stabilizing productivity. Using species or functional group richness as diversity measures there was no significant effect on spatial variability of biomass production, demonstrating the importance of the biodiversity component considered. Moreover, we found a significant increase in spatial variability of productivity with decreasing size of harvested area, suggesting small-scale heterogeneity as an important driver. The ability of diverse communities to stabilize biomass production across spatial heterogeneity may be due to complementary use of spatial niches. Nevertheless, the positive effect of functional diversity on spatial stability appears to be less pronounced than previously reported effects on temporal stability.

MASSIVE AND DISTINCTIVE EFFECTS OF MEADOW VOLES ON GRASSLAND VEGETATION

We ask whether vole herbivory in experimental grassland plots is sufficient to create an unpalatable community. In a six-year experiment, meadow voles (Microtus pennsylvanicus) reduced plant standing crop between 30% and 72%, well within the range of ungulate effects. Moreover, meadow voles reduced their available forage species by changing the plant community composition: four grass species and a legume upon which they foraged declined sharply in cover and/or number of individuals, five forbs avoided by voles increased, and two forbs neither declined nor increased with either measure. Reductions of diversity occurred when voles first defoliated the plots in 2000 but disappeared as plant species avoided by voles replaced vulnerable plants. Within six years, meadow voles created plant communities dominated by species that they did not eat.

The effects of plant diversity and insect herbivory on performance of individual plant species in experimental grassland

SummaryThere is increasing evidence that components of biodiversity affect processes at the ecosystem level; yet, the effects of biodiversity on the performance of individual organisms or particular trophic interactions are largely unexplored.We transplanted 10 individuals ofRumex acetosainto 82 experimental grassland plots differing in plant species and functional group richness. Half of the plants received an insecticide treatment to manipulate insect herbivory.We measured the amount of herbivory, plant size, survival and reproductive parameters in 2003 and 2004.Insect herbivores removed on average 4.3% (2003) and 5.1% (2004) of leaf area in unsprayed plants. Spraying significantly reduced damage levels on average by approximately 50%. Herbivory significantly decreased plant weight, leaf size and number, and inflorescence length and size.Plant height and inflorescence size ofR. acetosasignificantly decreased with an increase in species diversity. Mortality was slightly higher in the species‐poor mixtures. Plant functional group diversity had little effect on plant performance. The presence of legumes generally increased, while the presence of grasses generally decreased, morphological parameters and fitness inR. acetosa.Overall, the presence of particular plant functional groups was more important than functional group or species richnessper se, and insect herbivores had additive effects of the same magnitude as the presence of particular plant functional groups.Insect herbivory and plant functional identity, rather than species richness, determine the performance of individual plant species in temperate grasslands.