Grazing effects on plant functional group diversity in Mediterranean shrublands

The responses of three decomposer groups (earthworms, springtails and microorganisms) to manipulations in plant species diversity (1, 2, 4, 8), plant functional group diversity (1, 2, 3, 4) and functional group identity (grasses, legumes, small herbs, tall herbs) were studied in a microcosm experiment. Separate and combined treatments with earthworms and springtails were set up. Two earthworm species representing major functional groups of earthworms in grasslands were investigated, the endogeic species Aporrectodea caliginosa (Savigny) and the anecic species Lumbricus terrestris L. For springtails three species were investigated, the hemiedaphic species Heteromurus nitidus (Leleup) , Folsomia candida (Willem) and the euedaphic species Protaphorura fimata (Gisin). Plant species and functional group diversity beneficially affected A. caliginosa (increase in body weight and incorporation of 15 N from labelled litter) and P. fimata (density), presumably by changing the quality of belowground resources. In contrast, the biomass of L. terrestris decreased with plant species diversity but only in presence of legumes. For H. nitidus and F. candida the identity of plant functional groups was more important than plant species diversity per se. Also, the response of F. candida depended on earthworms. Microbial respiration was reduced by earthworms in more diverse plant communities, which correlated with root biomass. In contrast, microbial biomass was not affected by plant species diversity. The results suggest that belowground resource inputs from plant roots strongly modify decomposer performance and that the quality of the resources that enter the belowground subsystem is more important than their quantity. The responses of decomposers generally were not correlated with below‐ or aboveground plant productivity. In addition, the results document that effects of plant community composition on the performance of decomposer species depend on the presence of other decomposers.

The role of species and functional group diversity of primary producers for decomposers and decomposition processes is little understood. We made use of the "Jena Biodiversity Experiment" and tested the hypothesis that increasing plant species (1, 4, and 16 species) and functional group diversity (1, 2, 3, and 4 groups) beneficially affects decomposer density and activity and therefore the decomposition of plant litter material. Furthermore, by manipulating the densities of decomposers (earthworms and springtails) within the plant diversity gradient we investigated how the interactions between plant diversity and decomposer densities affect the decomposition of litter belonging to different plant functional groups (grasses, herbs, and legumes). Positive effects of increasing plant species or functional group diversity on earthworms (biomass and density) and microbial biomass were mainly due to the increased incidence of legumes with increasing diversity. Neither plant species diversity nor functional group diversity affected litter decomposition, However, litter decomposition varied with decomposer and plant functional group identity (of both living plants and plant litter). While springtail removal generally had little effect on decomposition, increased earthworm density accelerated the decomposition of nitrogen-rich legume litter, and this was more pronounced at higher plant diversity. The results suggest that earthworms (Lumbricus terrestris L.) and legumes function as keystone organisms for grassland decomposition processes and presumably contribute to the recorded increase in primary productivity with increasing plant diversity.

Plant diversity is known to influence the abundance and diversity of belowground biota; however, patterns are not well predictable and there is still much unknown about the driving mechanisms. We analyzed changes in soil nematode community composition as affected by long‐term manipulations of plant species and functional group diversity in a field experiment with plant species diversity controlled by sowing a range of 1–60 species mixtures and controlling non‐sown species by hand weeding. Nematode communities contain a variety of species feeding on bacteria, fungi, plants, invertebrates, while some are omnivorous. We analyzed responses of nematode abundance and diversity to plant species and functional diversity, and used structural equation modeling (SEM) to explore the possible mechanisms underlying the observed patterns. The abundance of individuals of all nematode feeding types, except for predatory nematodes, increased with both plant species and plant functional group diversity. The abundance of microbial‐feeding nematodes was related positively to aboveground plant community biomass, whereas abundance of plant‐feeding nematodes was related positively to shoot C:N ratio. The abundance of predatory nematodes, in turn, was positively related to numbers of plant‐feeding nematodes, but not to the abundance of microbial feeders. Interestingly, the numbers of plant‐feeding nematodes per unit root mass were lowest in the high‐diversity plant communities, pointing at reduced exposure to belowground herbivores when plants grow in species‐diverse communities. Taxon richness of plant‐feeding and microbial‐feeding nematodes increased with plant species and plant functional group diversity. Increasing plant functional group diversity also enhanced taxon richness of predatory nematodes. The SEM suggests that bottom‐up control effects of plant species and plant functional group diversity on abundance of nematodes in the various feeding types predominantly involve mechanistic linkages related to plant quality instead of plant quantity; especially, C:N ratios of the shoot tissues, and/or effects of plants on the soil habitat, rather than shoot quantity explained nematode abundance. Although aboveground plant properties may only partly serve as a proxy for belowground resource quality and quantity, our results encourage further studies on nematode responses to variations in plant species and plant functional diversity in relation to both quantity and quality of the belowground resources.

SummaryThe effects of the anecic earthwormLumbricus terrestrisL. on plant seedling recruitment and spatial aggregation were investigated in a microcosm glasshouse experiment by varying plant seed size (small and large); functional groups (grasses, legumes, herbs); plant species diversity (1, 3, 6); and plant functional group diversity (1, 3).Generally, earthworms buried seeds quickly irrespective of seed size and species. Secondary seed dispersal (phase II dispersal) by earthworms affected plant community composition depending mainly on seed size but less on plant functional group identity and diversity: small‐seeded species were repressed whereas large‐seeded were promoted.Although, in general, recruitment of seedlings was less in the presence ofL. terrestris, recruited seedlings benefited from establishing in the vicinity of earthworm burrows. The strong aggregation of plants in the vicinity of earthworm burrows resulted in plant communities with a more heterogeneous small‐scale architecture. Earthworm burrows and middens acted as an important regeneration niche for emergent seedlings by reducing microsite and nutrient limitations.In conclusion, seed dispersal, seed burial, seedling recruitment, and the spatial distribution of seedlings of plant species of different functional groups and with a wide range of seed size are strongly affected byL. terrestris, and this probably affects plant community composition.

The response of species numbers and density of Collembola to manipulation of plant species richness (1, 2, 4, 8, 32 species) and number of plant functional groups (grasses, legumes and non‐legume herbs) was studied in an experimental grassland at the Swiss BIODEPTH site (Lupsingen, Switzerland) in October 1997. Plant species richness or number of plant functional groups did not affect total diversity of Collembola, however, the number of Collembola species increased in the presence of legumes and the grass Trisetum flavescens . The abundance of Protaphorura armata increased but that of Hypogastruridae/Neanuridae significantly decreased with increasing number of plant functional groups. Other groups including the herbivorous Symphypleona did not respond to plant species richness and plant functional groups. Possibly, Hypogastruridae/Neanuridae species are weak competitors declining in density if the density of other Collembola groups increase. In general, the effect of the number of plant functional groups on the densities of collembolan taxa was stronger than that of plant species richness. Changes in Collembola density and diversity in part was likely caused by increased soil microbial and fine root biomass in treatments with higher plant functional group diversity. The presence of legumes resulted in an increase in the densities of total Collembola, Symphypleona/Neelipleona and Isotomidae indicating that they benefited from the high litter quality and the increased microbial biomass in the rhizosphere of legumes. The results suggest that microbivorous soil invertebrates are controlled by food quality rather than quantity. Furthermore, they indicate that presence of certain plant species and functional groups may be more important for collembolan community structure than the diversity of plant species and functional groups per se.

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.

Interrill erosion at disturbed alpine sites: Effects of plant functional diversity and vegetation cover

High yields are a priority in managing biomass for renewable energy, but the environmental impacts of various feedstocks and production systems should be equally considered. Mixed‐species, perennial grasslands enrolled in conservation programs are being considered as a source of biomass for renewable energy. Conservation grasslands are crucial in sustaining native biodiversity throughout the US Upper Midwest, and the effects of biomass harvest on biodiversity are largely unknown. We measured the effect of late‐season biomass harvest on plant community composition in conservation grasslands in three regions of Minnesota, USA from 2009 to 2012. Temporal trends in plant species composition within harvested grasslands were compared to unharvested grasslands using mixed effects models. A before‐after control‐impact approach using effect sizes was applied to focus on pre‐ and postharvest conditions. Production‐scale biomass harvest did not affect plant species richness, species or functional group diversity, nor change the relative abundance of the main plant functional groups. Differences in the relative abundances of plant functional groups were observed across locations; and at some locations, changed through time. The proportion of non‐native species remained constant, while the proportion of noxious weeds decreased through time in both harvested and unharvested grasslands at the central location. Ordination revealed patterns in species composition due to location, but not due to harvest treatment. Therefore, habitat and bioenergy characteristics related to grassland plant communities are not expected to change due to short‐term or intermittent late‐season biomass harvest.

The impacts of three kinds of human disturbances, i.e., non-grazing, reseeding and free-grazing, on plant community characters, i.e., species composition, composition of functional groups, species diversity, and aboveground biomass, were studied in a typical steppe in Inner Mongolia, China. The results showed that different disturbances had significant impacts on the structure and diversity of plant functional groups. The number of plant species, aboveground biomass, community diversity, and richness indices were the highest in the non-grazing treatment, with the fi-gures of 22, 171.32 g·m-2, 1.46 and 5.7, respectively. The importance value (IV), percentage, aboveground biomass, diversity, richness, and evenness indices of shrub and sub-shrub, and perennial grasses were also the highest in the non-grazing treatment. In contrast, the IV, species percentage, aboveground biomass, diversity, and evenness indices of perennial forbs were the lowest in the non-grazing treatment. The aboveground biomass of mesophyte, C3 and C4 plants was the highe-st with the figures of 22.22, 143.35 and 27.97 g·m-2 respectively in the non-grazing treatment. The aboveground biomass of mesoxerophyte was highest (13.60 g·m-2), the species percentage of xerophyte was lowest (48.5%) and that of C4 plants was highest (28.8%) in the reseeding treatment. The species percentage of annual or biennial herbs was highest (12.3%), while that of mesophyte and C4 plants was lowest (17.0% and 20.9%) in the free-grazing treatment. The non-grazing treatment showed a beneficial effect for restoration of degraded grassland system.

Plot-Measured Variables Indicate Landscape-Scale Patterns of Annual Grass Invasion in Northwestern US Rangelands

As an indicator and regulator of climate and environmental change, the Tibetan Plateau is an important barrier for ecological security. However, despite the importance of soil microbial communities in almost all soil biochemical processes and ecosystem functions, the biogeography of soil microbial communities on the Tibetan Plateau is poorly understood, especially at large scales over different ecosystem types. In this study, we collected samples from 64 sampling sites representing different grassland ecosystem types and spanning 2121 km across on the Tibetan Plateau. We then used next generation high-throughput sequencing to investigate the soil prokaryote community (i.e. bacteria and archaea) diversity and spatial patterns and to explore their relationship with biotic (e.g. plant functional group diversity and biomass) and abiotic (e.g. aridity index, soil carbon and nitrogen levels) factors. Among the four alpine grassland types (i.e. alpine meadow, alpine steppe, alpine shrub and alpine desert) sampled in this study, alpine meadow had the highest soil microbial biomass and alpine desert had the lowest soil microbial richness and Shannon diversity. The soil microbial diversity in the alpine grassland correlated with plant diversity and climate factors. Soil microbial diversity negatively correlated with the annual average air temperature, but was not correlated with the annual average precipitation, indicating that temperature, rather than precipitation, may be more important in controlling the soil microbial diversity in alpine grassland ecosystems at cold temperatures. Higher air temperature likely led to an intensified aridity under limited precipitation, and thus decreased microbial diversity. As a result, the aridity index combined with temperature and precipitation explained more of the variance in the soil microbial diversity than air temperature or precipitation did individually. Moreover, after separating plant species into four functional groups (grass, forb, legume and sedge), microbial diversity positively correlated with plant functional group diversity, explaining more of the variance in microbial diversity than plant species diversity did. Results of structural equation modeling revealed that the aridity index and annual air temperature affected soil microbial diversity, directly or indirectly, through influencing plant functional group diversity and aboveground biomass; while aboveground biomass changed the soil carbon to nitrogen ratio in the upper soil layers and thus impacted soil microbial diversity. However, in contrast to microbial diversity, soil microbial biomass carbon was not correlated with plant functional group diversity, plant species diversity, or the climate factors annual average air temperature, annual precipitation and aridity index, but were linked to soil nutrient status (e.g. soil dissolved organic carbon, ammonia, available phosphorus, and carbon to nitrogen ratio) and plant biomass of sedges and forbs, demonstrating that microbial biomass and diversity were likely controlled by different factors. In summary, this study investigated the spatial patterns of soil microbial communities across different alpine grassland ecosystem types on the Tibetan Plateau and enhanced our understanding of biotic and abiotic factors controlling microbial biomass and diversity, which will be important in predicting microbial changes on the Tibetan Plateau under future climate change. Under future warming and wetting scenarios on the Tibetan Plateau, it is possible that the aridity index would decrease, leading to increased soil microbial diversity. Results of this study also suggest a focus on the aridity index and plant functional group diversity in future microbial biogeography studies in order to further determine their roles in controlling or mediating soil microbial biomass and diversity.

Prescribed fire is an active management tool used to address wildfire hazard and ecological concerns associated with fire exclusion and suppression over the past century. Despite widespread application in the United States, there is considerable inconsistency and lack of information regarding the extent to which specific outcomes are achieved and under what prescribed fire regimes, particularly in regard to ecological goals related to plant community structure. We quantify differences and patterns in plant functional group abundance, species richness and diversity, and other key forest components through time from a unique long‐term (15‐yr) experiment within the Malheur National Forest of Oregon. Treatments included five fire regimes: fall and spring 5‐yr burning, fall and spring single burn, and no burning. Original burns were conducted in the fall of 1997 and spring of 1998, and plant data were collected every five years starting in 2002. Many perennial plant group responses were neutral, subtle, and ephemeral. Total cover increased marginally in response to the first burn, but this response disappeared within a decade. Three 5‐yr reburns did not increase or decrease total plant cover, richness, or diversity. Some plant groups with fire resistant and resilient traits, such as annual forbs, exotic forbs, open bunchgrasses, and rhizomatous and resprouting perennials, showed some short‐term positive responses to initial burning and 1–2 reburns, but not 3. The moderately more severe fall burns generally impacted plant responses more than spring burning, but many patterns were legacies from the original higher severity burns. Burn frequency was more important for less severe spring burning. Overall, we document that most native perennial plant functional groups were able to resist or recover from burning and reburning, but did not necessarily strongly respond. This may be due to the importance of other overriding forcing factors and ecosystem inertia not easily overcome by very low‐severity prescribed burns. Results from our study are important for practitioners using prescribed fire to achieve biodiversity, conservation, and habitat goals that hinge on a vigorous native perennial plant response, as such outcomes are not certain even with frequent burn regimes.

Summary 1. The benefits of plant functional group and plant species diversity for sustaining primary productivity have been extensively studied. However, few studies have simultaneously explored potential benefits of plant species and functional group richness and composition for the delivery of other ecosystem services and their dependency on resource availability. 2. Here, we investigated in soils of different fertility the effects of plant species and functional group richness and composition on carbon (C) and nitrogen (N) stocks in vegetation, soil and soil microbes and on CO2 exchange and the loss of C and N from soil through leaching. We established plant communities from a pool of six mesotrophic grassland species belonging to one of three functional groups (C3 grasses, forbs and legumes) in two soils of contrasting fertility. We varied species richness using one, two, three or six species and one, two or three functional groups. 3. After 2 years, vegetation C and N and soil microbial biomass were greater in the more fertile soil and increased significantly with greater numbers of plant species and functional group richness. The positive effect of plant diversity on vegetation C and N coincided with reduced loss of water and N through leaching, which was especially governed by forbs, and increased rates of net ecosystem CO2 exchange. 4. Soil C and N pools were not affected by the number of plant species or functional group richness per se after 2 years, but were enhanced by the presence and biomass of the legumes Lotus corniculatus and Trifolium repens. 5. Synthesis. Collectively, our findings indicate that changes in plant species and functional group richness influence the storage and loss of both C and N in model grassland communities but that these responses are related to the presence and biomass of certain plant species, notably N fixers and forbs. Our results therefore suggest that the co‐occurrence of species from specific functional groups is crucial for the maintenance of multifunctionality with respect to C and N storage in grasslands.

No consistent effects of plant diversity on root biomass, soil biota and soil abiotic conditions in temperate grassland communities

Plant diversity in wild and agricultural plant communities has been shown to protect plant communities from pathogens, and the effects of many aspects of diversity (genetic diversity, species richness, evenness and their combination) on plant pathogen epidemiology have been examined. Diversity among plant functional groups (e.g. grasses, legumes and non‐leguminous forbs) is known to reduce plant diseases in natural and agricultural plant communities. However, potential interactions among plant functional groups and their effects on plant virus infection dynamics have not been examined. Insect‐vectored plant virus infection dynamics may differ among distinct plant host functional types (C 3 vs C 4 ) due to differences in their palatability to aphid vectors and plant physiological responses to environmental factors. Here, we evaluated the role of plant functional group diversity (richness and composition), and host functional type (three C 3 and three C 4 species) on plant virus infection in experimental annual plant communities that were inoculated with aphids carrying barley yellow dwarf virus (BYDV), a widespread and economically important generalist grass virus. We also assessed the relationships between community‐level virus prevalence and the productivity of the total community (grasses + forbs + legumes), as well as the component plant functional types. In this study, virus infection was not mitigated or amplified by increasing functional group richness or altered functional group composition, nor did virus infection vary between C 3 and C 4 grasses. However, we found a positive correlation between community‐level virus prevalence and C 3 grass production (but not C 4 production). Synthesis . Our findings indicated that plant virus infection in grasses was not diluted by plant functional group diversity. However, we found a positive relationship between community‐level virus prevalence and C 3 grass productivity. Together, these findings suggest that while plant diversity did not reduce the infection levels of a cryptic plant virus, mutualistic plant–virus interactions may contribute to enhanced ecosystem functioning via increased productivity.