Species co‐occurrence shapes spatial variability in plant diversity–biomass relationships in natural rangelands under different grazing intensities

Grazing can alter plant species interactions in natural rangelands, which in turn might influence the productivity of the ecosystem but we do not fully understand how spatial variability in plant diversity-biomass relationships are modulated by grazing intensity. Here, we hypothesized that plant species co-occurrence in rangelands is mainly driven by niche segregation due to grazing and heterogeneity in local resources, and that grazing, therefore, modulates diversity–biomass relationships. We tested our hypothesis across 35 rangeland sites in Iran, using a species co-occurrence index to assess plant spatial aggregation within each site. At each site, we measured aboveground biomass, plant diversity, topography, soil nutrients, and three levels of grazing intensity. High spatial segregation of plant communities (low species co-occurrence) was found at heavily grazed sites, whereas greater spatial aggregation (high species co-occurrence) was found on low and moderate grazed sites, showing varied associational patterns of species with grazing intensity. Soil nutrients increased with grazing intensity and spatial segregation of plant communities was greater at sites with high soil nutrient concentrations, indicating that grazing intensity influences the spatial heterogeneity of plant communities via nutrients deposited in urine and feces. Declining plant biomass with grazing intensity was related to a strong decline in graminoid species diversity, which suggests that the diversity-biomass relationship is influenced by selective grazing of palatable species. The relationships between species co-occurrence and biomass or plant diversity suggest non-random patterns in species co-occurrences with grazing intensity, which could be the result of competition driven by high livestock grazing intensity. We, therefore, suggest that rangeland stocking rates should be managed properly to maintain rangeland production while promoting plant diversity

Biocrust diversity enhances dryland saline soil multifunctionality

Biocrusts are multifaceted communities including mosses, lichens, and cyanobacteria that are crucial for sustaining soil functions in drylands. Most studies on biocrust functions to date have focused on biocrust cover and development, largely in non-saline soils, and we know very little about the importance of biocrust diversity for maintaining multifunctionality in saline dryland soils. We assessed the direct and indirect linkages between biocrust richness, soil texture and salinity and soil multifunctionality by measuring 13 variables characterizing soil biological, nutrient and hydrological functions across 32 plots in a salinized dryland in northeastern Iran. We assessed the species richness of biocrust patches and characterized soil functions in bare soils. Overall, biocrust species richness declined with soil clay content and soil salinity, whereas soil salinity increased with soil clay content. Structural equation modelling showed a strong positive association between biocrust species richness and all measured dryland soil functions (soil biological, nutrient and hydrological functions), but soil hydrological function declined with soil salinity. Overall, dryland soil multifunctionality was positively associated with biocrust species richness but negatively associated with soil clay content. Biocrust species richness likely enhances soil multifunctionality via the distinct roles of species and biocrust functional groups in providing carbon and nutrient inputs, creating favorable microsites, enhancing infiltration, and facilitating soil microbial colonization in saline dryland soils. Overall, our findings highlight a key role for biocrust diversity in facilitating and maintaining soil multifunctionality in drylands affected by soil salinity

Soil stoichiometry mediates links between tree functional diversity and soil microbial diversity in a temperate forest

Interactions between plants and soil microbial communities underpin soil processes and forest ecosystem function, but the links between tree diversity and soil microbial diversity are poorly characterized. Differences in both the taxonomic and functional diversity of trees and microbes can shape soil nutrient status and carbon storage, but the stoichiometry of carbon and nutrients in the soil also influences resource availability to plant and microbial communities. Given the key role of resource availability in plant–soil interactions, we hypothesized that relationships between tree diversity metrics and soil bacterial or fungal diversity are mediated by soil stoichiometry. To test our hypothesis, we measured tree diversity metrics (tree species richness, functional trait diversity and functional trait composition) and soil stoichiometry in a temperate forest in China, and we determined soil microbial diversity by Illumina sequencing. We used structural equation models to assess the relationships between tree diversity metrics and soil bacterial or fungal diversity and to evaluate the influence of soil stoichiometry. Overall, microbial diversity was strongly related to soil stoichiometry, whereby fungal diversity was associated with high soil N/P ratios, whereas bacterial diversity was related to high soil C/P ratios. Soil bacterial and fungal diversity were more closely related to tree functional trait diversity and composition than to tree species richness, and the links between tree and soil microbial diversity were mediated by soil stoichiometry. The strong links between tree functional traits, soil stoichiometry and soil bacteria or fungi suggest that resource quality plays a key role in plant–microbial interactions. Our results highlight the importance of nutrient stoichiometry in linkages between tree functional diversity and soil microbial diversity

Differential responses of forest strata species richness to paleoclimate and forest structure

Regional factors, such as historical and contemporary climate conditions, and local factors, such as vegetation structural attributes, can influence current patterns of plant species richness but their relative roles remain unknown, particularly across forest strata. Here, we used a multi-scale survey of temperate forest plots across a large region of Northeast China to explore the relative importance of environmental factors (paleoclimate, contemporary climate, topography and anthropogenic disturbance) and forest structural attributes (stem abundance, stand basal area and tree size variation) on tree, shrub, and herb species richness. Although environmental and forest structural factors all played a role in explaining plant species richness patterns, their relative roles varied in direction and magnitude depending on forest stratum. Tree species richness increased with the magnitude of change in temperature since the Last Glacial Maximum (AnomalyMAT) but declined with increasing magnitude of change in precipitation (AnomalyMAP). By contrast, herb species richness declined with increasing AnomalyMAT but increased with AnomalyMAP, highlighting contrasting processes for tree and herb species richness driven by paleoclimate. Contemporary climate played a lesser role in explaining species richness, but tree species richness increased with diurnal temperature range, shrub species richness increased with the climatic moisture index, and herb species richness increased with both climatic variables. Herb species richness also increased with disturbance intensity, whereas tree and shrub richness declined. Overall, plant species richness increased significantly with all forest structural attributes, except for stem abundance, which had a negative effect on herb species richness, and forest structure mediated the linkages between plant species richness and disturbance or climate. The pronounced influence of paleoclimate on forest plant species richness highlights the potential threat of current climate change for forest diversity. Together, simultaneous consideration of past and current climate as well as forest structural attributes could improve our understanding of the complex mechanisms shaping patterns of plant species richness across forest strata

Tree species diversity enhances plant-soil interactions in a temperate forest in northeast China

International audienceThe plant-soil interactions may drive the diversity and functioning of forests, but we do not fully understand how interrelationships between plant and soil compartments are underlined by multiple ecological mechanisms. Here, we hypothesize that positive plant-soil interactions enhance biodiversity and functioning in a temperate forest. To do so, we tested the relationships between plant diversity (i.e., tree and herb species richness) and functions (i.e., coarse woody productivity and litterfall productivity), and soil diversity (i.e. bacterial, fungal and nematode) and functions (i.e. soil nutrient and carbon stock), and their interrelationships in a temperate forest in northeast China. The positive relationship between diversity and functioning was predominant within plant and soil compartments, and hence, provide support to the niche complementarity effect. As such, the positive interrelationships between the diversity of soil and plant compartments provide support to the positive plant-soil interactions. Tree species diversity was positively related with herb species diversity and coarse-woody productivity. Importantly, tree species diversity had pronounced positive effect on soil biodiversity resulting in increased soil carbon stocks, indicating that tree species diversity effect matters for linking positive interrelationships between plant and soil compartments of a temperate forest. This study shows that tree diversity effect is the main regulating biotic mechanism for linking the positive connections between plant and soil compartments of a temperate forest, and hence, the niche complementarity effect can enhance forest functioning through positive interactions on resource supply. We argue that linking the multiple key functions and diversity indices of forests can enhance our knowledge on the main influential factors and underlying ecological mechanisms

Divergent above‐ and below‐ground biodiversity pathways mediate disturbance impacts on temperate forest multifunctionality

International audienceBiodiversity plays a fundamental role in provisioning and regulating forest ecosystem functions and services. Above-ground (plants) and below-ground (soil microbes) biodiversity could have asynchronous change paces to human-driven land-use impacts. Yet, we know very little how they affect the provision of multiple forest functions related to carbon accumulation, water retention capacity and nutrient cycling simultaneously (i.e. ecosystem multifunctionality; EMF). We used a dataset of 22,000 temperate forest trees from 260 plots within 11 permanent forest sites in Northeastern China, which are recovering from three post-logging disturbances. We assessed the direct and mediating effects of multiple attributes of plant biodiversity (taxonomic, phylogenetic, functional and stand structure) and soil biodiversity (bacteria and fungi) on EMF under the three disturbance levels. We found the highest EMF in highly disturbed rather than undisturbed mature forests. Plant taxonomic, phylogenetic, functional and stand structural diversity had both positive and negative effects on EMF, depending on how the EMF index was quantified, whereas soil microbial diversity exhibited a consistent positive impact. Biodiversity indices explained on average 45% (26%–58%) of the variation in EMF, whereas climate and disturbance together explained on average 7% (0.4%–15%). Our result highlighted that the tremendous effect of biodiversity on EMF, largely overpassing those of both climate and disturbance. While above- (β = 0.02–0.19) and below-ground (β = 0.16–0.26) biodiversity had direct positive effects on EMF, their opposite mediating effects (β = −0.22 vs. β = 0.35 respectively) played as divergent pathways to human disturbance impacts on EMF. Our study sheds light on the need for integrative frameworks simultaneously considering above- and below-ground attributes to grasp the global picture of biodiversity effects on ecosystem functioning and services. Suitable management interventions could maintain both plant and soil microbial biodiversity, and thus guarantee a long-term functioning and provisioning of ecosystem services in an increasing disturbance frequency world

Grazing intensity alters the plant diversity‐ecosystem carbon storage relationship in rangelands across topographic and climatic gradients

1. Plant diversity supports multiple ecosystem functions, including carbon sequestration. Recent shifts in plant diversity in rangelands due to increased grazing pressure and climate changes have the potential to impact the sequestration of carbon in arid to semi-humid regions worldwide. However, plant diversity, grazing intensity and carbon storage are also influenced by environmental factors such as nutrient availability, climate, and topography. The complexity of these interactions limits our ability to fully assess the impacts of grazing on biodiversity-ecosystem function (BEF) relationships. Read the free Plain Language Summary for this article on the Journal blog. 2. We assessed how grazing intensity modifies BEF relationships by determining the links between plant diversity and ecosystem carbon stocks (plant and soil carbon) across broad environmental gradients and different plant growth forms. To achieve this, we surveyed 1493 quadrats across 10 rangelands, covering an area of 23,756 ha in northern Iran. 3. We show that aboveground carbon stocks increased with plant diversity across topographic, climatic and soil fertility gradients. The relationship between aboveground carbon stocks and plant diversity was strongest for forbs, followed by shrubs and grasses. Soil carbon stocks increased strongly with soil fertility across sites, but aridity, grazing, plant diversity and topography were also important in explaining variation in soil carbon stocks. 4. Importantly, aboveground and soil carbon stocks declined at high grazing intensity, and grazing modified the relationship between plant diversity and carbon stocks regardless of differences in abiotic conditions across sites. 4. Our study demonstrates that relationships between plant diversity and ecosystem carbon stocks persist across gradients of aridity, topography, and soil fertility, but the relationships are modified by grazing intensity. Our findings suggest that potential losses in plant diversity under grazing intensification could reduce ecosystem carbon storage across wide areas of arid to semi-humid rangelands. We discuss the potential mechanisms underpinning rangeland BEF relationships to stimulate future research