Disentangling the effects of environmental and communities' factors on species' biomass inequality in Qinghai-Tibetan grassland systems

Plant biomass (size) inequality among and within species is central to maintenance of species diversity. Understanding its role in community dynamic and variation with environments in stressful alpine systems where plant communities are structured by a number of different interactions remains a critical challenge for ecologists. Using data from 531 Qinghai-Tibetan grassland communities, we analyzed community-scale plant biomass inequality both in individual(i.e. inter-specific difference in mean individual biomass) and species(i.e. mean difference in species' overall biomass) dimension, and used regression, two-level general linear models (GLM), and structural equation models (SEMs) to evaluate their association with communities' species richness, productivity, abundance and environmental factors. We found that individual-dimension biomass inequality increased with increasing air temperature but decreasing soil water content (SWC), while species-dimension inequality increased with increasing SWC, resulting in a higher individual-dimension biomass inequality than species-dimension one toward warm and arid environments but lower toward cold and (or) wet conditions. The effects of community factors on plant biomass inequality operated in different dimensions, in which individualdimension inequality decreased with abundance but species-dimension inequality increased with species richness and especial above-ground biomass. The SEMs showed that individual-dimension biomass inequality decreased mainly with abundance indirectly via decreasing temperature but increasing precipitation as well as with SWC directly; while species-dimension biomass inequality increased mainly with productivity indirectly via increasing SWC and secondarily with species diversity indirectly via increasing precipitation. Overall, our results imply that both 'environment constraint' and 'resource competition' mechanism operate simultaneously driving plant biomass difference in the Qinghai-Tibetan grassland systems. We also evidence the importance of community biomass and abundance in regulating species' and individual biomass variation, respectively. Furthermore, our results demonstrate that a reduction in interspecific individual biomass difference acts as a common mechanism in maintaining species coexistence for grassland community under stressful environments.

Automated summary

The study analyzed plant biomass inequality in Qinghai-Tibetan grassland communities and found that the individual-dimension biomass inequality increased with air temperature but decreased with soil water content, while species-dimension inequality increased with higher soil water content. Community factors such as abundance, species richness, and above-ground biomass also influenced plant biomass inequality in different dimensions. This study suggests that both 'environment constraint' and 'resource competition' mechanisms operate simultaneously in driving plant biomass difference in alpine grassland systems.