Interaction between light availability and grazing enhances species richness and turnover of vascular plants in shrubby pastures in Romania

Abstract

Shrubs in pastures are expected to adversely affect plant species diversity and community composition at various spatial scales. We recorded vegetation composition as well as light conditions and other environmental variables in 105 1 m2 plots nested in 35 sampling sites in shrubby pastures in Romanian Banat. We used generalized linear models, generalized linear mixed-effects models, and linear regression to find determinants of species turnover and richness at patch and community level. We expected shrubs to negatively influence species richness on the patch level but to introduce spatial heterogeneity to the light availability resulting in a unimodal relationship of light availability with species richness on the community level. Species turnover was significantly influenced by light conditions, grazing variability, and distance between the plots. We found a unimodal relationship between light availability and species richness consistently across the two spatial scales in grazed sites. However, the same relationship changed its shape from linear to unimodal in non-grazed sites. Shrubs in pastures may not be considered as a threat to plant species diversity. Instead, they contribute significantly to plant species diversity and vegetation heterogeneity.