Influence of grazing intensity on richness, phylogenetic and functional dimensions of highly diverse mountainous grasslands from Mazandaran, Iran

Abstract

Recently, more studies have focused on predicting plant response to livestock grazing by using plant functional traits as indicators to study species adaptation under grazing disturbance. This study examines how different types of plant diversity (taxonomic, phylogenetic, and functional diversity) and strategies (Competitive, Stress-tolerant, and Ruderal) respond to grazing intensity. The study was conducted in the Siah Bisheh Rangelands, located in the southeast of Mazandaran province, Iran. In the region, three adjacent sites were chosen for vegetation sampling. Each site had low and high grazing intensity (LG and HG) areas. For each site and grazing intensity, three 100 m transects were randomly placed. Along each transect, 5 plots (1 × 1 m²) were sampled at 20 m intervals, resulting in a total of 90 plots. The plant coverage was recorded for each plot. To assess the evolutionary relationship among the 76 and 66 species in LG and HG grazing intensities, respectively, a phylogenetic tree was generated. To estimate plant strategy and calculate the overall functional distance between species in this study, four quantitative traits were used: canopy height, leaf area, leaf dry matter content (LDMC), and specific leaf area (SLA). Additionally, three characteristics of CSR strategy classification (Competitive, Stress-tolerance, and Ruderal), were considered. To evaluate functional differences between the two groups with different grazing intensities, we computed functional indices for each sampling plot based on the relative cover of species. The communities subjected to two grazing intensities (LG and HG) were compared using the T-test with a significance level of 5%. The study found that higher grazing intensity caused a shift from the ruderal to the stress-tolerance strategy at both the species and community levels. Furthermore, increased grazing intensity led to a decrease in species richness and diversity, while increasing functional divergence, functional evenness, and community LDMC. Ultimately, stress-tolerant and ruderal strategies dominated under high and low grazing intensities, respectively. The study suggests that plant functional traits are essential in evaluating plant strategies. Analyzing traits like leaf size, SLA, and LDMC can help understand plant survival and growth in different ecosystems. These findings highlight the importance of considering grazing intensity when evaluating plant strategies and the need for further research on how different plant functional traits impact plant responses to grazing pressure. Understanding how various traits contribute to plant performance and ecosystem dynamics can inform decisions about conservation priorities and management strategies. Plant functional traits are a powerful tool for ecologists and conservationists.