Effect of altitude and soil properties on biomass and plant richness in the grasslands of Tibet, China, and Manang District, Nepal

A positive relationship between plant species richness and biomass has often documented in temperate grassland systems but is still largely lacking in the Himalayan region. Considering altitude and soil properties as major factors influencing species richness and biomass in the Himalayan grasslands, we tested the hypothesis that peak aboveground biomass and species richness increase with higher levels of soil nutrients and decrease with altitude. Our study was conducted in the grasslands of Tibet, China, and Manang District, Nepal, which represent, respectively, the northern and southern slopes of the central Himalayan grasslands. During the growing seasons, we measured vegetation aboveground biomass and soil properties along an altitudinal gradient on the northern slope in 2011 and 2012, and on the grasslands of the southern slope in 2017. Data were analyzed with a Spearman correlation analysis, classification and regression tree model, and the structural equation modeling (SEM) to identify the key factors determining aboveground biomass and species richness. The results demonstrated that aboveground biomass and species richness on the southern slope were significantly greater at the lowest altitude zone compared to middle and higher zones, whereas on the northern slope, aboveground biomass and species richness were not significantly different at three altitudinal zones. There was a significant positive correlation between aboveground biomass and species richness on both northern and southern slopes. However, the slope was found higher in grasslands of southern slope compared to the grasslands of northern slope. Classification and regression tree analysis and SEM indicated that altitude is most closely associated with large-scale variations in aboveground biomass and species richness. The Spearman correlation analysis revealed that altitude and soil pH were negatively related to aboveground biomass in these grasslands. This study demonstrates that on northern and southern slopes of the Himalayan Range, a positive relation between plant species richness and biomass is determined more by altitude than soil nutrients.