The Complex Biodiversity-Ecosystem Function Relationships for the Qinghai-Tibetan Grassland Community

Abstract

Despite the long history of the study of the biodiversity-ecosystem function relationship, uncertainty remains about the relationship of natural grassland ecosystems under stressful conditions. Recently, trait- and phylogenetic-based tests provide a powerful way to detect the relationship in different spaces but have seldom been applied to stressful zones on a large spatial scale. We selected Qinghai-Tibetan as the study area and collected a grassland community database involving 581 communities. We calculated biomass and species', functional, and phylogenetic diversity of each community and examined their relationships by using linear and non-linear regression models. Results showed an overall positive biodiversity-productivity relationship in species', functional and phylogenetic space. The relationship, however, was non-linear, in which biodiversity explained better the variation in community biomass when species diversity was more than a threshold, showing a weak effect of biodiversity on ecosystem function in low species diversity communities. We also found a filled triangle for the limit of the relationship between species and functional diversity, implying that functional diversity differs significantly among communities when their species diversity is low but finally converges to be a constant with increasing communities' species diversity. Our study suggests that multiple niche processes may structure the grassland communities, and their forces tend to balance in high-biodiversity communities.

Keywords: Tibetan grassland; biodiversity; community assembly; ecosystem function; functional diversity; phylogenetic diversity; seed mass; specific leaf area.

FIGURE 1

The geographic variation in aboveground biomass (A) and species richness (B), and the distribution of the 118 study sites. The map showed the outline of the Qinghai-Tibetan Plateau, which was edited and generated with ArcGIS 10.2 software, http://www.esri.com/. Coordinate system: E, east longitude; N, north latitude.

FIGURE 2

Relationships between aboveground biomass production (log-scale, y-axis, g/m²) and species richness (A), species/m²) and Shannon-Weaver index (B). Lines showed the best models fitting the relationship, and their significance and explanatory power were shown in Table 1.

FIGURE 3

Relationships between aboveground biomass production (log-scale, y-axis) and the standardized effect size (SES) of Rao’s quadratic diversity (FD_Q) for leaf size (A), specific leaf area (SLA, B), plant height (C), seed mass (D), multiple traits (E), and phylogenetic diversity (MPD, F). Lines showed the best models fitting the relationship (the segmented line in piecewise regression was shown as dotted lines when its slope was non-significantly different from zero), and their significance and explanatory power were shown in Table 1.

FIGURE 4

SES of FD_Q for leaf size (A), specific leaf area (B), plant height (C), seed mass (D), multiple traits (E), and phylogenetic diversity (F) in relation to the species richness (SR, log-scale, species/m², x-axis) at the upper (95th), median (50th), and lower (5th) quantile levels. Positive (or negative) SES value indicated greater (or lower) functional and phylogenetic diversity than null. Significant and non-significant linear relationships (at α = 0.05) were shown as solid and dashed lines, respectively.