Linking Tree and Shrub Biomass to Plant Diversity During Early-Stage Alpine Afforestation

Abstract

Afforestation has considerable potential to restore and maintain plant diversity, which is closely associated with ecosystem functions and services. However, there remain numerous uncertainties regarding alpine afforestation performance. Hence, it is necessary to determine the factors contributing to plant diversity during the early stages of afforestation in alpine regions. Our main objective of this study was to examine the effects of tree and shrub biomass on plant diversity, namely, species (SD) and phylogenetic (PD) diversity, during the early period of afforestation. We used a general linear mixed model (GLMM) to determine the associations between tree and shrub biomass and multiple indices (Shannon, Simpson's diversity, species richness, Pielou's evenness, Faith's phylogenetic diversity, and net relatedness) at two spatial scales (1 × 1 m² and 10 × 10 m²), based on 78 10 × 10 m² sites, each containing five 1 × 1 m² plots. On the basis of the GLMM results, we established that both tree and shrub biomass had significant effects on plant diversity at the site and plot scales, and found that the responses of SD and PD to tree and shrub biomass were non-linear. Overall, whereas at the site scale, SD (i.e., Pielou's evenness index) was highest at a median level of tree biomass, there were negative relationships between shrub biomass and the Shannon index, species richness, and Faith's PD at the plot scale. The effects of biomass on SD and PD were found to be dependent on different spatial scales (i.e., plot and site) and life form (i.e., tree or shrub), thereby providing further evidence regarding the efficacy of ecological restoration subsequent to alpine afforestation. The findings of this study will provide a novel empirical reference for optimal tree species selection and planning for planting practices in alpine regions.

Keywords: aboveground biomass; afforestation; alpine ecosystem; general linear mixed model; phylogenetic diversity; species diversity.

FIGURE 1

A map of the study sites within the Huangshuihe River Valley in Qinghai Province, China. In each study area, we investigated six sites. In each of these sites, with due north as a reference, plots 1, 2, 3, and 4 are set from right to left in four counterclockwise corners, and plot 5 is sited in the center of the investigation site. The color from greed to red indicates the increasing altitude (m) of study sites.

FIGURE 2

Intra‐class correlation coefficients of tree and shrub biomass, species diversity (SD), and phylogenetic diversity (PD) based on the 13 study sites. Boxes show the intra‐class correlation coefficients (white lines), and lowest and upper (borders).

FIGURE 3

The marginal and conditional coefficient of determination (R ²) of general linear mixed models of plant diversity, including species diversity (SD) (assessed using the Shannon index, Simpson's index, species richness, and Pielou's evenness) and phylogenetic diversity (PD) (assessed using Faith's PD and the net relatedness index [NRI]) with tree and shrub biomass and environmental conditions (PCs 1 and 2) at the site scale (a); The importance of tree and shrub biomass and environmental conditions to SD and PD (b). The blue lines represent conditional R ² and the red lines represent conditional R ² in panel (a). The color from blue to red represents increasing importance of tree and shrub biomass and environmental conditions (PCs 1 and 2) to SD and PD. PC1 explains the soil factors including soil organic carbon, nitrogen, sand, and silt contents, and PC2 explains the climatic factors including annual mean temperature and temperature seasonality and declining precipitation seasonality. Tbiomass and Sbiomass represent tree and shrub biomass respectively.

FIGURE 4

Associations between the biomasses of trees and shrubs biomass and species (SD) and phylogenetic (PD) diversities at the site and plot scales. Shaded areas over the dashed regression lines represent the 95% confidence interval of fitted index values. Site represents SD at the site scale and plot represents SD and PD at plot scale. (a) Association between tree biomass and Pielou's evenness, (b) Association between shrub biomass and PD, (c) Association between shrub biomass and species richness, and (d) Association between shrub biomass and Shannon index.