Data_Sheet_1_Abiotic Drivers of Seedling Bank Diversity in Subtropical Forests of Southern China.docx

Abiotic factors are important to shape plant community composition and diversity through processes described as environmental filtering. Most studies on plant diversity in forests focus on adult trees, while the abiotic drivers of forest seedling community characteristics are less understood. Here, we studied seedling banks’ composition, richness, diversity, and abundance, and investigated their relationships with microsite abiotic conditions along a wide elevational gradient. We sampled seedling communities in 312 1-m2 quadrats, distributed in 13 one-ha plots in four subtropical forests in south China, covering an elevation gradient of 1500 m, for 2 years. We measured light availability, slope, and 11 soil nutrients for each seedling quadrat. We used analysis of similarities and multivariate analysis of variance to compare the composition and abiotic drivers of the four forests’ seedling communities. We then used mixed models and structural equation modeling to test the direct and indirect effects of abiotic factors on seedling species richness, diversity, and abundance. The differences in seedling community composition among these forests were mostly explained by differences in elevations and soil nutrients. Seedling diversity as Shannon and Simpson diversity index decreased with increasing elevation and increased with increasing slope, but seedling abundance and species richness did not. Elevation had an indirect effect on Simpson’s diversity index through modulating the direct effects of soil properties. Our findings show that soil properties play a prominent role in favoring differentiation in species composition among the four forests we studied and provide additional evidence to decreasing species diversity with elevation. However, this was reflected in decreasing Shannon and Simpson indices rather than species richness, which is more commonly studied. Whether and to what extent future environmental changes in climate and soil acidification will alter future forest composition and diversity needs to be investigated.

非生物因素在塑造植物群落组成和多样性方面发挥着至关重要的作用，这一过程被称为环境筛选。大多数关于森林植物多样性的研究集中于成年树木，而对于影响森林幼苗群落特性的非生物驱动因素，理解尚显不足。本研究中，我们探讨了幼苗库的组成、丰富度、多样性和丰度，并调查了它们与广泛海拔梯度上的微生境非生物条件之间的关系。我们在南中国四个亚热带森林中的13个一公顷样地内，共312个1平方米样方中采样幼苗群落，覆盖了1500米的海拔梯度，为期两年。我们对每个幼苗样方进行了光照可用性、坡度和11种土壤养分的测量。利用相似性分析和多元方差分析来比较四个森林幼苗群落的组成和非生物驱动因素。随后，我们运用混合模型和结构方程模型来检验非生物因素对幼苗物种丰富度、多样性和丰度的直接和间接影响。这些森林中幼苗群落组成的差异主要可由海拔和土壤养分差异来解释。幼苗多样性（以香农指数和辛普森指数衡量）随海拔升高而降低，随坡度增加而升高，但幼苗丰度和物种丰富度并未呈现明显变化。海拔通过调节土壤特性的直接影响，对辛普森多样性指数产生间接影响。我们的研究结果表明，土壤特性在促进我们所研究四个森林物种组成分化方面发挥着显著作用，并为进一步证实随着海拔升高物种多样性下降提供了额外证据。然而，这一结果反映在香农和辛普森指数的降低上，而非物种丰富度的变化，后者是研究中的常见指标。未来气候变化和土壤酸化对森林组成和多样性可能产生的影响，以及影响的程度，尚需进一步研究。