Data_Sheet_1_Elevation, aspect, and slope influence woody vegetation structure and composition but not species richness in a human-influenced landscape in northwestern Yunnan, China.docx

Elevation-for-latitude substitution offers a tool for studying the influence of temperature and precipitation variability on vegetation structure and composition. Understanding how elevation, aspect, and slope influence vegetation patterns may help in predicting how climate change influences human forest usage and in developing strategies for ensuring the sustained provision of ecosystem services. However, most ecological studies have been carried out in protected areas, leaving forest areas used by humans to lesser attention. Therefore, we asked how elevation, aspect, and slope impact the vegetation on a human-influenced mountain. We measured woody vegetation size, richness, and composition on a mountain with plots set systematically in four cardinal directions at 100-m elevational intervals from the peak, from 1900 to 4200 m above sea level, in the Hengduan Mountains in eastern Himalaya, southwestern China. We quantified how tree maximum height, basal area, aboveground biomass (AGB), tree and shrub species richness, and woody species composition changed with elevation, aspect, and slope. Based on generalized linear models, the maximum tree height, tree basal area, and woody species AGB followed a unimodal trend along elevational gradients, with tree height and basal area peaking at 3100 m, while AGB was highest at 3300 m and somewhat higher on the southern slope. Basal area increased with slope degree. Neither tree nor shrub species richness was influenced by elevation, aspect, or slope. According to canonical correspondence analysis and TWINSPAN classification, elevation and north-south orientation of the slope were major factors influencing woody species compositions, and vegetation was classified into five types of communities. Our results indicated that the influences of elevation, aspect, and slope on woody vegetation structure were similar in a human-influenced forested mountain area as in protected mountain landscapes based on the literature. However, as forests in this area are used more intensively at low and middle elevations of the southern and western slopes, where aridity restricts tree size and AGB, climate change is likely to challenge traditional harvesting practices and place pressure on moving forest usage to higher altitudes.

海拔替代纬度法（Elevation-for-latitude substitution）为研究温度与降水变异性对植被结构与组成的影响提供了有效工具。明晰海拔、坡向与坡度对植被分布格局的影响机制，有助于预测气候变化对人类林业利用的影响，并为保障生态系统服务的持续供给制定策略。然而，当前多数生态学研究均聚焦于保护区，人类经营的林区却未得到足够关注。因此，本研究旨在探讨海拔、坡向与坡度对受人类活动干扰山地的植被的影响。本研究在中国西南喜马拉雅东部的横断山脉开展，以海拔1900至4200米的山地为研究对象，沿海拔梯度以100米为间隔，在四个主方位（four cardinal directions）系统布设样地，测定了木本植被的体量、物种丰富度与组成。本研究量化了乔木最大树高、胸高断面积、地上生物量（aboveground biomass, AGB）、乔灌物种丰富度以及木本物种组成随海拔、坡向与坡度的变化规律。基于广义线性模型（generalized linear models）的分析结果显示，乔木最大树高、乔木胸高断面积以及木本植物地上生物量沿海拔梯度均呈单峰分布格局：树高与胸高断面积的峰值出现在3100米海拔处，而地上生物量的峰值则位于3300米海拔处，且南坡的地上生物量略高。胸高断面积随坡度增大而增加。乔灌物种丰富度均不受海拔、坡向与坡度的影响。通过典范对应分析（canonical correspondence analysis）与双向指示种分类法（TWINSPAN classification）分析可知，海拔与坡向的南北梯度是影响木本物种组成的主要因子，研究区植被可划分为5种群落类型。本研究结果表明，在受人类活动干扰的林区山地中，海拔、坡向与坡度对木本植被结构的影响规律，与已有文献记载的保护区山地景观中的结果一致。但由于该区域南坡与西坡的中低海拔林区利用强度较高，且干旱环境限制了乔木体量与地上生物量的积累，因此气候变化可能会对传统采伐模式造成冲击，并迫使林业利用向更高海拔区域转移。