Data from: Factors determining forest diversity and biomass on a tropical volcano, Mt. Rinjani, Lombok, Indonesia

Tropical volcanoes are an important but understudied ecosystem, and the relationships between plant species diversity and compositional change and elevation may differ from mountains created by uplift, because of their younger and more homogeneous soils. We sampled vegetation over an altitudinal gradient on Mt. Rinjani, Lombok, Indonesia. We modeled alpha- (plot) and beta- (among plot) diversity (Fisher’s alpha), compositional change, and biomass against elevation and selected covariates. We also examined community phylogenetic structure across the elevational gradient. We recorded 902 trees and shrubs among 92 species, and 67 species of ground-cover plants. For understorey, subcanopy and canopy plants, an increase in elevation was associated with a decline in alpha-diversity, whereas data for ground-cover plants suggested a hump-shaped pattern. Elevation was consistently the most important factor in determining alpha-diversity for all components. The alpha-diversity of ground-cover vegetation was also negatively correlated with leaf area index, which suggests low light conditions in the understorey may limit diversity at lower elevations. Beta-diversity increased with elevation for ground-cover plants and declined at higher elevations for other components of the vegetation. However, statistical power was low and we could not resolve the relative importance to beta-diversity of different factors. Multivariate GLMs of variation in community composition among plots explained 67.05%, 27.63%, 18.24%, and 19.80% of the variation (deviance) for ground-cover, understorey, subcanopy and canopy plants, respectively, and demonstrated that elevation was a consistently important factor in determining community composition. Above-ground biomass showed no significant pattern with elevation and was also not significantly associated with alpha-diversity. At lower elevations communities had a random phylogenetic structure, but from 1600 m communities were phylogenetically clustered. This suggests a greater role of environmental filtering at higher elevations, and thus provides a possible explanation for the observed decline in diversity with elevation.

热带火山是一类重要但研究不足的生态系统。由于其土壤更为年轻且均一，植物物种多样性、群落组成变化与海拔之间的关联模式可能不同于造山抬升形成的山地。本研究在印度尼西亚龙目岛的林贾尼火山沿海拔梯度开展植被调查，分别以样地内α多样性（费希尔α指数）、样地间β多样性、群落组成变化及地上生物量为建模对象，结合选定的协变量开展建模分析；同时探究了全海拔梯度下的群落系统发育结构。本研究共记录到92个物种的902株乔灌木，以及67种地被植物。针对林下层、亚冠层与冠层植物，海拔升高与α多样性下降呈显著关联；而地被植物的α多样性则呈现驼峰型分布模式。海拔始终是调控所有植被层α多样性的最关键因子。地被植物的α多样性还与叶面积指数（Leaf Area Index, LAI）呈负相关，表明林下低光环境可能限制了低海拔区域的地被植物多样性。地被植物的β多样性随海拔升高而上升，而其他植被层的β多样性则在高海拔区域出现下降。但本研究统计效力有限，无法厘清不同因子对β多样性的相对贡献度。样地间群落组成变异的多变量广义线性模型（Generalized Linear Model, GLM）分别解释了地被植物、林下层、亚冠层与冠层植物群落67.05%、27.63%、18.24%及19.80%的偏差变异（deviance），结果证实海拔始终是调控群落组成的核心因子。地上生物量未呈现随海拔变化的显著趋势，且与α多样性无显著关联。低海拔区域群落呈现随机系统发育结构，而海拔1600米以上的群落则表现为系统发育聚集。这表明高海拔区域环境过滤作用更强，可为观测到的“海拔升高伴随多样性下降”的现象提供合理的解释机制。