Vertical and Horizontal Vegetation Structure across Natural and Modified Habitat Types at Mount Kilimanjaro

In most habitats, vegetation provides the main structure of the environment. This complexity can facilitate biodiversity and ecosystem services. Therefore, measures of vegetation structure can serve as indicators in ecosystem management. However, many structural measures are laborious and require expert knowledge. Here, we used consistent and convenient measures to assess vegetation structure over an exceptionally broad elevation gradient of 866–4550m above sea level at Mount Kilimanjaro, Tanzania. Additionally, we compared (human)-modified habitats, including maize fields, traditionally managed home gardens, grasslands, commercial coffee farms and logged and burned forests with natural habitats along this elevation gradient. We distinguished vertical and horizontal vegetation structure to account for habitat complexity and heterogeneity. Vertical vegetation structure (assessed as number, width and density of vegetation layers, maximum canopy height, leaf area index and vegetation cover) displayed a unimodal elevation pattern, peaking at intermediate elevations in montane forests, whereas horizontal structure (assessed as coefficient of variation of number, width and density of vegetation layers, maximum canopy height, leaf area index and vegetation cover) was lowest at intermediate altitudes. Overall, vertical structure was consistently lower in modified than in natural habitat types, whereas horizontal structure was inconsistently different in modified than in natural habitat types, depending on the specific structural measure and habitat type. Our study shows how vertical and horizontal vegetation structure can be assessed efficiently in various habitat types in tropical mountain regions, and we suggest to apply this as a tool for informing future biodiversity and ecosystem service studies.在多数生境中，植被构成了环境的主体框架。这类结构复杂性可助力生物多样性（biodiversity）与生态系统服务（ecosystem services）的提升，因此植被结构（vegetation structure）指标可作为生态系统管理（ecosystem management）的指示因子。然而，多数植被结构测量方法耗时费力且需专业知识支撑。本研究于坦桑尼亚乞力马扎罗山（Mount Kilimanjaro）选取了海拔跨度为866–4550米的极宽海拔梯度（elevation gradient），采用统一且便捷的方法评估植被结构。此外，本研究对比了经人类改造的生境，包括玉米田（maize fields）、传统管理家庭菜园（traditionally managed home gardens）、草原（grasslands）、商业咖啡种植园（commercial coffee farms）以及择伐焚烧林（logged and burned forests），并将其与该海拔梯度上的自然生境（natural habitats）进行对照。为考量生境的复杂性与异质性，我们区分了垂直植被结构（vertical vegetation structure）与水平植被结构（horizontal vegetation structure）。 垂直植被结构通过植被层（vegetation layers）的数量、宽度与密度、最大冠层高度（maximum canopy height）、叶面积指数（leaf area index）及植被覆盖度进行评估，其呈现单峰海拔格局，在山地森林（montane forests）的中等海拔处达到峰值；而水平植被结构通过植被层数量、宽度与密度、最大冠层高度、叶面积指数及植被覆盖度的变异系数（coefficient of variation）进行评估，其在中等海拔处最低。 总体而言，经改造生境的垂直植被结构始终低于自然生境类型，而水平植被结构在改造与生境间的差异并不一致，具体取决于所采用的结构指标与生境类型。本研究展示了如何在热带山区的各类生境中高效评估垂直与水平植被结构，并建议将该方法作为工具，为未来的生物多样性与生态系统服务研究提供参考。