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.

在绝大多数生境中，植被构成了环境的主体框架。此类结构复杂性可促进生物多样性提升与生态系统服务功能的实现。因此，植被结构的量化指标可作为生态系统管理的重要指示因子。然而，多数植被结构量化方法耗时费力，且需具备专业知识的人员方可开展。

本研究针对坦桑尼亚乞力马扎罗山范围内、海拔跨度达866至4550米的极宽梯度生境，采用统一且便捷的方法开展植被结构评估。此外，本研究沿该海拔梯度，将自然生境与各类人为改造生境进行对比，后者涵盖玉米农田、传统管理的家庭菜园、草原、商业咖啡种植园以及经采伐与火烧的林地。

为充分表征生境的复杂性与异质性，本研究分别考量植被垂直结构与水平结构。垂直植被结构（通过植被层数量、宽度与密度、最大冠层高度、叶面积指数（Leaf Area Index）及植被盖度进行量化）呈现出单峰型海拔分布格局，在山地森林的中等海拔区域达到峰值；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、叶面积指数及植被盖度的变异系数进行量化）则在中等海拔处达到最低值。

整体而言，人为改造生境的垂直植被结构普遍低于自然生境；但水平植被结构在人为与自然生境间的差异并不统一，具体取决于所选取的结构量化指标与生境类型。

本研究证实了热带山地不同生境类型中植被垂直与水平结构的高效评估方法，并建议将该方法作为工具，为后续生物多样性与生态系统服务相关研究提供支撑。