Tree plot sampling data and forest fragment parameters in the East Usambara Mountains, Tanzania

Ongoing habitat loss and fragmentation alter the functional diversity of forests. Generalising the magnitude of change in functional diversity of fragmented landscapes and its drivers is challenging because of the multiple scales at which landscape fragmentation takes place. Here we propose a multi-scale approach to determine whether fragmentation processes at the local and landscape scales are reducing functional diversity of trees in the East Usambara Mountains, Tanzania. We employ a structural equation modelling approach using five key plant traits (seed length, dispersal mode, shade tolerance, maximum tree height, and wood density) to better understand the functional responses of trees to fragmentation at multiple scales. Our results suggest both direct and indirect effects of forest fragmentation on tree functional richness and evenness. A reduction in fragment area appears to exacerbate the negative effects resulting from an increased amount of edge habitat and loss of shape complexity, further reducing richness and evenness of traits related to resource acquisition and favouring tree species with fast growth. As anthropogenic disturbances affect forests around the world, we advocate to include the direct and indirect effects of forest fragmentation processes to gain a better understanding of shifts in functional diversity that can inform future management efforts.

持续的栖息地丧失与片段化正改变森林的功能多样性。由于景观片段化发生于多个尺度，想要量化概括破碎化景观的功能多样性变化幅度及其驱动机制，极具挑战性。本研究提出一种多尺度研究方法，旨在探明坦桑尼亚东部乌桑巴拉山脉中，局地与景观尺度上的片段化过程是否降低了当地树木的功能多样性。我们采用结构方程模型方法，选取种子长度、扩散模式、耐阴性、最大树高与木材密度共5种关键植物功能性状，以更清晰地解析树木在多尺度下对森林片段化的功能响应。研究结果表明，森林片段化对树木功能丰富度与功能均匀度同时存在直接与间接影响。片段面积的缩减似乎会加剧由边缘生境占比提升与形状复杂性丧失所带来的负面影响，进一步降低与资源获取相关的功能性状的丰富度与均匀度，并更倾向于筛选出生长速度较快的树木物种。鉴于人为干扰正影响着全球范围内的森林，我们呼吁将森林片段化过程的直接与间接影响纳入考量，以更深入地解析功能多样性的变化格局，从而为未来的森林管理实践提供科学参考。