Intraspecific leaf trait variation mediates edge effects on litter decomposition rate in fragmented forests

There is strong trait dependence in species-level responses to environmental change and their cascading effects on ecosystem functioning. However, there is little understanding of whether intraspecific trait variation (ITV) can also be an important mechanism mediating environmental effects on ecosystem functioning. This is surprising, given that global change processes such as habitat fragmentation and the creation of forest edges drive strong trait shifts within species. On 20 islands in the Thousand Island Lake, China, we quantified intraspecific leaf trait shifts of a widely distributed shrub species, Vaccinium carlesii, in response to habitat fragmentation. Using a reciprocal transplant decomposition experiment between forest edge and interior on 11 islands with varying areas, we disentangled the relative effects of intraspecific leaf trait variation vs. altered environmental conditions on leaf decomposition rates in forest fragments. We found strong intraspecific variation in leaf traits in response to edge effects, with a shift towards recalcitrant leaves with low specific leaf area and high leaf dry matter content from forest interior to the edge. Using structural equation modelling, we showed that such intraspecific leaf trait response to habitat fragmentation had translated into significant plant afterlife effects on leaf decomposition, leading to decreased leaf decomposition rates from the forest interior to the edge. Importantly, the effects of intraspecific leaf trait variation were additive to and stronger than the effects from local environmental changes due to edge effects and habitat loss. Our experiment provides the first quantitative study showing that intraspecific leaf trait response to edge effects is an important driver of the decrease in leaf decomposition rate in fragmented forests. By extending the trait-based response-effect framework towards the individual level, intraspecific variation in leaf economics traits can provide the missing functional link between environmental change and ecological processes. These findings suggest an important area for future research on incorporating ITV to understand and predict changes in ecosystem functioning in the context of global change.物种对环境变化的响应及其对生态系统功能的级联效应存在显著的性状依赖性。然而，目前尚不清楚种内性状变异（intraspecific trait variation, ITV）是否同样是介导环境变化对生态系统功能产生影响的重要机制。考虑到栖息地破碎化与林缘形成等全球变化过程会驱动物种内部产生显著的性状转变，这一认知空白令人意外。本研究在中国千岛湖的20座岛屿上，针对广泛分布的灌木物种乌饭树（Vaccinium carlesii），量化了其响应栖息地破碎化的种内叶片性状变化。通过在面积各异的11座岛屿上开展林缘与林内的互易移植分解实验，我们厘清了种内叶片性状变异与改变后的环境条件对森林片段内叶片分解速率的相对贡献。我们发现，叶片性状的种内变异对林缘效应存在显著响应：从林内到林缘，叶片逐渐转变为比叶面积（specific leaf area, SLA）较低、叶片干物质含量较高的难分解叶片。本研究通过结构方程模型（structural equation modelling, SEM）证实，这类响应栖息地破碎化的种内叶片性状响应，已转化为对叶片分解的显著植物残体效应，使得从林内到林缘的叶片分解速率出现下降。尤为重要的是，种内叶片性状变异所产生的效应具有叠加性，且强于林缘效应与栖息地丧失所引发的局地环境变化带来的效应。本实验为首项定量研究，证实了种内叶片性状对林缘效应的响应，是破碎化森林中叶片分解速率下降的重要驱动因素。通过将基于性状的响应-效应框架拓展至个体层面，叶片经济性状的种内变异能够填补环境变化与生态过程之间缺失的功能联系。上述研究结果表明，未来可将种内性状变异（ITV）纳入研究范畴，以理解并预测全球变化背景下的生态系统功能变化，这是一个极具研究价值的方向。