Landscape configuration alters spatial arrangement of terrestrial-aquatic subsidies in headwater streams

Context: Freshwater ecosystems depend on surrounding terrestrial landscape for resources. Most important are terrestrial leaf litter subsidies, which differ depending on land use. We lack a good understanding of the variation of these inputs across spatial scales. Objectives: We sought to determine: (1) the relative importance of local versus catchment-level forestation for benthic leaf litter biomass in streams, (2) how landscape configuration alters these relationships, and (3) how land use affects the quality and diversity of leaf litter subsidies. Methods: We measured biomass and identity of benthic leaf litter in 121 reaches in 10 independent catchments seasonally over the course of a year. We assessed direct and indirect effects of forestation, reach position, and seasonality on leaf litter biomass using structural equation models, and assessed how leaf litter diversity varied with land use. Results: In catchments with forested headwaters, the degree of forestation and reach position in the catchment influenced benthic leaf litter biomass indirectly through local reach-scale forestation. In catchments where forest was only located downstream, or with minimal forest, none of these factors influenced reach-level benthic leaf litter. Leaf litter diversity peaked in fall in all land use types, but was generally lowest in forested reaches. Conclusions: Not only habitat amount, but its location relative to other habitats is important for ecosystem function in the context of cross-ecosystem material flows. Here, lack of upstream forest altered spatial patterns of leaf litter storage. Studies with high spatiotemporal resolution may further reveal effects of landscape configuration on other ecosystems.

研究背景：淡水生态系统依赖周边陆地景观获取各类资源，其中最为关键的是陆地叶凋落物补给（terrestrial leaf litter subsidies），这类输入会因土地利用方式的不同而存在显著差异。目前学界对这类输入在不同空间尺度上的变异规律仍缺乏充分的认知。
研究目标：本研究旨在明确三个核心科学问题：（1）溪流底栖叶凋落物生物量相关的本地尺度与集水区尺度森林覆盖度的相对重要性；（2）景观格局如何改变上述二者的关系；（3）土地利用方式如何影响叶凋落物补给的质量与物种多样性。
研究方法：本研究于一年内分季节对10个独立集水区内的121个河段开展了底栖叶凋落物生物量与物种组成的测定。通过结构方程模型（structural equation models）量化了森林覆盖度、河段位置以及季节变化对叶凋落物生物量的直接与间接效应，并分析了叶凋落物多样性随土地利用类型的变化特征。
研究结果：在源头区域具备森林覆盖的集水区中，集水区整体森林覆盖度与河段在集水区内的空间位置，可通过本地河段尺度的森林覆盖度间接影响底栖叶凋落物生物量。而在森林仅分布于下游区域或森林覆盖率极低的集水区中，上述因素均不会对河段底栖叶凋落物生物量产生显著影响。所有土地利用类型下，叶凋落物多样性均在秋季达到峰值，但在森林覆盖的河段中通常处于最低水平。
研究结论：在跨生态系统的物质流动情境下，不仅栖息地的面积规模，其相对于其他栖息地的空间位置对生态系统功能同样至关重要。本研究结果表明，上游森林的缺失会显著改变叶凋落物存储的空间格局。未来采用高时空分辨率的研究方案，可进一步揭示景观格局对其他类型生态系统的影响效应。