Litter movement pathways across terrestrial-aquatic ecosystem boundaries affect litter colonization and decomposition in streams

1. Streams and their riparian zones are connected by spatial flows of organic matter, and constitute a model example of a meta-ecosystem. Fluxes of leaf litter from the riparian zone to the stream are a major energy source in stream food webs. Leaf litter can enter the stream vertically, falling from the tree and into the stream, or laterally, washing into the stream after a period of exposure in the terrestrial ecosystem. The latter can contribute up to 23% to the total amount of litterfall entering streams. 2. To determine if decomposition, microbial and invertebrate colonization of lateral litter inputs are similar to those of vertical inputs, we assessed leaf decomposition of alder, poplar and a 1:1 mixture of the two species in three scenarios across a gradient of terrestrial:aquatic exposures. 3. Overall decomposition was explained by a negative exponential model and decreased with the increase in the period of terrestrial exposure in all cases. Invertebrate colonization tended to decrease with the increase in the period of terrestrial exposure, but total invertebrate richness and biomass were more affected by litter type than by the exposure scenario, attaining higher values in the mixture than in the species alone. 4. As the length of exposure in the terrestrial ecosystem increased, in-stream decomposition rates of leaf litter decreased. Comparing leaf species treatments, alder decomposition rates were faster than poplar and the alder-poplar mixture. 5. The richness of the aquatic hyphomycete community colonizing leaf litter after submergence decreased and sporulation rates were strongly inhibited with an increasing terrestrial exposure period. While fungi colonizing leaf litter exposed only in the stream invested in rapid reproduction, fungi colonizing litter with prior terrestrial exposure built up more biomass. 6. We conclude that the path taken by the litter fluxes has important effects on the functioning of the receiving ecosystem. Studies relying only on the fate of freshly abscissed leaf litter (vertical inputs) may not present a complete picture of the decomposition process in streams and may have been overestimating the overall richness and reproductive activity of the aquatic hyphomycetes colonizing leaf litter.

1. 溪流及其河岸带（riparian zone）通过有机物质的空间流动相互连通，是元生态系统（meta-ecosystem）的典型范例。从河岸带输入溪流的叶凋落物通量，是溪流食物网的主要能量来源。叶凋落物可通过两种方式进入溪流：一是垂直输入，即直接从树木掉落至水中；二是侧向输入，即先在陆地生态系统中暴露一段时间后被冲刷进入溪流。后者占进入溪流的总凋落物量的比例可达23%。
2. 为明确侧向输入的叶凋落物的分解过程、微生物与无脊椎动物定殖特征是否与垂直输入的凋落物一致，我们针对桤木、杨树以及二者1:1混合的叶凋落物，在3种沿陆地-水生暴露梯度设置的处理场景中开展了分解实验。
3. 整体分解过程符合负指数模型（negative exponential model），且在所有处理中，分解速率均随陆地暴露时长的增加而降低。无脊椎动物的定殖密度也随陆地暴露时长增加呈下降趋势，但无脊椎动物的总丰富度与生物量受凋落物类型的影响大于暴露场景，在混合凋落物处理中的数值高于单一物种处理。
4. 随着陆地生态系统中暴露时长的增加，叶凋落物在溪流中的分解速率逐渐降低。不同物种凋落物处理的对比结果显示，桤木的分解速率快于杨树以及桤木-杨树混合凋落物。
5. 凋落物浸没溪流后，定殖其上的水生丝孢菌（aquatic hyphomycete）群落丰富度随陆地暴露时长增加而降低，产孢率也受到显著抑制。仅在溪流中暴露的凋落物所定殖的真菌以快速繁殖为策略，而预先经陆地暴露的凋落物所定殖的真菌则积累了更多生物量。
6. 本研究表明，凋落物的输入路径对接收生态系统的功能具有显著影响。仅以新鲜脱落的叶凋落物（即垂直输入）为研究对象的相关实验，可能无法完整反映溪流中凋落物的分解过程，且此前或高估了定殖于叶凋落物的水生丝孢菌的总丰富度与繁殖活性。