Do tropical forest streams function like their temperate counterparts? Insights from litter decomposition

Current stream ecosystem theory has been developed mainly based on data from temperate streams. It emphasizes the importance of pulsed litter inputs for aquatic food webs and ecosystem functioning. We explore to what extent patterns known from temperate forest streams can be extended to streams in tropical dry forests. To this end we determined decomposition rates and fungal and detritivore colonization of litter in a cross-wise experiment conducted in a series or tropical (10°N, Venezuela), warm-temperate (41°N, Portugal) and cool-temperate (52°N, Germany) streams. Exponential decay rates of both a tropical and temperate litter species varied about fourfold along the latitudinal gradient. However, these differences largely disappeared when decomposition rates were temperature-normalized. Rates in fine-mesh bags, indicative of microbial decomposition, were not systematically different from rates in coarse-mesh bags that allow detritivore feeding on leaves, even in temperate streams. Fungal biomass and sporulation reached higher maximum values and showed greater dynamics in the tropical streams. Like patterns were observed for litter-colonizing invertebrates. However, as with decomposition rate, assessment of time-temperature integrated impacts of fungi and detritivores on litter mass loss revealed no striking differences. Collectively this data suggests that key functional features of forest streams do not fundamentally differ between tropical and temperate biomes when differences in temperature and associated difference in absolute process rates are accounted for. Indeed, after temperature normalization of data, variation within biomes may be as large as between them, depending on the abundance of leaf shredders at particular locations.