Leaf litter decomposition and decomposer communities in streams affected by intensive forest biomass removal

Abstract

The complexity and multidimensionality of freshwater processes require multiple metrics to assess their responses to disturbances. However, studies have historically relied on structural attributes, while functional attributes have received much less attention. The relative ease of implementing functional attributes, like leaf decomposition assays, make them an attractive choice to complement structural attributes (e.g. species diversity and community composition). However, the congruence of these disparate attributes is still debatable, as studies have reported results to be strongly context-dependent. The concept of using forest biomass as a source of alternative energy instead of fossil fuel has led to rapid changes in forestry practices. One of these practices involves removal of logging residues that would have otherwise been left on site (logging residue removal, LRR), potentially leaving the landscape prone to high erosion, run-off and sedimentation. This study investigated the decomposition rates of alder leaves in sites affected by LRR compared with conventionally logged (CL) and near-natural, non-harvested sites. Our results revealed CL as having the strongest effects, while LRR had an intermediate response, both in terms of decomposition rates (ktotal, kmicrobial and kshredders) and microbial biomass. These results match those for structural attributes reported previously at the same sites, thus providing multiple lines of evidence for the ecosystem-level impacts of forestry activities. Bacterial biomass on decomposing leaves was consistently several orders of magnitude greater than that of fungi in all study streams and treatments. Most previous research on the relative contributions of shredders, fungi and bacteria to leaf decomposition has focused on fast-flowing riffles and more research is needed to clarify their relative roles in other types of running waters, such as the slow-flowing, strongly allochthonous and humic forest streams studied by here. In conclusion, this study points to further need to revise the current practice of forestry activities in light of its negative impact on biodiversity.