Summer flower pulses: Catkin litter processing in headwater streams

Abstract

The influences of inter- and intraspecific variation in leaf litter quality on aquatic food webs are well understood. However, the timing and type of organic matter inputs also varies within a species based on plant phenophase, including litterfall and flowering, which may influence instream communities and ecosystem processes including decomposition. We investigated how leaf and flower litters of a willow shrub species (Salix sitchensis) influenced decomposition and aquatic invertebrate colonization in a headwater stream. We collected abscised willow catkins and leaves from female plants, analyzed their initial chemistry, constructed litterbags, and incubated them at our stream site for five weeks. A subset of litterbags of each litter type was retrieved at 7, 22, and 37 days and analyzed for litter decomposition and invertebrate colonization. We found significant differences between catkins and leaves. Catkins had lower initial condensed tannin concentrations and higher invertebrate colonization compared to leaves. Initial leaching losses were faster for leaves, but overall decomposition rates were comparable. Catkin structural complexity supported higher invertebrate abundance and diversity as well as altered community composition, indicating their unique contribution to stream ecosystems. Although the ratio of reproductive to leaf inputs is low, we argue that the sequential availability of these two litter types may provide complementary energy inputs to consumers in headwater stream ecosystems dependent on organic matter.