Consequences of riparian invasion for in-stream litter processing

We tested the hypothesis that invasions of riparian zones by Japanese knotweed (Fallopia japonica) alter the phenology of in-stream litter processing.  We used 0.1m² buckets to collect dehiscing leaves along a transect that included invaded and non-invaded riparian habitat abutting Kayaderosseras Creek, NY, and weighed the leaves that fell weekly from 21 Sept to 25 Nov, 2008.  Owing to among-species differences in leaf-fall phenology (median loss prior to Oct 14 for Populus, Acer, Rhamnus, Tilia and Ulmus spp, Oct 28 for Quercus spp and Nov 4 for Fallopia), leaves in knotweed-invaded microsites entered streams later in the autumn. We created leaf-packs that mimicked the changing composition of in-stream litter derived from these two habitats, systematically added leaf material and/or collected bags over a 40 day interval.  Litter from invaded microsites was processed more slowly (over a given time interval) and retained mass for longer periods of time.  We attribute these differences to the combined effects of differences in leaf quality (based on earlier work) and progressively falling water temperatures when late autumn leaves, such as knotweed, enter the stream.  Ongoing work is testing whether leaf packs derived from the invaded versus non-invaded microsites host different aquatic macro-invertebrate communities.