Leaf decomposition and stream macroinvertebrate colonisation of Japanese knotweed, an invasive plant species

Japanese knotweed (Fallopia japonica HouTT. RONSE DECRANE) is a highly invasive exotic plant that forms monocultures in riparian areas, effectively reducing plant diversity. This change in riparian plant composition alters the allocthonous input of leaf litter into adjacent streams. A field experiment was completed to understand how leaf decomposition and macroinvertebrate colonisation associated with the incorporation of exotic leaf litter. Leaf packs of Japanese knotweed, native alder (Alnus incana L.), native cottonwood (Populus trichocarpa TORR. and GRAY), and two additional mixed pack types (alder and cottonwood; alder, cottonwood, and Japanese knotweed) were placed into a 50 in stream reach in Clear Creek, Idaho, and removed over a three-month period. Leaf decomposition and macroinvertebrate assemblages were similar between leaf types, despite differences in nitrogen and phosphorus content. The diversity of leaf types within a given leaf pack also had no effect on leaf decomposition or macroinvertebrate dynamics. These findings suggest that allochthonous inputs of Japanese knotweed fulfill a detrital function similar to that of native leaf litter.