Ecological Impacts of Invasive Knotweed on Stream Processes

Invasive Japanese knotweed (<i>Polygonum cuspidatum</i>) and cogeners spread aggressively along streams and establish dense stands of knotweed, thereby reducing riparian plant species diversity.  Allochthonous inputs from riparian plants are an important source of nutrients and organic matter for aquatic organisms in small streams.  When the riparian vegetation changes from a mixed composition of native plants to a monoculture of knotweed the quantity, quality, and timing of allochthonous inputs is altered and may negatively affect aquatic insects and fish.  Senesced knotweed leaves are typically low in nitrogen, but highly variable between plants (C:N 32-97:1), compared to western red alder (C:N 25-31:1), and similar to black cottonwood (C:N 66-87:1).  Leaf litter with lower C:N ratios are expected to decay more rapidly than litter with higher C:N ratios.  However, little information is available concerning the influence of knotweed on stream processes.  This study examines the influence of decomposing knotweed leaves on aquatic fungi and macroinvertebrates.  Aquatic fungi are important in the litter decomposition process and as a food resource for detritivores invertebrates.  We present results from a field study along 3 streams in western Washington.  Specifically, this study compares senesced leaves of  knotweed to native red alder and black cottonwood in terms of (1) the nutrient quality (C,N,P) and leaf structure (lignin, cellulose), (2) rate of decomposition in streams, and (3) the biomass (ergosterol) of aquatic fungi and the quantity/diversity of macroinvertebrates on decomposing leaves.  Changes to lower trophic levels in the food web (litter inputs and fungi) have the potential to impact higher trophic levels (macroinvertebrates and fish).