Decomposition rates and phosphorus concentrations of purple loosestrife (Lythrum salicaria) and cattail (Typha spp.) in fourteen Minnesota wetlands

Purple Loosestrife is rapidly displacing native vegetation in North American wetlands. Associated changes in wetland plant communities are well understood. Effects of Loosestrife invasion on nutrient cycling and decomposition rates in affected wetlands are unknown, though potentially of significance to wetland function. We used litter bag methods to quantify decomposition rates and phosphorus concentrations of purple Loosestrife (Lythrum salicaria) and native cattails (Typha spp.) in fourteen Minnesota wetlands. A 170-day study that began in autumn modeled decomposition of Loosestrife leaves. Loosestrife stems and Typha shoots that had overwintered and fragmented were measured in a 280- day study that began in spring. In general Loosestrife leaves decomposed most rapidly of the three; Typha shoots decomposed faster than Loosestrife stems. Significant decay coefficients (k- values) were determined by F-testing single exponential model regressions of different vegetation types in the fourteen wetlands. Significant decay coefficients were: k=2.5 x 10^-3 and 4.32 x 10^-3 for all Loosestrife leaves (170 d); k = 7.2 x 10^-4 and 1.11 x 10^-3 for overwintered Loosestrife stems (280-d) and k = 7.9 x 10^-4, 1.42 x 10^-3 and 2.24 x 10^{- 3} for overwintered Typha shoots (280-d). Phosphorus concentrations of plant tissue showed an initial leaching followed by stabilization or increase probably associated with microbial growth. Loosestrife leaves had twice the phosphorus concentration of Loosestrife stems and Typha shoots. Our results indicate that conversion of wetland vegetation from cattails to Loosestrife may result in significant change in wetland function by altering timing of litter input and downstream phosphorus loads. Conversion of a riverine, flow- through wetland from Typha to Loosestrife may effectively accelerate eutrophication of downstream water bodies. Impacts of Loosestrife invasion must be considered when wetlands are managed for wildlife or for improvement of downstream water quality.