We compared hybrid and mixed leaf litter for two Populus species to determine additive and non-additive effects on in-stream decomposition and macroinvertebrate communities. Leaf litter was collected at a common garden for a Populus maximowiczii (Hokkaido, Japan) x Populus balsamifera ssp. trichocarpa (Chilliwack, British Columbia, Canada) hybrid and both parental species. Litterbags made from leaves (2 g ± 0.05 g) of each species in isolation, the hybrid and a mixture of both parental types were packed in polyurethane mesh bags and installed into Snyder Creek, Olympia, WA to determine leaf litter mass loss and macroinvertebrate colonization. Leaf litter bags were removed from the stream after 10, 24, and 41 days. Initial phytochemistry and mass loss through time were determined, and macroinvertebrates were identified.
In this study, we found that leaf litter from two Populus parental species and their hybrid differed significantly in terms of initial condensed tannin (CT) concentrations (R2 = 0.92, F(2,10) = 64.66, p < 0.0001) and % C (R2 = 0.99, F(2,6) = 417.41, p < 0.0001). Interestingly, the hybrid did not demonstrate intermediate values of CT compared to parent species. Decomposition rates (k day-1) for P. maximowiczii, P. trichocarpa, P. trichocarpa x P. maximowiczii hybrid and a mixture of both parents were -0.0071, -0.0126, -0.0110, and -0.0121, respectively. Although we predicted that decomposition for the hybrid and mixture would be intermediate between the two parents, we show non-additive decomposition for the mixed litter, but additive decomposition for the hybrid. In terms of macroinvertebrate colonization, trichopterans were significant indicator species for the mixture of both P. trichocarpa and P. maximowiczii litter. These organisms may be responding to the variable resources available in mixed litter bags. These results have implications for the ecosystem influences of both species diversity and hybrids in ecosystems because they suggest that mixture and hybrid effects on community and ecosystem processes differ in their influences.