Do Riparian Plant Hybrids Mimic Leaf Mixtures in Terms of In-Stream Litter Dynamics?

Both riparian tree species diversity and genetic diversity can influence streams through leaf litter decomposition; however, these two sources of variation have not been compared directly. Here, we compare leaf litter decomposition for a physical mixture of two Populus species to the litter from an F1 hybrid (a genetic mixture) of the same two species. Leaf litter was collected from a common garden for an F1 hybrid between Japanese poplar (Populus maximowiczii A. Henry) and black cottonwood (Populus trichocarpa Torr. & A. Gray ex Hook.), as well as both parent species. Four litterbag treatments consisted of litter from the F1 hybrid, an equal-part mixture treatment of litters from both parents, and litter from each parent in isolation. The hybrid litter had higher C:N content and lower % condensed tannins than either parent species, or the average of the two parents that represented the mixture. While the hybrid and mixture treatments both lost more mass than expected by day 42, the mixture lost relatively more mass than the hybrid and roughly as much as the faster-decomposing P. trichocarpa parent. The hybrid mimicked the mixture, and both supported higher aquatic macroinvertebrate richness and less litter mass remaining than expected based on parent species values, despite differences between the hybrid and mixture in initial phytochemistry.