Two major forms of allochthonous inputs are important for Pacific Northwest streams: leaf litter and salmon carcasses. Although much is known about these two input types in isolation, few studies have examined their interaction, and no studies have examined interactions between salmon carcasses and genetically distinct litter types within tree species. In this study, we created leaf litter bags of seven genotypes of black cottonwood (Populus balsamifera L. ssp. trichocarpa [Torr. & A. Gray ex Hook.] Brayshaw) made from litter collected in a 25-year old common garden, four mixtures of two genotypes each, and one mixture with six genotypes. All leaf litter treatments were placed in blocks in McKenna Creek (Yelm, WA) directly downstream of chum salmon (Oncorhynchus keta) carcasses or no-salmon controls. Our major research questions included: 1) Does leaf litter from genotypes of P. trichocarpa collected from across a latitudinal gradient from OR, USA to BC, Canada decompose at different rates based on inherent differences in phytochemistry? 2) Do mixtures of litter from different genotypes decompose additively, and 3) Does the presence of salmon carcasses interact with these litter treatments to alter patterns seen among genotypes or mixtures?

Results/Conclusions
Genotypes of black cottonwood significantly differed in initial litter % condensed tannins (R² = 0.83, F_(6,14) = 11.89, p < 0.0001), % phosphorus (R² = 0.74, F_(7,19) = 7.88, p = 0.0002), and % lignin (R² = 0.35, F_(6,43) = 3.80, p = 0.004). These genetic differences in initial phytochemistry were related to significant differences in leaf litter mass loss at harvest day 14 (R² = 0.57, F_(5,25) = 6.68, p = 0.0004) and day 28 (R² = 0.40, F_(5,40 = 5.42, p = 0.0007. Additionally, we show evidence for non-additive decomposition (both synergism and antagonism) in genotype mixtures and a significant interaction of salmon carcasses on leaf decomposition at day 14 (R² = 0.19, F_(5,48) = 3.57, p = 0.0084).These findings have important restoration implications because restoration projects often use salmon carcass “tosses” and/or the replanting of riparian vegetation to improve riparian and stream habitats, while interactions between these restoration actions in a community and ecosystem context are understudied. Further, genetic-based variation in community and ecosystem responses suggests that intraspecific variation may play a large role in determining the nature of ecosystem processes and ecological interactions in stream environments of the Pacific Northwest.