Changing climates threaten to disrupt the function of riparian areas as biodiversity hotspots and providers of important ecosystem services. In the American Southwest, riparian trees, such as the foundation species Populus fremontii, cycle terrestrial nutrients into support for aquatic species, linking these ecosystems. Changes in the trait characteristics of Populus fremontii leaf litter can therefore have consequences for aquatic functioning and foodwebs. Here, we investigated how interactive genetic and environmental effects alter the leaf traits of these trees, and subsequently how this shapes the shredding rate of an aquatic invertebrate shredder, the caddisfly larvae Phylloicus mexicanus. We used an experimental design of two common gardens spanning a temperature gradient on the warmer end of the Populus fremontii range, each containing six clonal genotypes originating from across the species’ climatic range. On some clonal replicates of each genotype, we applied a simulated herbivory treatment to simulate the increased insect activity predicted to accompany warming climates. We found that genetics and growing environment shaped both leaf traits and shredding rates more than simulated herbivory. Though simulated herbivory did have some small impacts on defensive leaf chemistry, this did not translate into effects on shredding. Interestingly, the highest shredding rates were observed when the growing climate was closer to the climate at the genotype's origin. Variation in shredding rates was best explained by a combination of genetics, condensed tannins, carbon to nitrogen ratio, and specific leaf area, with most of these traits themselves influenced by garden/environment. Our findings confirm that genetic and environmental variation in this foundation tree species directly influence changing nutrient flows to aquatic systems by altering trait-based species interactions.