Intraspecific leaf chemistry drives locally accelerated ecosystem function in aquatic and terrestrial communities

Resource patchiness influences consumer foraging, movement, and physiology. Fluxes across ecosystem boundaries can extend these effects to otherwise distinct food webs. Intraspecific diversity of these cross‐ecosystem subsidies can have large consequences for recipient systems. Here, we show intraspecific variation in leaf defensive chemistry of riparian trees drives local adaptation among terrestrial and riverine decomposers that consume shed leaf litter. We found extensive geographic structuring of ellagitannins, diarylheptanoids, and flavonoids in red alder trees. Ellagitannins, particularly those with strong oxidative activity, drive aquatic leaf decomposition. Further, spatial variation in these leaf components drives local ecological matching: in experiments using artificial food sources distinguished only by the chemical content of individual trees, we found decomposers both on land and in rivers more quickly consumed locally derived food sources. These results illustrate that terrestrial processes can change the chemistry of cross‐ecosystem subsidies in ways that ultimately alter ecosystem function in donor and recipient systems.