Warming and drying slow temperate-boreal tree litter decomposition, while warm-grown litter highlights an important research frontier

Rachel A King, Samuel Powers Reed , Habacuc Flores-Moreno, Raimundo Bermudez Villanueva, Artur Stefanski, Laura J Williams, Sarah E Hobbie, Peter G Kennedy, Peter B. Reich

Plant litter decomposition is a primary control on terrestrial carbon fluxes and is critical to soil temperature, fauna, and nutrients.  Individually, key mediators of decomposition are well documented, with litter traits, temperature and moisture each known to affect decomposition. However, the combined effects of these mediators on decomposition remains unclear: in situ experiments that test how combined warming and rainfall reduction impact decomposition are rare, and few studies have tested how litter formed under elevated temperature subsequently influences decomposition. To this end, we used a unique open-air climate manipulation experiment located in the temperate-boreal ecotone of North America to test how warming and rainfall reduction affect the decomposition of leaf litter from eight boreal and temperate tree species. We found that warming and rainfall reduction increased litter half-life by 11% and 28%, respectively, in comparison to litter exposed to ambient climatic conditions. However, only rainfall reduction influenced litter mean residence time, increasing it by 37% relative to ambient rainfall. We also tested how leaf litter formed in ambient versus warmed growing conditions decomposed when transplanted into ambient and warmed environments. We found that warm-grown litter had a 23% lower half life than ambient grown litter under ambient temperatures. Ambient-grown and warm-grown litter had slower, but similar decomposition rates in warmed environments. Synthesis: Our findings indicate that climate change may slow carbon cycling in systems where moisture becomes a limiting factor. Additionally, our finding that warm-grown litter decomposition is more sensitive to temperature points to a new ecological knowledge gap with ramifications for carbon modeling under global change and highlights the need to simultaneously consider climate change effects on litter quality and subsequent decomposition.