Combined warming and drying slow temperate-boreal tree litter decomposition, while warm-grown leaf litter foreshadows an unexpected decomposition signal

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, among many other biotic and abiotic factors.  Individually, the key mediators of decomposition—litter traits, temperature, and moisture—are relatively well understood. However, our understanding of how combined climate drivers influence decomposition remains limited, as in situ experiments testing how combined warming and rainfall reduction impact decomposition are rare. Additionally, despite our knowledge that warming temperatures can alter leaf traits, few studies test how changes in leaf traits with increasing temperature can then influence decomposition. To this end, using the Boreal Forest at an Ecotone in Danger (B4warmED) experiment, we tested how warming and rainfall reduction impact the decomposition of leaf litter from eight boreal and temperate tree species. We found that combined warming and rainfall reduction increased litter half-life by 42% ± 11% in comparison to litter exposed to ambient climatic conditions. However, only rainfall reduction increased litter mean residence time by 37% ± 18% in comparison to ambient rainfall plots. We also tested how leaf litter grown in ambient and warmed growing conditions decomposed when transplanted into ambient and warmed environments. We found that warm-grown litter had a 22.4% ± 6.5% lower half life than ambient grown litter under ambient temperatures. Ambient-grown and warm-grown litter had slower, but equal decomposition rates in warmed environments. Our research indicates 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 highlights a key limitation of many decomposition studies that only use ambient-grown litter. This result also points to a new ecological knowledge gap with ramifications for carbon modeling under global change.