Plasticity in leaf litter traits partly mitigates the impact of thinning on forest floor carbon cycling

Abstract

Abstract Reducing stand density by thinning intensification has been emphasized as an efficient strategy of forest adaptation to climate change as it improves stand resistance to drought. Yet, it is still unclear how it could affect litter carbon (C) cycling processes. Recent evidence indicates that the plasticity of an oak tree species can lead to a decline in its leaf litter quality and decomposability following thinning. The consequences for litter decomposition and forest floor C storage at the ecosystem scale remain largely unexplored. In this study, we took advantage of a regional‐scale, multi‐site network of long‐term thinning experiments in temperate oak (Quercus petraea) forests to address this issue. We measured ecosystem properties related to forest floor C cycling in 19 plots across eight experimental sites covering a large gradient of stand density and age. Though we expected thinning to affect in situ litter decomposition by altering oak leaf litter quality, we conducted complementary experiments exploring additional mechanisms, that is alterations of microenvironmental conditions and soil faunal activity. Thinning intensification induced a strong decline in tree canopy leaf area index, above‐ground tree litter production and forest floor decomposition rate in early “aggradation” stage of forest development. This slower litter decomposition was mainly driven by plasticity of oak trees that produced leaf litter of poorer quality and decomposability following thinning, for example, litter richer in secondary metabolites such as condensed tannins. Change in microenvironmental conditions also contributed to the slowdown of litter decomposition, likely as a result of the less buffered microclimate associated with larger tree canopy opening. No change in soil faunal effect induced by thinning was observed. Thinning intensification resulted in a limited decrease in forest floor C stock. Indeed, the slower litter decomposition offset nearly half of the forest floor C loss associated to the reduced litterfall in “aggradation” stage. Our study demonstrated that phenotypic plasticity in leaf litter traits of a dominant tree species can strongly affect ecosystem functioning by slowing forest floor decomposition following thinning intensification, in turn partly mitigating the negative effect of thinning on forest floor C storage.