Context dependency of tree diversity effects on standardized substrates decomposition: Role of tree functional composition, mycorrhizal type and climatic conditions

Tree diversity influences litter decomposition both directly, through changes in litter quality and composition, and indirectly, by altering the local decomposition environment (LDE). However, the role of the LDE in shaping litter decomposition rates remains less explored than the direct effects. A standardized decomposition experiment using cellulose and wood substrates was conducted over the course of a year across seven tree diversity experiments in Europe and North America to explore how tree diversity, through its influence on the LDE, impacts decomposition rates. Tree functional diversity enhanced the decomposition rate of high-quality substrate (cellulose) but had no effect on the decomposition rate of low-quality substrate (wood). The impact of LDE was context-dependent, with decomposition rates being highest under favourable climatic conditions, such as moderate temperatures and high precipitation. Contrary to the common assumption that litter decomposes faster in broadleaved and arbuscular (AM)-dominated stands, our findings show that decomposition was faster in mixtures containing coniferous species and ectomycorrhizal (EM)-associated trees, suggesting that LDE plays a larger role than initially thought. Synthesis. This study highlights the crucial role of LDE in shaping decomposition rates. While tree functional diversity generally enhances decomposition under favourable climatic conditions, LDE played a more significant role than previously recognized in EM stands, suggesting that faster decomposition rates in AM stands are primarily due to litter quality. These findings emphasize the context-dependent nature of decomposition and the importance of considering LDE in understanding how tree diversity influences decomposition processes.