The ongoing decline in biodiversity has fuelled concerns about its impact on ecosystem functioning. Mediterranean oak forests may prove very sensitive to global change, which could strongly influence the species composition of plant communities and thereby affect ecosystem processes. To determine the potential outcome of shifts in species composition on litter decomposition dynamics, we conducted a fullfactorial decomposition experiment over a gradient of litter species diversity in a Mediterranean Downy oak (Quercus pubescens Willd.) forest. We used litter from the three dominant tree species naturally present in the Downy oak forest and litter from Aleppo pine (Pinus halepensis Mill.) in anticipation of its possible spread in the future with global change. Litter water holding capacity and N/P ratio were the most important and positive drivers of decomposition process. In contrast to other ecosystems where synergistic non-additive effects are prevalent, we observed 54% of additive and 46% of non-additive effects on litter mass loss in our Mediterranean ecosystem. These results could indicate less complementarity among decomposers for decomposition of diverse plant material in such a stressful climate. Moreover, dominant tree species are of key importance for nutrient availability, and the arrival of Aleppo pine would strongly reduce the N release during the decomposition process. Based on calculations of an annual partial nutrient budget at the ecosystem level, we showed that a shift in plant communities could affect nutrient release, ranging from 5 to 36% for N and 63 to 83% for P depending on the co-occurring tree species in the Downy oak forest.