In contrast to plant-animal interactions, the conceptual framework regarding the impact of secondary metabolites in mediating plant-plant interference is currently less well defined. Here, we address hypotheses about the role of chemically-mediated plant-plant interference (i.e. allelopathy) as a driver of Mediterranean forest dynamics. Growth and defense abilities of a pioneer (Pinus halepensis) and a late-successional (Quercus pubescens) Mediterranean forest species were evaluated under three different plant interference conditions: i) allelopathy simulated by application of aqueous needle extracts of Pinus, ii) resource competition created by the physical presence of a neighboring species (Pinus or Quercus), and iii) a combination of both allelopathy and competition. After 24 months of experimentation in simulated field conditions, Quercus was more affected by plant interference treatments than was Pinus, and a hierarchical response to biotic interference (allelopathy < competition < allelopathy + competition) was observed in terms of relative impact on growth and plant defense. Both species modulated their respective metabolic profiles according to plant interference treatment and thus their inherent chemical defense status, resulting in a physiological trade-off between plant growth and production of defense metabolites. For Quercus, an increase in secondary metabolite production and a decrease in plant growth were observed in all treatments. In contrast, this trade-off in Pinus was only observed in competition and allelopathy + competition treatments. Although Pinus and Quercus expressed differential responses when subjected to a single interference condition, either allelopathy or competition, species responses were similar or positively correlated when strong interference conditions (allelopathy + competition) were imposed.

Litter decomposition is a fundamental process of biogeochemical cycles, and there is a strong consensus that litter mixture interactions are one of the factors driving the decomposition process. A better understanding of how climate change can alter interactions between species and the litter decomposition process could facilitate projections of ecosystem functioning into the future. A 24-month litterbag decomposition experiment was carried out in a Mediterranean forest to analyze the effects of climate and species diversity changes on litter mixture interactions and the decomposition process. In the control plot, synergistic interactions increased with time and species diversity in litter mixtures, leading to more efficient litter decomposition. Drier conditions obtained in the field with a rain exclusion device decreased decomposition rates, resulting in three-fold less synergistic interactions and five-fold more antagonistic interactions during the decomposition process. Furthermore, synergistic interactions were better preserved in the drought conditions with increasing number of species. Our findings underline how a longer drought season could strongly affect the relationship between biodiversity and ecosystem functioning. Drier climate led to slower mass loss rates and a strong shift in the litter mixture interactions, with fewer synergistic interactions and more antagonistic interactions.