Phenology, growth, and drought: how species interactions shape intra-annual tree dynamics in Mediterranean Forests

In Mediterranean forests, where seasonal drought imposes severe physiological constraints, the timing and dynamics of tree growth are shaped not only by climate but also by interspecific interactions. However, how these factors influence intra-annual growth phenology remains poorly understood. Here, we used biweekly band dendrometer data from 154 trees across seven pure and mixed sample plots in the Spanish Northern Plateau to investigate how stand composition and interspecific competition affect phenological events, seasonal growth rates, and tree water deficit (TWD) in four tree species: Pinus pinea , Pinus pinaster , Quercus ilex and Juniperus thurifera . We found that the identity of surrounding species significantly altered the timing and magnitude of growth, particularly in spring and early summer. Species mixing tended to advance spring onset in J. thurifera , but delayed it in Q. ilex and P. pinaster . Under strong interspecific competition most species showed increased growth rates during dry springs, consistent with the stress-gradient hypothesis. In contrast, Q. ilex exhibited the opposite pattern under wetter conditions, suffering increased TWD and reduced growth in mixtures. These divergent responses were driven by species-specific water-use strategies and seasonal asynchrony in root activity. Our findings demonstrate that interspecific interactions can either buffer or amplify climatic stress, depending on species identity and season. By linking cambial phenology, growth dynamics, and water deficit under varying competition contexts, this study provides new insight into the mechanisms of climate buffering in Mediterranean forests – and underscores the importance of species composition in supporting forest resilience under climate change.