Climatic stress and species interactions shape tree vitality decline in Iberian pine species

Understanding vitality decline is essential, as it precedes tree mortality. Anticipating this process requires the identification of its main driving factors, including environmental conditions, stand structure, and species-specific responses to species mixing, whose combined effects remain poorly understood. To address this gap, tree vitality decline in the Iberian pine forests ( Pinus halepensis, P. nigra, P. pinaster, P. pinea, P. sylvestris, and P. uncinata ) was analysed by developing vitality decline models using a large Spanish National Forest Inventory dataset (∼280000 trees), including 15638 plots in monospecific stands and 5958 plots in 25 representative two-species mixtures spanning all climatic conditions across the Iberian Peninsula. First, we examined monospecific stands to disentangle the role of environmental and endogenous drivers of vitality decline. In these stands, tree vitality decline was mainly driven by temperature and moderated by site quality, with species-specific sensitivities influenced by endogenous factors such as tree size and stand development. Building on these results, we assessed the effect of mixed stands and climatic conditions on vitality decline. The influence of species mixing depended on species composition, suggesting that admixture can either buffer or exacerbate vitality decline depending on asymmetric competition, vertical stand structure, and the proportion of the target species, which in certain mixtures may result in greater conspecific dominance and intensified competitive stress. Climatic conditions, and specifically temperature, strongly influence patterns of tree vitality decline. Overall, tree vitality is shaped by climatic stress, species identity, and forest composition, suggesting that careful selection of species mixtures could enhance forest resilience.