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Mediterranean trees and shrubs form intra-annual density fluctuations (IADFs) in tree rings as a sign of their plasticity in wood formation in response to intraseasonal variations of environmental conditions. Different species show a different aptitude to form IADFs, due to their diverse ability to cope with climate stressors, since the occurrence of IADFs may affect plant hydraulics. Dendroecology and quantitative wood anatomy were used to characterise IADFs in Pinar pinea and Arbutus unedo co-occurring at a Mediterranean site in Italy. The relations between climate parameters (i.e. temperature and precipitation) and intra-annual tree-ring traits (i.e. IADF frequency and conduit size) were analysed to highlight the main triggers for IADF formation and their functional role.

Data showed that both species are characterised by a high plastic response to climate and formed a high frequency of L-IADFs (occurrence of earlywood-like conduits in latewood). The two species, although forming the same type of IADFs, showed different sensitivity to environmental factors. Pinus pinea showed a high dependence of tracheid size on temperature, while Arbutus unedo was more sensitive to precipitation in spring and autumn. Arbutus unedo promptly developed more than one IADF per year in response to rainfall events following drought periods.

The overall results were useful to compare the aptitude of the two species in forming IADFs and to highlight the factors priming their formation. This is useful to understand wood growth reactions to environmental drivers and to evaluate the adaptive capabilities in these two species, and thus to predict forest reactions after climate changes.