Long-Term Cambial Phenology Reveals Diverging Growth Responses of Two Tree Species in a Mixed Forest Under Climate Change.

The net effect of stress induced by climate change on forest functional dynamics remains uncertain. We monitored the dynamics of wood formation and cambial phenology for 11 consecutive years in two co-occurring tree species with different drought tolerance, Pinus sylvestris and Quercus pyrenaica, providing a unique long-term xylogenesis dataset (2012-2022). To assess the influence of climate on cambial and xylem developmental phases, we analyzed biologically meaningful climatic covariates across different time windows. In pine, late-winter temperatures strongly regulated the onset of cambial reactivation, advancing it 5.5 days per°C of warming, with reactivation occurring between early April and mid-May depending on winter thermal conditions. The onset of cambial reactivation in oaks was influenced both by soil water content and late-winter temperature, although the effect of temperature was weaker and restricted to a narrower time window than in pines. The effect of climate on the end of enlargement was nearly identical in both species, consistent with a turgor-driven regulation: higher maximum temperatures accelerated the process, whereas late-spring precipitation in late spring delayed it. In oaks and pines, the end of wood formation was advanced under hot and dry summers, inducing the early cessation of secondary wall lignification and, thus, reducing the length of xylogenesis. Despite the positive effect of warmer winters on earlier cambial resumption in pines, the duration of the enlargement phase (i.e., radial growth period) remained consistently shorter than in the more drought-tolerant oaks. Yet, the high phenological pasticity of pines to winter temperatures may also increase their growth duration, thereby partially buffering the negative effects of hotter droughts. The long dataset analyzed provided a robust assessment of species-specific phenological plasticity under climate change. Disentangling the net effect of climate on xylogenesis is crucial to understand future growth dynamics in mixed forests where more drought-tolerant species are becoming increasingly dominant.