Contrasting plastic and genetic responses of leaf unfolding and senescence to temperature in Quercus petraea.

Understanding the relative roles of phenotypic plasticity and genetic differentiation in shaping plant phenological responses is essential for predicting forest tree responses to climate warming. Although spring and autumn phenophases respond sensitively to rising temperatures, the underlying regulatory mechanisms may differ substantially, particularly across environmental gradients. We investigated intra-specific phenological variation in Quercus petraea across the French Pyrenees using a multi-environment experimental framework. This approach integrated 15 years of in situ phenological monitoring with common garden (CG) and reciprocal transplant experiments spanning an elevational gradient. We quantified key phenological traits-leaf unfolding, leaf senescence, and growing season length-and applied generalized linear mixed models to evaluate the relative effects of temperature, provenance-level differentiation, and their interactions. Leaf unfolding showed strong temperature-associated plastic responses and comparatively limited provenance-level differentiation. In contrast, leaf senescence showed weaker and less consistent temperature responses. Growing season length increased under warmer conditions, largely reflecting the combined effects of earlier leaf unfolding and variable senescence timing. Our findings reveal phase-specific differences in phenological responses, with stronger temperature-associated plasticity in spring than in autumn phenology. These results highlight the need for long-term, multi-environmental data for understanding tree phenology under a changing climate and provide insights for developing adaptive forest management strategies that maintain adaptive capacity.