Lower growth and production of latewood intra-annual density fluctuations due to drought-triggered forest die-off

Hotter droughts and aridification are causing forest die-off episodes worldwide. These events are characterized by canopy dieback and elevated mortality rates, but not all trees are equally affected with neighboring conspecifics showing contrasting vigor. Tree-ring data have been used to forecast die-off because of the contrasting growth rates declining (D) and non-declining (ND) trees show before tree death. However, discrete wood features such as latewood intra-annual density fluctuations (IADFs) have not been considered despite they record specific climate events. We hypothesized that D trees were less able to use water from rare summer-autumn rainfall pulses, which trigger IADF formation. We evaluated radial growth (tree-ring width) and latewood IADFs production in D and ND trees of two Mediterranean Pinus pinaster stands, which showed a strong die-off episode after the extreme 2017 drought. Wood anatomy and intra-annual growth rates and their climate drivers were also studied in one site, where dieback and mortality were annually monitored. The D trees significantly grew more in the past, particularly during wet decades, but tended to form less IADFs than ND trees, albeit differences were not significant. In the two decades before the die-off onset, ND trees formed tracheids with wider lumen and thicker walls than D trees. High precipitation in late summer and early autumn enhanced the formation of IADFs, a result supported by simulations of the Vaganov-Shashkin model. These findings suggest a greater ability of D trees to grow more in response to spring rainfall during wet periods, but a higher vulnerability in response to recent drought stress. In contrast, ND trees show a higher capacity to recover after the summer drought and to form latewood IADFs, a feature which can be further investigated as prognostic tool for die-off.