Growth phenology adjusts to seasonal changes in water availability in coexisting evergreen and deciduous mediterranean oaks

Abstract

Different leaf (evergreen vs. deciduous habit) and xylem (diffuse-vs. ring-porous wood) traits represent contrasting strategies to face seasonal changes in water availability and temperature. However, how contrasting leaf and xylem habits of coexisting tree species affect stem wood formation and tree-ring development remains poorly understood. Here, we investigated the spatio-temporal patterns of wood formation in two deciduous oaks (Quercus faginea and Quercus petraea) and two evergreen oaks (Quercus ilex and Quercus suber) coexisting in seasonally dry Mediterranean forests along an aridity gradient in Spain. We hypothesized that growth responses to drought and intra- and inter-annual growth patterns would differ between functional groups. We simulated intra- and interannual growth using a modified version of the Vaganov-Shashkin (VS) process-based, growth model. The VS model simulations were used to estimate growth changes under a high emission scenario (RCP 8.5) for the current distribution of the study oak species and to forecast their future performance under warm (4.8 C) conditions in the Iberian Peninsula. Our simulations indicate that climate warming would induce a shortening of the ringgrowth season and a reduction of radial growth in evergreen and deciduous Mediterranean oaks, particularly in dry sites from southern and eastern Iberia currently occupied by Q. ilex and Q. faginea. Evergreen oaks may better recover after dry periods than deciduous oaks by resuming growth after the summer drought. Low soil water availability in spring would be more detrimental to growth of deciduous oaks. Process-based growth models should be refined and validated to better forecast changes in tree growth as a function of climate.