Xylem anatomical traits reveal different strategies of two Mediterranean oaks to cope with drought and warming

Performance and survival of Mediterranean trees depend on their capacity to cope with water stress. In harsh environments, xylem growth strategies are critical to ensure efficient water transport, though these are largely unknown for many species, including most Mediterranean oaks. We investigated xylem anatomical traits in tree-ring series of deciduous Quercus ithaburensis and Q. boissieri in the South-Eastern Mediterranean Basin to assess xylem response to climate variability, with special attention to dry and warm years and to temperature increase over the last decades. Abundant precipitation and low temperature during the wet season (November–April) benefited xylem formation of both species. However, intra-ring analysis revealed different patterns, as the size of early-formed vessels in Q. boissieri was related to previous-year ring, while in Q. ithaburensis it was strongly affected by current wet season conditions. During dry years, vessel size and number in Q. ithaburensis were strongly reduced, but they fully recovered in the following year (high resilience). Conversely, Q. boissieri vessels were barely reduced during dry or warm years (high resistance). Different strategies allow both species to overcome dry and warm years, but their capacity to withstand long-term warming seems to differ. Q. ithaburensis xylem traits did not evidence any trend over the last few decades, while Q. boissieri was facing a decline in some traits. Our analysis suggests that the growth strategies of Q. ithaburensis would be effective under future drier conditions, while temperature increase could induce a hydraulic deterioration and a potential decline in the less fit Q. boissieri.