Xylem resistance to cavitation increases during summer in Pinus halepensis.

Cavitation resistance has often been viewed as a relatively static trait, especially for stems of forest trees. Meanwhile, other hydraulic traits, such as turgor loss point (Ψ tlp ) and xylem anatomy, change during the season. In this study, we hypothesized that cavitation resistance is also dynamic, changing in coordination with Ψ tlp . We began with a comparison of optical vulnerability (OV), microcomputed tomography (µCT) and cavitron methods. All three methods significantly differed in the slope of the curve,Ψ 12 and Ψ 88 , but not in Ψ 50 (xylem pressures that cause 12%, 88%, 50% cavitation, respectively). Thus, we followed the seasonal dynamics (across 2 years) of Ψ 50 in Pinus halepensis under Mediterranean climate using the OV method. We found that Ψ 50 is a plastic trait with a reduction of approximately 1 MPa from the end of the wet season to the end of the dry season, in coordination with the dynamics of the midday xylem water potential (Ψ midday ) and the Ψ tlp . The observed plasticity enabled the trees to maintain a stable positive hydraulic safety margin and avoid cavitation during the long dry season. Seasonal plasticity is vital for understanding the actual risk of cavitation to plants and for modeling species' ability to tolerate harsh environments.