Xylem Pit Anatomy and Minimum Leaf Conductance Drive Drought Mortality in Pinus pinaster

Drought‐triggered forest die‐off events are commonly attributed to hydraulic failure, carbon starvation, or a combination of the two. Nevertheless, the anatomical and physiological traits that make trees vulnerable to drought in the field are often unknown, hindering predictive efforts. To identify these traits, we compared coexisting declining (D, heavily defoliated) and non‐declining (ND, lightly defoliated) trees. We studied a recent die‐off event affecting maritime pine (Pinus pinaster) in north‐eastern Spain that started after the severe 2017 drought. We compared the depth of soil water uptake, estimated using δ18O and δ2H in soil and xylem water samples, as well as field measurements. We also measured anatomical and physiological wood and leaf variables, paying particular attention to pit anatomy and minimum leaf conductance (gmin). The D trees were smaller in terms of diameter and height, and exhibited lower growth rates. They also formed tracheids with smaller lumen diameters and thinner cell walls than the ND trees. The measured soil depth was greater for ND than for D trees. Isotope data also indicated that ND trees used water from deeper soil layers than D trees during the late summer period of peak drought severity. No differences in the sapwood concentrations of non‐structural carbohydrates were found between the two tree types. The D trees had lower midday water potentials than ND trees, and the pressure inducing 50% loss of hydraulic conductance (P50) and gmin were higher in D trees. The D trees also exhibited lower torus overlap, margo flexibility and valve effect than ND trees. However, these differences in pit anatomy were observed in the 2010s when ND trees exhibited higher δ13C‐derived intrinsic water‐use efficiency. A combination of traits, such as a large pit aperture and a high gmin makes trees vulnerable to drought stress.