Xylem pit anatomy and minimum leaf conductance drive drought mortality in Pinus pinaster

Abstract

Drought‐triggered forest die‐off events are commonly attributed to hydraulic failure, carbon starvation, or a combination of thetwo. 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 Spainthat started after the severe 2017 drought. We compared the depth of soil water uptake, estimated using δ18 O and δ2H in soil andxylem 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 diameterand height, and exhibited lower growth rates. They also formed tracheids with smaller lumen diameters and thinner cell wallsthan the ND trees. The measured soil depth was greater for ND than for D trees. Isotope data also indicated that ND trees usedwater from deeper soil layers than D trees during the late summer period of peak drought severity. No differences in thesapwood concentrations of non‐structural carbohydrates were found between the two tree types. The D trees had lower middaywater potentials than ND trees, and the pressure inducing 50% loss of hydraulic conductance (P 50 ) and g min were higher in Dtrees. The D trees also exhibited lower torus overlap, margo flexibility and valve effect than ND trees. However, these differencesin pit anatomy were observed in the 2010s when ND trees exhibited higher δ13C‐derived intrinsic water‐use efficiency. Acombination of traits, such as a large pit aperture and a high gmin makes trees vulnerable to drought stress.

This study was funded by projects PID2021‐123675OB‐C43 andTED2021‐129770B‐C21 (Spanish Ministry of Science and Innovation[Ministerio de Ciencia, Innovación y Universidades]). We sincerelythank Francisco José García Breijo (Univ. Politécnica de Valencia,Spain) for his help with wood anatomy analyses and Aragón Govt. forsampling permissions and facilities