A retrospective, dual-isotope approach reveals individual predispositions to winter-drought induced tree dieback in the southernmost distribution limit of Scots pine

Winter-drought induced forest diebacks in the low-latitude margins of species’ distribution ranges can provide new insights into the mechanisms (carbon starvation, hydraulic failure) underlying contrasting tree reactions.We analysed a winter-drought induced dieback at the Scots pine’s southern edge through a dual-isotope approach (D13C and d18O in treering cellulose).We hypothesized that a differential long-term performance, mediated by the interaction between CO2 and climate, determined the fates of individuals during dieback. Declining trees showed a stronger coupling between climate, growth and intrinsic water-use efficiency (WUEi) than nondeclining individuals that was noticeable for 25 years prior to dieback. The rising stomatal control of water losses with time in declining trees, indicated by negative D13C-d18O relationships, was likely associated with their native aptitude to grow more and take up more water (suggested by larger tracheid lumen widths) than non-declining trees and, therefore, to exhibit a greater cavitation risk. Freeze-thaw episodes occurring in winter 2001 unveiled such physiological differences by triggering dieback in those trees more vulnerable to hydraulic failure. Thus, WUEi tightly modulated growth responses to long-term warming in declining trees, indicating that co-occurring individuals were differentially predisposed to winter-drought mortality. These different performances were unconnected to the depletion of stored carbohydrates.

This work was partially supported by the DGI projects CGL2009-13079-C02-01 and CGL2011-26654 and grant FP7- PEOPLE-2009-RG-246725 (SMARTREES). We acknowledge R. Hernández, M. Maestro, V. Pérez-Fortea and P. Sopeña for field and technical assistance. J.P.F. is supported by a postdoctoral contract from the ‘Ramon y Cajal’ programme (MCINN).