Data from:Quantitative wood anatomical characteristics, basal area increments (BAI) and tree-ring derived intrinsic water-use efficiency (iWUE) for three coniferous tree species in Central Spain

Increasing aridity is a major threat to forests worldwide. Understanding
tree functional constraints under drought and their impacts on resilience
and mortality among species is crucial to assess global change on forests.
We analyzed the long-term drought and atmospheric CO₂ responses in three
Mediterranean co-occurring species with differing drought tolerances
(Pinus pinaster < Pinus pinea < Juniperus oxycedrus). In
this mixed forest, P. pinaster exhibited widespread mortality and
mistletoe infection, P. pinea showed scattered mortality, and J. oxycedrus
showed no decline. Using tree-ring data (1978–2016), we compared intrinsic
water-use efficiency (iWUE) and xylem hydraulic traits in healthy and
non-healthy individuals of both pine species and healthy junipers. Healthy
P. pinaster trees produced a more hydraulically efficient xylem, with
wider lumen tracheids, than non-healthy trees, whereas P. pinea showed no
anatomical differences between health status. Healthy P. pinaster
displayed greater anatomical plasticity, adjusting hydraulic conductivity
and cell-wall thickness to water availability. Despite small differences
in average iWUE, the response of iWUE to rising CO2 and drought differed
between species and health status. J. oxycedrus and P. pinea showed steady
iWUE increases, but P. pinea experienced periods of stagnation following
an extreme drought, later recovering regardless of health status. In
contrast, iWUE in P. pinaster plateaued for over 20 years after a
drought-inducing drought, particularly in non-healthy, mistletoe-infected
trees. Differences in iWUE response to CO2 and anatomical plasticity to
drought may explain the contrasting mortality patterns among these
coniferous species. Our results suggest a long-term decline spiral in P.
pinaster induced by low hydraulic efficiency in drought-induced defoliated
trees and limited physiological response to rising CO₂ and drought.
Increasing drought stress makes pine recovery increasingly unlikely.