Data from: The effect of competition on responses to drought and interannual climate variability of a dominant conifer tree of western North America

To accurately predict how ecosystems will respond to climate change – and
how management actions can influence such responses – scientists and
managers need a better understanding of how and when biotic interactions
modify climate–growth relationships. However, current research has largely
ignored the role of competition in modulating climate–growth relationships
of mature trees. In this study, we assessed the effect of competition on
tree responses to drought and interannual climate variability as well as
linkages between climate sensitivity and morphological characteristics of
the stem wood. We sampled 10 sites in north-eastern Washington, USA, and
used dendroecology to quantify responses of Douglas-fir (Pseudotsuga
menziesii) to drought and climate variability. Tree-ring series were
converted to basal area increment series, and the effects of competition
on climate–growth relationships were assessed at the tree and site levels
using a combination of correlation analyses and linear mixed-effects
models. Competition did not affect tree responses to extreme drought. When
soil moisture was below average, tree growth was tightly coupled to
climate variability for all trees, regardless of their competitive status.
However, in wet years, competition had a pronounced, positive effect on
climate sensitivity of growth. Trees with more competition from neighbours
exhibited a significantly higher proportion of sapwood area in latewood (a
morphological trait associated with greater drought resistance).
Synthesis. Our results suggest that a tree's ability to cope with
environmental variability is driven not just by the proximate effects of
neighbours on resource availability, but also by phenotypic plasticity and
long-term adaptations to competitive stress (such as changes in carbon
allocation). Findings have important implications for individual-tree and
stand-level growth models and may help managers better understand how
their activities will modify tree responses to climate change.