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

Abstract

SummaryTo 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.