Growth responses of old growth trees to climate along a vertical canopy gradient
Large old-growth trees play critical roles in forests, sequestering carbon, providing habitat, and shaping future forest dynamics. Understanding growth-climate relationships of large old-growth trees is therefore important for assessing future forest productivity, while changes in growth-climate relationships may be precursors to reduced vigor and mortality which have implications for future forest composition and structure. In tall trees, hydraulic constraints can limit growth and photosynthesis along a vertical canopy gradient, but relationships between tree growth and climate are poorly understood along vertical canopy gradients because dendrochronology studies of growth-climate relationships typically rely on increment cores taken at the base of trees. In this study we examined long- and short-term growth responses of old growth trees to climate along a vertical canopy gradient in a mixed-conifer forest in the California Sierra Nevada, USA. We used a unique tree-ring data set collected at three heights (base of tree, base of live crown, and top of tree) in tall old-growth trees of four conifer species (Abies concolor, Calocedrus decurrens, Pinus jeffreyi, and Pinus lambertiana). Relationships between climate variables and detrended growth chronologies were quantified using bootstrapped correlation coefficients. Response, resistance, and resilience to drought events were calculated from series basal area increments
Detrended growth chronologies were positively associated with precipitation and negatively associated with summer temperature. These relationships varied by species, but growth at higher canopy positions was generally more sensitive to climate. Growth responses to drought were more pronounced higher along the vertical canopy gradient, although again this varied by species. Species differences in growth-climate relationships along a vertical canopy gradient were broadly consistent with ecophysiological differences in water regulation between the species studied. This study suggests growth-climate relationships determined using standard dendrochronology methods at the base of trees may underestimate the sensitivity of large old-growth trees to climate stress. This may be especially true in relation to tree responses to drought events, which are projected in the future to become more frequent and severe in the American southwest.