Altered climatic sensitivity of tree growth following drought: a synthesis of tree-ring data from multiple species across the Southwest
Understanding the impacts of drought on forest health and tree growth is of major concern given projected climate change. For example, droughts may become more common in the Southwest due to extreme temperatures that will drive increased evapotranspiration and lower soil moisture, in combination with uncertain precipitation changes. We downloaded ~1.3 million tree-ring widths from the International Tree Ring Data Bank representing 11 species (9 conifers, 2 oaks) in the Southwest to evaluate the effects of drought on tree growth (ring widths). We categorized ring widths by the formation year in relation to drought (e.g., pre-drought, drought year, 1, 2, …, 10 years after drought), and we used a non-linear mixed effects model to estimate the effects of current and antecedent precipitation and temperature on tree growth during the post-drought recovery period. Our model simultaneously accounts for the effects of tree age, past growth rates, and current and antecedent climate to explore changes in tree growth behavior that are not typically evaluated in traditional dendrochronological analysis. This allowed us to assess changes in sensitivity of tree growth to precipitation and temperature at multiple time-scales following multiple droughts, and to evaluate drought resilience and resistance in these species.
After accounting for the effects of age and past ring-widths (autocorrelation), the effects of precipitation and temperature on ring widths following drought varied among species and time-since drought. Across species, ~27% of the climate effects (i.e., “sensitivities”) were significantly different from their pre-drought values. Species responses differed, with some showing increased sensitivities to precipitation and temperature following drought, and others showing decreased sensitivities. Furthermore, some species (e.g. Pseudotsuga menziesii) showed low resilience, with changes in growth sensitivities persisting up to 8 years following drought; others (e.g. Juniper spp.) showed relatively high resistance, such that their climatic sensitivities did not change following drought. Among species, different antecedent climate variables were more important during different periods following drought. Though only one species (Quercus lobata) responded positively to same-year precipitation before drought, eight species were positively affected by same-year precipitation the second year after drought. Similarly, ring width was positively correlated with previous year precipitation for six species before drought, but <3 species in each of the following eight years. Our results demonstrate that tree growth sensitivities to climate vary among species and with time-since-drought, raising questions about physiological mechanisms and implications for forest health under future drought and climate.