Climatic memory of tree growth in the southwestern US

The controls on forest carbon cycling are the focus of much interest, as forests store nearly half of terrestrial carbon, can exhibit large temporal variability in flux rates, and will undergo significant stresses, and likely state shifts under climate change. Towards this, a large body of research on the mechanisms and controls on tree mortality from drought has given us insight into the physiological processes taking place in dying or declining trees. Despite this, understanding of the processes controlling tree growth, including the temporal and spatial variability among different species could be improved, particularly in the southwestern US (“Southwest”), where tree growth is quite variable. This dissertation addresses three broad weaknesses in current understanding of the climatic controls on tree growth in the Southwest. In particular, understanding of the physiological processes occurring in trees surviving drought, particularly in the years that follow droughts stress, is a key weakness in current knowledge. Second, understanding of the importance of antecedent climate, that is, conditions occurring multiple years before ring formation, for tree growth is also limited. Third, time-invariant approaches to modelling growth are likely insufficient to capture the variability in tree growth responses to climate, and recognition of plasticity in growth-climate responses and their temporal characteristics is likely needed. I review some of these limitation in chapter 1. In chapter 2, I quantify the legacy effects of drought across 10 species using a temporally correlated mixed effects model. I document long and complex legacies in tree growth-climate sensitivities (recovery times often ~ 5 years) and important species differences. Given this lagged importance of drought, in chapter 3 I apply a more temporally flexible stochastic antecedent model (SAM) to tree-ring chronologies to show that more than 50% of growth-drought sensitivities are driven by conditions occurring more than 2 years prior to ring formation, and that growth sensitivities vary strongly across the region, likely related to the North American Monsoon gradient. In chapter 4, I expand the SAM approach applied in chapter 3 to account for variation in growth-climate relationships related to La Nina years (dry winters) and their legacies, and show that both these relationships and their temporal characteristics (“memory”) are plastic. Taken together, the results from this dissertation call for a need to include more complex temporal variability in growth into predictive models. These results also demonstrate a continuing need for experiments to identify the mechanisms underlying drought legacy effects and complex tree growth patterns, with a focus on understanding the role of non-structural carbohydrates in driving variability in tree growth through time and space.