Variable Effects of Climate on Forest Growth in Relation to Ecosystem State

1 Abstract Changes in the frequency, duration, and severity of climate extremes are forecast to occur under global climate change. The impacts of climate extremes on forest productivity and health are complicated by potential interactions with disturbance events and forest stand dynamics. Such interactions may lead to non-linear forest growth responses to climate and disturbance involving thresholds and lag effects. The effects of stand dynamics on forest responses to climate and disturbance are particularly important given forest characteristics driven by stand dynamics can be modified through forest management with the goal of increasing forest resistance and resilience to climate change. We develop a hierarchical Bayesian state-space model in which climate effects on tree growth are allowed to vary over time and in relation to climate extremes, disturbance events, and stand dynamics. The model is a first step toward integrating stand dynamics into predictions of forest growth responses to climate extremes and disturbance. We demonstrate the insights afforded by the model through its application to a dendrochronology dataset comprising measurements from forest stands of varying composition, structure, and development stage in northeastern Minnesota. Results indicate that average forest growth was most sensitive to water balance variables describing climatic water deficit. Forest growth responses to water deficit over time were partitioned into responses driven by climatic threshold exceedances and interactions with forest tent caterpillar defoliation. Forest growth was both resistant and resilient to climate extremes with the majority of forest growth responses occurring after multiple climatic threshold exceedances across seasons and years. Interactions between climate and insect defoliation were observed in a subset of years with forest growth responses to climatic growing conditions in the opposite direction than expected. Forest growth was most sensitive to climate, regardless of driving factor, during periods of high stem density following major regeneration events when average inter-tree competition was high. Results suggest that forest growth resistance and resilience to interactions between climate extremes and insect defoliation can be increased through management steps such as thinning to reduce competition during early stages of stand development and small-group selection harvests to maintain forest structures characteristic of older, mature stands. Understanding the effects of climate on forest productivity is integral to predicting the response of forest ecosystems to global climate change. Forest growth responses to changing climatic conditions have important implications for sustainable forest management, a fundamental goal of which is to maintain healthy and productive forests in perpetuity. …