Variable Effects of Climate on Forest Growth in Relation to Climate Extremes, Disturbance, and Forest Stand Dynamics

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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 stand dynamics. 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 allowing climate effects on tree growth to vary over time and in relation to climate extremes, disturbance events, and stand dynamics. We apply the model to a dendrochronology dataset comprising measurements from forest stands of varying composition, structure, and development stage in northeastern Minnesota. Results indicate average forest growth was most sensitive to variables describing climatic water deficit. Forest growth responses to water deficit 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 or insect defoliation events. Forest growth was most sensitive to water deficit 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.

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