Components of tree resilience: Effects of successive low-growth episodes in old ponderosa pine forests

Recent world-wide episodes of tree dieback have been attributed to increasing temperatures and associated drought. Because these events are likely to become more common, improved knowledge of their cumulative effects on resilience and the ability to recover pre-disturbance conditions is important for forest management. Here we propose several indices to examine components of individual tree resilience based on tree ring growth: resistance (inverse of growth reduction during the episode), recovery (growth increase relative to the minimum growth during the episode), resilience (capacity to reach pre-episode growth levels) and relative resilience (resilience weighted by the damage incurred during the episode). Based on tree ring analyses, we analyzed historical patterns of tree resilience to successive drought-induced low growth periods in ponderosa pine trees growing in unmanaged, remote forests of the Rocky Mountains. Low-growth periods registered in tree rings were related to anomalies in the Palmer drought severity index (PDSI) and were attributed to drought. Independently of the impact of a specific event, subsequent growth after a single low-growth episode was related to the growth prior to the event. Growth performance differed with tree age: young trees were overall more resistant to low-growth periods, but older trees recovered better from more recent events. Regardless of tree age, recently burned sites exhibited lower post-episode growth and lower resistance and resilience than unburned ones. We found mixed evidence for the cumulative effect of past low-growth episodes: overall, greater impacts of a prior event and greater cumulative effects of past low-growth periods caused a decrease in resistance. However, we did not find a progressive decrease in resilience over time in old trees. Our results highlight the value of using a combination of estimators to evaluate the different components of resilience. Specifically, while tree responses to disturbance depend on past disturbance episodes, the response is context-specific and depends on the impact the capacity to recover after disturbance. This suggests that recent increases in forest mortality under current climate trends could relate to thresholds on specific components of resilience (resistance, recovery, resilience itself) rather than to an overall loss of resilience over time. Identifying such thresholds and their underlying mechanisms is a promising area of research with important implications for forest management.