Components of tree resilience: effects of successive low‐growth episodes in old ponderosa pine forests

Abstract

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.

Recent Drought-Induced Vitality Decline of Black Pine (Pinus nigra Arn.) in South-West Hungary—Is This Drought-Resistant Species under Threat by Climate Change?

“…For the drought years or multi-year drought periods we calculated four indices of drought reaction (resistance, recovery, resilience and relative resilience) following Lloret et al [63]. Resistance means that the trees are able to withstand a drought without showing a drop in ring width, thus, it can be computed as a ratio of ring widths during and before a drought period.…”

Assessing future suitability of tree species under climate change by multiple methods: a case study in southern Germany

“…Arguably, the diameter increment reaction in drought years may not unambiguously indicate a species' vulnerability to drought (Breda et al 2006, McDowell et al 2008, Lloret et al 2011). In our case, for instance, even the more drought-sensitive beech and maple recovered to the pre-drought increment level or even overshot , Zimmermann et al 2015.…”

“…Therefore, a superposed epoch analysis (SEA) was performed for each tree ring chronology with 10,000 bootstrapped resamples and 5-year lag before and after the drought events (Bunn 2008). Tolerance indices for resistance and recovery were calculated for every drought year and focal species using a five year reference window (Lloret et al 2011). Significant differences between species were revealed with a one-way analysis of variance (p < 0.05).…”

Forest Growth Responses to Drought at Short- and Long-Term Scales in Spain: Squeezing the Stress Memory from Tree Rings

“…In a short-term perspective (1-5 years), the comparison of forest growth before, during, and after the drought event using resilience indices provides information on forest recovery after drought (Lloret et al, 2011). However, this approach lacks a longterm (10-30 years) perspective of forest response to drought that can only be detected by comparing growth trends of different tree individuals several years or decades after drought started and considering the cumulative effects of consecutive droughts (Peltier et al, 2016).…”