Does the legacy of historical thinning treatments foster resilience to bark beetle outbreaks in subalpine forests?

Abstract

Promoting ecological resilience to increasing disturbance activity is a key management priority under warming climate. Across the Northern Hemisphere, tree mortality from widespread bark beetle outbreaks raises concerns for how forest management can foster resilience to future outbreaks. Density reduction (i.e., thinning) treatments can increase vigor of remaining trees, but the longevity of treatment efficacy for reducing susceptibility to future disturbance remains a key knowledge gap. Using one of the longest‐running replicated experiments in old‐growth subalpine forests, we measured stand structure following a recent (early 2000s) severe mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak to examine the legacy of historical (1940s) thinning treatments on two components of resilience. We asked: ‘How did historical thinning intensity affect (1) tree‐scale survival probability and stand‐scale survival proportion (collectively “resistance” to outbreak) for susceptible trees (lodgepole pine [Pinus contorta] ≥ 12 cm diameter) and (2) post‐outbreak stand successional trajectories?’ Overall outbreak severity was high (MPB killed 59% of susceptible individuals and 78% of susceptible basal area), and historical thinning had little effect on tree‐scale and stand‐scale resistance. Tree‐scale survival probability decreased sharply with increasing tree diameter and did not differ from the control (uncut stands) in the historical thinning treatments. Stand‐scale proportion of surviving susceptible trees and basal area did not differ from the control in historically thinned stands, except for treatments that removed nearly all susceptible trees, in which survival proportion approximately doubled. Despite limited effects on resistance to MPB outbreak, the legacy of historical treatments shifted dominance from large‐diameter to small‐diameter lodgepole pine by the time of outbreak, resulting in historically thinned stands with ~2× greater post‐outbreak live basal area than control stands. MPB‐driven mortality of large‐diameter lodgepole pine in control stands and density‐dependent mortality of small‐diameter trees in historically thinned stands led to convergence in post‐outbreak live tree stand structure. One exception was the heaviest historical thinning treatments (59–77% basal area removed), for which sapling dominance of shade‐tolerant, unsusceptible conifers was lower than control stands. After six decades, thinning treatments have had minimal effect on resistance to bark beetle outbreaks, but leave persistent legacies in shaping post‐outbreak successional trajectories.

Rocky Mountain forests are poised to recover following bark beetle outbreaks but with altered composition

“…We compiled field datasets that were collected for 16 studies of bark beetle effects on subalpine forests throughout the SRM (Andrus, Chai, et al, 2021; Andrus, Hart, & Veblen, 2020; Audley et al, 2020; Carlson et al, 2020; Carter et al, 2022; Chapman, Unpublished Data; Coop et al, Unpublished Data; Fornwalt et al, 2018; Gill et al, 2017; Harvey et al, 2021; Morris et al, 2022; Nigro et al, 2022; Pappas et al, 2020; Redmond & Kelsey, 2018; Rhoades et al, 2020; Schapira, Stevens‐Rumann, Shorrock, Hoffman, & Chambers, 2021). This combined dataset includes 969 field plots with nearly 40,000 records of trees (≥3 m in height) and saplings (≥1.4 m and <3 m), tallies of over 40,000 juveniles (<1.4 m) and height growth rates for nearly 2000 saplings and juveniles (Figure 1; Appendix S1).…”

Spatial interactions among short‐interval fires reshape forest landscapes

“…Initially, we used a test statistic [Menning Departure Index ( M ); Menning et al, 2007] that characterizes the degree of difference in each pair of distributions. The value of M is useful in comparing differences in shape and skewness direction between two distributions that can have different absolute frequencies or ranges, such as tree size distributions among forests (Morris et al, 2022). Differences between two distributions are indicated by the sign of M (a right shift from a reference to a test distribution is positive, whereas a left shift from a reference to a test distribution is negative), with the magnitude of M indicating the distance of the shift between distributions (Supporting Information Figure S2).…”

The importance of natural forest stewardship in adaptation planning in the United States

“…Although improved resilience and protection of biodiversity are goals of proposed adaptation management, active management may, in some cases, have little effect on future stand resistance (Morris et al, 2022), is often unnecessary for natural forest resilience (e.g., Cansler et al, 2022; Hart et al, 2015) and biodiversity (Thom & Seidl, 2016; Viljur et al, 2022), and is generally counterproductive to carbon storage, structural complexity, tree diversity, and resistance to invasive species. (Donato et al, 2013; Miller et al, 2018; Patton et al, 2022; Schwilk et al, 2009; Young et al, 2017; Table 1).…”

“…A recent review article found that “most of the inference about intervention options has been drawn from theory rather than empiricism” (Prober et al, 2019); and according to the latest IPCC report, there is almost no evaluation of the success of adaptation approaches in the scientific literature (Parmesan et al, 2022). Establishing more long‐term experiments with adaptation treatments and unmanaged controls (e.g., Morris et al, 2022) would provide much‐needed information on this topic.…”

“…Certainly, insects, disease, wind, and wildfire account for current and future tree death and carbon losses in forests (Thom & Seidl, 2016); however, in many cases disturbances such as insect outbreaks that target dominant tree species result in increased tree diversity in the post‐outbreak stand (Morris et al, 2022). Additionally, carbon losses from fire and insects are often much less than models predict.…”