Understanding drivers of disturbances across scales is critical as environmental constraints change in a warming climate. Outbreaks of native bark beetles (Curculionidae: Scolytinae) are key natural disturbances that shape the structure and function of conifer forests across the northern hemisphere. While drivers of bark beetle outbreaks have been studied extensively at spatial scales ranging from stands to continents, within‐stand processes governing individual tree mortality in an outbreak are less well understood. Here, we use a spatially explicit long‐term monitoring dataset of a lodgepole pine (Pinus contorta var. latifolia) forest (>9000 individually mapped trees in three 2‐ha plots) impacted by a severe mountain pine beetle (Dendroctonus ponderosae) outbreak to explore interactions among fine scale drivers of beetle‐caused tree mortality. Using a Bayesian spatial modeling approach, we evaluated how tree scale and tree neighborhood scale characteristics interact with tree size to mediate host tree susceptibility to mountain pine beetle outbreak in the Southern Rocky Mountains (USA). We found evidence that both tree growth rate preceding the outbreak and neighborhood structure (within a 10 meter radius of the host tree) mediate the effect of tree size, and that the direction and magnitude of these mediating effects vary with tree size. Tree scale mortality probability increased with pre‐outbreak growth rate for small to medium sized host trees (~10–25 cm diameter), but that same effect was not detected for large trees. Conversely, tree scale mortality probability increased with greater neighborhood density, with the most pronounced effects for medium to large sized host trees (~15–30 cm diameter). Within‐stand topographic variability was not an important predictor of mortality probability; among stands, however, the stand in the driest topographic position experienced the greatest overall mortality. By explicitly considering how within‐stand heterogeneity mediates individual tree scale susceptibility to bark beetle outbreak, our findings bridge an important gap in understanding multi‐scale drivers of disturbance dynamics.

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.

Aim Ecological disturbances are increasing as climate warms, and how multiple disturbances interact spatially to drive landscape change is poorly understood. We quantified burn severity across fire regimes in reburned forest landscapes to ask how spatial patterns of high‐severity fire differ between sequential overlapping fires and how landscape heterogeneity is shaped by cumulative disturbance patterns. We also characterized the amount and configuration of an emerging phenomenon: areas burned as high‐severity fire twice in successive fires. Location Northwest USA. Time period 1984–2020. Major taxa studied Forests of western continental USA. Methods We used a field‐calibrated atlas of satellite‐measured burn severity across diverse fire regimes (more than three decades, >200 short‐interval fires) to quantify landscape metrics of high‐severity (>75% tree mortality) fire in sequential overlapping short‐interval fires. We used generalized linear models to test differences in individual and cumulative landscape patterns of burn severity following the first and second fires. Results The amount of severe wildfire and patch size/configuration were generally similar between successive overlapping fires and across fire regimes. However, overlapping individual fires produced cumulative landscape patterns of recent high‐severity fire that were consistently more homogeneous after two fires, with greater distances to remaining mature forest. Additionally, 19–25% of landscapes affected by short‐interval fires burned at high severity in both fires, highlighting the spatial extent of repeatedly and severely disturbed forests. Main conclusions When two individually heterogeneous fires overlap, burn mosaics can fit together like puzzle pieces, whereby twice‐burned landscapes are composed of large and simple‐shaped patches of cumulative recent high‐severity fire interspersed with small patches of mature/old forest. These cumulative spatial outcomes of interacting disturbances can be mechanisms of shifting ecosystem dynamics as global change unfolds and reburns continue.

Increasing wildfire activity in forests worldwide has driven urgency in understanding current and future fire regimes. Spatial patterns of area burned at high severity strongly shape forest resilience and constitute a key dimension of fire regimes, yet remain difficult to predict. To characterize the range of burn severity patterns expected within contemporary fire regimes, we quantified scaling relationships relating fire size to patterns of burn severity. Using 1615 fires occurring across the Northwest United States between 1985 and 2020, we evaluated scaling relationships within fire regimes and tested whether relationships vary across space and time. Patterns of high‐severity fire demonstrate consistent scaling behaviour; as fire size increases, high‐severity patches consistently increase in size and homogeneity. Scaling relationships did not differ substantially across space or time at the scales considered here, suggesting that as fire‐size distributions potentially shift, stationarity in patch‐size scaling can be used to infer future patterns of burn severity.

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