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Amplified by warming temperatures and drought, recent outbreaks of native bark beetles (Curculionidae: Scolytinae) have caused extensive tree mortality throughout Europe and North America. Despite their ubiquitous nature and important effects on ecosystems, forest recovery following such disturbances is poorly understood, particularly across regions with varying abiotic conditions and outbreak effects.To better understand post‐outbreak recovery across a topographically complex region, we synthesized data from 16 field studies spanning subalpine forests in the Southern Rocky Mountains, USA. From 1997 to 2019, these forests were heavily affected by outbreaks of three native bark beetle species (Dendroctonus ponderosae, Dendroctonus rufipennis and Dryocoetes confusus). We compared pre‐ and post‐outbreak forest conditions and developed region‐wide predictive maps of post‐outbreak (1) live basal areas, (2) juvenile densities and (3) height growth rates for the most abundant tree species – aspen (Populus tremuloides), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta) and subalpine fir (Abies lasiocarpa).Beetle‐caused tree mortality reduced the average diameter of live trees by 28.4% (5.6 cm), and species dominance was altered on 27.8% of field plots with shifts away from pine and spruce. However, most plots (82.1%) were likely to recover towards pre‐outbreak tree densities without additional regeneration. Region‐wide maps indicated that fir and aspen, non‐host species for bark beetle species with the most severe effects (i.e. Dendroctonus spp.), will benefit from outbreaks through increased compositional dominance. After accounting for individual size, height growth for all conifer species was more rapid in sites with low winter precipitation, high winter temperatures and severe outbreaks.Synthesis. In subalpine forests of the US Rocky Mountains, recent bark beetle outbreaks have reduced tree size and altered species composition. While eventual recovery of the pre‐outbreak forest structure is likely in most places, changes in species composition may persist for decades. Still, forest communities following bark beetle outbreaks are widely variable due to differences in pre‐outbreak conditions, outbreak severity and abiotic gradients. This regional variability has critical implications for ecosystem services and susceptibility to future disturbances. [ABSTRACT FROM AUTHOR]

Climate-driven increases in disturbance frequency and extent augment the potential for compounded disturbances. Drawing on well-studied forests that experienced successive disturbances, we asked: (1) how does post-fire cover of litter, herbaceous cover and bare ground vary between stands affected by combinations of blow-down, insect outbreak, and fire? (2) How do post-fire relationships between ground cover and conifer regeneration vary with recent disturbance history? We measured ground cover and conifer regeneration from 2003 to 2014 following stand-replacing fires in 2002. Burned stands were either blown down in 1997, affected by a 1940s Dendroctonus rufipennis (spruce beetle; SB) outbreak, or neither. Implementing mixed-effects models, we measured the relationships between pre-fire stand attributes (structural stage, canopy dominance and combination of disturbances) and post-fire ground cover and between post-fire ground cover and conifer regeneration. Fire-only stands had more litter and herbaceous cover post fire than other stands (P < 0.05). Fir regeneration increased with litter in stands that only burned, but decreased with litter in stands that were first blown down. Similarly, pine and fir regeneration increased with herbaceous cover after fire-only, but did the opposite in stands affected by the SB outbreak. Pre-fire legacies can modulate the effects of ground cover on plant regeneration. Post-fire organic litter and herbaceous cover depend on whether or not other disturbances precede fire. Resulting differences in ground cover affect post-fire regeneration of conifers. Litter cover promoted high-density conifer regeneration after fire-only, but inhibited it when blow-down or beetle outbreak preceded fire. Likewise, herbaceous cover promoted conifer regeneration after fire-only, but inhibited it when bark beetle outbreak preceded fire. [ABSTRACT FROM AUTHOR]

Since the late 1990s, extensive outbreaks of native bark beetles (Curculionidae: Scolytinae) have affected coniferous forests throughout Europe and North America, driving changes in carbon storage, wildlife habitat, nutrient cycling, and water resource provisioning. Remote sensing is a crucial tool for quantifying the effects of these disturbances across broad landscapes. In particular, Landsat time series (LTS) are increasingly used to characterize outbreak dynamics, including the presence and severity of bark beetle-caused tree mortality, though broad-scale LTS-based maps are rarely informed by detailed field validation. Here we used spatial and temporal information from LTS products, in combination with extensive field data and Random Forest (RF) models, to develop 30-m maps of the presence (i.e., any occurrence) and severity (i.e., cumulative percent basal area mortality) of beetle-caused tree mortality 1997–2019 in subalpine forests throughout the Southern Rocky Mountains, USA. Using resultant maps, we also quantified spatial patterns of cumulative tree mortality throughout the region, an important yet poorly understood concept in beetle-affected forests. RF models using LTS products to predict presence and severity performed well, with 80.3% correctly classified (Kappa = 0.61) and R2 = 0.68 (RMSE = 17.3), respectively. We found that ≥10,256 km2 of subalpine forest area (39.5% of the study area) was affected by bark beetles and 19.3% of the study area experienced ≥70% tree mortality over the twenty-three year period. Variograms indicated that severity was autocorrelated at scales < 250 km. Interestingly, cumulative patch-size distributions showed that areas with a near-total loss of the overstory canopy (i.e., ≥90% mortality) were relatively small (<0.24 km2) and isolated throughout the study area. Our findings help to inform an understanding of the variable effects of bark beetle outbreaks across complex forested regions and provide insight into patterns of disturbance legacies, landscape connectivity, and susceptibility to future disturbance. [ABSTRACT FROM AUTHOR]