Abstract
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.
Highlights
Native bark beetles (Curculionidae: Scolytinae) are key drivers of ecosystem structure and function in Earth’s temperate and boreal forests [1,2,3]
The methods used in the present study provide a framework that can be extended to other regions by leveraging freely available spatial data (e.g., Landsat imagery, land cover maps, insect and disease surveys) and existing field datasets to quantify the effects of biotic disturbances on forest structure and landscape pattern
By developing a mapping approach that links field data with Landsat image time series (LTS) products, we found that over 10,000 km2 of subalpine forest area has been affected by three important bark beetle species—mountain pine beetle, spruce beetle, and western balsam bark beetle—in just two decades
Summary
Native bark beetles (Curculionidae: Scolytinae) are key drivers of ecosystem structure and function in Earth’s temperate and boreal forests [1,2,3]. Bark beetles can cause extensive tree mortality through coordinated mass attacks, altering important ecosystem services such as carbon storage, wildlife habitat, nutrient cycling, and water resource provisioning [4,5,6]. Despite the ubiquity of these insects and their importance for forest dynamics, characterizing even basic attributes such as the affected area and severity of effects remains challenging, at relatively fine While the spatial patterns of ecological disturbances are known to influence vegetation dynamics and a wide range of ecosystem processes [9,10,11], the spatial patterns of bark beetle outbreaks have rarely been explored, across regions with irruptions of multiple beetle species. Improved mapping efforts that combine detailed field data and remotely sensed products are critical to refining understanding of the extent, severity, and spatial patterns of outbreaks
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