Effects of Bark Beetle Outbreaks on Forest Landscape Pattern in the Southern Rocky Mountains, U.S.A.

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.

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

“…Outbreak severity has also shaped forest conditions and processes following recent outbreaks in the SRM, with remotely sensed outbreak severity improving our predictions of post‐outbreak basal area and height growth rates (Figures 5a and 6a). The severity of recent bark beetle outbreaks has been widely variable throughout the SRM, with stands ranging from low‐level mortality (<5% basal area), to a near‐total loss of overstory canopy (>90%; Rodman, Andrus, et al, 2021). Indeed, severity had very limited autocorrelation at spatial scales >5 km and patches of near‐total canopy loss were small (<0.24 km 2 ) and isolated (Rodman, Andrus, et al, 2021).…”

“…The severity of recent bark beetle outbreaks has been widely variable throughout the SRM, with stands ranging from low‐level mortality (<5% basal area), to a near‐total loss of overstory canopy (>90%; Rodman, Andrus, et al, 2021). Indeed, severity had very limited autocorrelation at spatial scales >5 km and patches of near‐total canopy loss were small (<0.24 km 2 ) and isolated (Rodman, Andrus, et al, 2021). Such variability has created heterogeneous forest overstory conditions throughout subalpine forests of the SRM, a pattern which is further elucidated by our species‐specific maps of live basal area.…”

“…Map of the study area and sampled field plots (white circles) in the Southern Rocky Mountains, USA (EPA Level III Ecoregion #21; grey polygon). Colours (orange to purple) indicate the remotely sensed severity of bark beetle outbreaks from 1997 to 2019 (Rodman, Andrus, et al, 2021). Grey areas within the regional boundary were not analysed because they were unforested, recently logged, recently burned, outside the subalpine zone or unaffected by the most abundant bark beetle species (USFS, 2020).…”

Overlapping outbreaks of multiple bark beetle species are rarely more severe than single‐species outbreaks

“…Colored areas show the cumulative disturbance severity (percent of pre‐outbreak tree basal area killed by bark beetles) during the 1999–2019 study period. Disturbance severity data are from Rodman et al (2021).…”

The Impact of the Neophyte Tree Fraxinus pennsylvanica [Marshall] on Beetle Diversity under Climate Change

“…Climate change facilitates the distribution of pest species, allowing them to expand into new ranges [2,53,54]. Bark beetles (Scolytinae) attacking conifers are among the best-known examples [55]; they can cause extensive tree mortality through coordinated mass attacks, whilst their effects on important ecosystem services such as carbon storage, nutrient cycling, and water provisioning are highly variable depending on the structure, composition, and spatial patterns of the vegetation [55].…”