Long-term landscape changes in a subalpine spruce-fir forest in central Utah, USA

Background: In Western North America, increasing wildfire and outbreaks of native bark beetles have been mediated by warming climate conditions. Bioclimatic models forecast the loss of key high elevation species throughout the region. This study uses retrospective vegetation and fire history data to reconstruct the drivers of past disturbance and environmental change. Understanding the relationship among climate, antecedent disturbances, and the legacy effects of settlement-era logging can help identify the patterns and processes that create landscapes susceptible to bark beetle epidemics. Methods: Our analysis uses data from lake sediment cores, stand inventories, and historical records. Sediment cores were dated with radiometric techniques (C-14 and Pb-210/Cs-137) and subsampled for pollen and charcoal to maximize the temporal resolution during the historical period (1800 CE to present) and to provide environmental baseline data (last 10,500 years). Pollen data for spruce were calibrated to carbon biomass (C t/ha) using standard allometric equations and a transfer function. Charcoal samples were analyzed with statistical models to facilitate peak detection and determine fire recurrence intervals. Results: The Wasatch Plateau has been dominated by Engelmann spruce forests for the last similar to 10,500 years, with subalpine fir becoming more prominent since 6000 years ago. This landscape has experienced a dynamic fire regime, where burning events are more frequent and of higher magnitude during the last 3000 years. Two important disturbances have impacted Engelmann spruce in the historical period: 1) high-grade logging during the late 19th century; and (2) a high severity spruce beetle outbreak in the late 20th century that killed >90 % of mature spruce (> 10 cm dbh). Conclusions: Our study shows that spruce-dominated forests in this region are resilient to a range of climate and disturbance regimes. Several lines of evidence suggest that 19th century logging promoted a legacy of simplified stand structure and composition such that, when climate became favorable for accelerated beetle population growth, the result was a landscape-scale spruce beetle outbreak. The lasting impacts of settlement-era landscape history from the Wasatch Plateau, UT may be relevant for other areas of western North America and Europe where sufficient host carrying capacity is important in managing for resistance and resilience to outbreaks.