Subalpine fir mortality in Colorado is associated with stand density, warming climates and interactions among fungal diseases and the western balsam bark beetle

Abstract

Subalpine fir mortality complex has caused significant damage to high elevation forests within Colorado. For the past two decades the climate of spruce-fir forests has trended towards being warmer and drier, which indirectly affects subalpine fir mortality. We examined potential links among abiotic (i.e. deviations in temperature and precipitation), biotic [Armillaria root disease (Armillaria spp.), Western balsam bark beetle (Dryocoetes confusus), a black stain fungus (Ophiostoma dryocoetidis)] and forest structure factors with mortality in subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii) dominated forests of Colorado. Sampling was performed in two separate field surveys: 1. Statewide characterization plots and 2. Stand health monitoring plots. The statewide characterization plots were compared to aerial surveys and to establish suitable locations for stand health plots. Stand health monitoring plots collected direct measurements of abiotic and biotic factors affecting forests, with an emphasis on identifying the presence of D. confusus and Armillaria spp. The objectives of this study were to determine: (1) Is spruce-fir morality observed at ground level similar to that estimated through aerial surveys? (2) Is the presence dead subalpine fir and Englemann spruce on a plot associated with site and stand characteristics?, (3) What factors are associated with the incidence of Armillaria spp., Dr. confusus, and Dendroctonus rufipennis?, and (4) Do trends in warming temperatures and reductions in precipitation in the past five years influence subalpine fir mortality and/or the presence of biotic agents? Our results suggested that the presence of biotic agents (Dr. confusus, De. rufipennis, Armillaria spp., and O. dryocoetidis) and stand density are associated with spruce-fir mortality, whereas climatic factors had a direct association to the presence of Armillaria and were indirectly associated with mortality via Dr. confusus. Increasing maximum summer temperatures were found to be associated with the presence of Armillaria spp. The climatic variables investigated in this study did not significantly link to incidence of Dr. confusus, yet stand density was associated with increasing prevalence of Dr. confusus. The presence of Armillaria spp. and Dr. confusus were significantly related to tree decline, but several other factors are also associated with subalpine fir mortality, suggesting complex interactions of abiotic and biotic factors are likely involved. We hypothesized that increasing stand density, drought and the presence of Dr. confusus and Armillaria spp. are threatening Colorado spruce-fir forests. Understanding the factors that are involved in subalpine fir mortality are important for management.