Modeling the interactive effects of spruce beetle infestation and climate on subalpine vegetation

Abstract

In the subalpine zone of the Rocky Mountains, climate change is predicted to result in an increase in the frequency and severity of spruce beetle outbreaks. Climate change itself may affect vegetation, potentially leading to changes in species composition. The direct and indirect effects of climate and disturbances on forest composition, biomass, and dynamics open the possibility for non-linear ecosystem responses. Modeling studies allow for the study of the interaction of these effects and their impact on the forest system. University of Virginia Forest Model Enhanced (UVAFME), an individual-based gap model that simulates forest dynamics and characteristics, is updated with a spruce beetle subroutine that calculates the probability for beetle infestation and potential mortality of each tree on a plot. The updated model is then run with multiple scenarios that combine beetle infestation with current or altered climate at sites across the southern Rocky Mountains. Results show that spruce beetle infestations acted to facilitate competition with invading lower-elevation species, resulting in an increase in the biomass of historically lowerelevation species and a further decline in Engelmann spruce biomass than occurred with solely bark beetle disturbance or solely climate change. We also found an initial enhancing effect between spruce beetle infestation and climate change; however, by the end of 100 yr of climate change and potential beetle infestation, climate had a dampening effect on spruce beetle infestation, through loss of host trees. These results are an important step in understanding the possible futures for vegetation of the Rocky Mountains as well as for spruce forests across the western United States and Canada.

Forest recovery following synchronous outbreaks of spruce and western balsam bark beetle is slowed by ungulate browsing

“…The presence of nonhost species and of understory populations of juveniles not killed by bark beetles provide more robust mechanisms of post‐disturbance recovery and maintenance of forest cover under climate warming. Though increases in severity and spatial extent of bark beetle outbreaks are expected (Raffa et al , Temperli et al , Reich et al , Foster et al ), currently available evidence indicates that individual and multiple, simultaneous bark beetle outbreaks are much less effective at converting forest to non‐forest vegetation types than wildfire.…”

Gap models across micro- to mega-scales of time and space: examples of Tansley’s ecosystem concept

“…An example landscape-scale gap model application Foster et al (2018) utilized the individual-based gap model, UVAFME, to predict interactions between vegetation, bark beetle infestation, climate change, and other disturbances in the subalpine zone of the US Rocky Mountains. Spruce beetles (Dendroctonus rufipennis (Kirby)) are an aggressive bark beetle species that infests spruce (Picea spp.)…”

“…Beetle population size is additionally impacted by the availability of suitable hosts and climate impacts on survivorship and population growth rate (Berg et al 2006;Hansen et al 2011Hansen et al , 2016Hart et al 2015). Through explicit simulation of individual trees and individual stands, UVAFME captured the multi-scale factors that influenced infestation rate under current and future climate scenarios (Foster et al 2018). The infestation probability of a spruce tree was determined by the tree-level characteristics such as size, stress-level, and proximity to other infested trees, standlevel characteristics such as basal area of spruce and down woody debris, and site-wide climate characteristics.…”

“…Through a Monte Carlo-style aggregation of several hundred plots, UVAFME additionally represented landscape-level forest properties and dynamics over time in response to spruce beetle outbreaks. At this scale the model produced, without prescription, emergent properties of these interactions, predicting the rising and falling of infestation rates over time (Foster et al 2018) (Fig. 4).…”

“…Follow-ons from these models are numerous and have been developed for use in the forests of China and Russia (Yan and Shugart 2005;Shuman et al 2017), the western US (Bugmann 2001;Foster et al 2017), the tropics (Huth and Ditzer 2000;Fischer et al 2016), Europe (Bugmann and Solomon 2000), and boreal North America (Bonan 1989;Foster et al 2019). Gap models simulate vegetation-soil interactions (Pastor and Post 1985;Bonan 1989;Foster et al 2019), wildfire, windthrow and insect outbreak impacts on vegetation (Schumacher et al 2006;Shuman et al 2017;Foster et al 2018), and volatile organic carbon (VOC) emissions from forests (Wang et al 2017a).…”