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

Jacquelyn K Shuman,Herman H Shugart,Jose Negron,Adrianna C Foster

doi:10.1002/ecs2.2437

Abstract

AbstractIn 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 lower‐elevation 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.

Highlights

Disturbances such as fire, windthrow, and insect outbreaks are principal drivers of vegetation dynamics within high-elevation and high-latitude spruce forests and interact to affect forest composition and dynamics as well as ecosystem processes and biogeochemical cycling (Veblen et al 1991a, Edburg et al 2012, Goetz et al 2012, Hansen 2013, Frank et al 2014, O’Halloran et al 2014, Seidl et al 2017)
Outbreaks of the spruce beetle (Dendroctonus rufipennis Kirby), which infests spruce (Picea spp.) species and whose native range extends from the southern Rocky Mountains in Arizona to boreal Alaska and Canada (Wood 1982), have increased in recent years (O’Connor et al 2015, USFS 2016), leading to widespread mortality and carbon losses throughout the western United States, Alaska, and
Fraser Experimental Forest and Niwot Ridge are slightly drier sites (~70 and ~50 cm annual precipitation, respectively) compared to Glacier Lakes Ecosystem Experimental Site (GLEES) and Wolf Creek (~100 and ~110 cm, respectively), and this difference in overall climate may be driving the increased effect of elevated temperatures at Niwot Ridge and FEF. These results indicate that fine-scale patterns in climate, weather, and disturbance regimes should be considered when predicting the future state of vegetation within the Rocky Mountains

Summary

Introduction

Disturbances such as fire, windthrow, and insect outbreaks are principal drivers of vegetation dynamics within high-elevation and high-latitude spruce forests and interact to affect forest composition and dynamics as well as ecosystem processes and biogeochemical cycling (Veblen et al 1991a, Edburg et al 2012, Goetz et al 2012, Hansen 2013, Frank et al 2014, O’Halloran et al 2014, Seidl et al 2017). A mass attack of many beetles, overwhelms trees’ efforts, leading to successful infestations and subsequent tree mortality (Raffa et al 2008). Weather and environmental/forest stand factors that increase susceptibility to spruce beetle attack (i.e., a high availability of weakened trees, high density and proportion of spruce, or droughts) can allow for more successful mass attacks that can build into widespread outbreaks (Schmid and Frye 1976, Berg et al 2006, DeRose and Long 2012)

Methods

Results

Discussion

Conclusion