Outbreaks of Douglas-Fir Beetle Follow Western Spruce Budworm Defoliation in the Southern Rocky Mountains, USA

Hailey M Cole,Sarah J Hart,Kyle C Rodman,Olivia Santiago,Cori Butkiewicz,Robert A Andrus,Angela Waupochick,Niko J Tutland

doi:10.3390/f13030371

Abstract

Changes in climate are altering disturbance regimes in forests of western North America, leading to increases in the potential for disturbance events to overlap in time and space. Though interactions between abiotic and biotic disturbance (e.g., the effect of bark beetle outbreak on subsequent wildfire) have been widely studied, interactions between multiple biotic disturbances are poorly understood. Defoliating insects, such as the western spruce budworm (WSB; Choristoneura freemanni), have been widely suggested to predispose trees to secondary colonization by bark beetles, such as the Douglas-fir beetle (DFB; Dendroctonus pseudotsugae). However, there is little quantitative research that supports this observation. Here, we asked: Does previous WSB damage increase the likelihood of subsequent DFB outbreak in Douglas-fir (Pseudotsuga menziesii) forests of the Southern Rocky Mountains, USA? To quantify areas affected by WSB and then DFB, we analyzed Aerial Detection Survey data from 1999–2019. We found that a DFB presence followed WSB defoliation more often than expected under a null model (i.e., random distribution). With climate change expected to intensify some biotic disturbances, an understanding of the interactions between insect outbreaks is important for forest management planning, as well as for improving our understanding of forest change.

Highlights

Publisher’s Note: MDPI stays neutralOver the past two decades, forests across the western United States have experienced widespread reductions in growth and increases in tree mortality due to outbreaks of native insects [1]
The western spruce budworm (WSB) activity exceeded that of Douglas-fir beetle (DFB)
From 1999 to 2019, 32.0% of Douglas-fir forests were affected by WSB, 18.4% were affected by DFB, and 9.5%

Summary

Introduction

Over the past two decades, forests across the western United States have experienced widespread reductions in growth and increases in tree mortality due to outbreaks of native insects [1]. This has created cascading consequences for key ecosystem services, including timber supply [2], recreation [3], wildlife habitat [4], and carbon sequestration [5]. Due to the fact that warming is forecasted to increase throughout the 21st century [14], future climate conditions are expected to facilitate more extensive and frequent outbreaks [15,16]. There is considerable uncertainty about the consequences of increased rates of disturbance interactions [18,19], when considering interactions between irruptive insects

Results

Discussion

Conclusion