Understanding interactions between bark beetle outbreaks in subalpine forests of the Rocky Mountains

Abstract:

Coincident with recent warm and dry conditions, native bark beetles have killed conifer trees across 22.5 M hectares of forest in the western United States. In the Interior West, much of the tree mortality has been concentrated in subalpine forests, where the mountain pine beetle (MPB; Dendroctonus ponderosae), spruce beetle (SB; D. rufipennis), and western balsam bark beetle (WBBB; Dryocoetes confusus), have caused extensive mortality of pine, spruce, and fir, respectively. While much recent research has sought to understand the causes and consequences of outbreaks of individual bark beetle species, little work has examined interactions between bark beetle outbreaks. To address this uncertainty, here I use data from the USFS Forest Inventory and Analysis (FIA) Program to understand how stand composition influences patterns of cumulative MPB, SB, and WBBB activity across subalpine forests with lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and subalpine fir (Abies lasiocarpa). I found the probability of either MPB, SB or WBBB occurring within a stand was greatest when all three hosts were present, but the severity of cumulative bark beetle activity was greatest when only one host was present. In stands with multiple hosts the co-occurrence of multiple bark beetle species occurred infrequently (ca. 5% of plots), but generally resulted in greater densities of affected trees and higher proportions of bark beetle-affected basal area. These results highlight the importance of managing forests in the context of multiple bark beetle species.