The effects of multiple coincident bark beetle outbreaks on subalpine forests in the Intermountain West

Authors: Sarah Hart*, University of Wisconsin-Madison
Topics: Biogeography
Keywords: disturbance, forest, biogeography
Session Type: Virtual Paper
Day: 4/7/2021
Start / End Time: 9:35 AM / 10:50 AM
Room: Virtual 9
Presentation File: No File Uploaded


Coincident with recent warm and dry conditions, native bark beetles have killed conifer trees across 23.8 M hectares of forest across the western United States over the past two decades. 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; Dendroctonus rufipennis), and western balsam bark beetle (WBBB; Dryocoetes confusus), have caused extensive mortality of pine, spruce, and fir, respectively. Most bark beetles generally prefer stands that are enriched in their focal host and thus more diverse stands are less likely to experience outbreak. Yet, it remains poorly understood how species diversity affects tree mortality in the face of multiple 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 tree mortality in the Intermountain West. 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, with strong effects of lodgepole pine presence on both presence and severity of outbreak. In stands with multiple hosts the co-occurrence of multiple bark beetle species occurred infrequently (ca. 5% of plots), but generally resulted in more severe mortality. These results highlight the importance of managing forests in the context of multiple bark beetle species.

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