Friday's Findings - April 2 2021

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Background: Whitebark pine (Pinus albicaulis) is a foundational species in high-elevation forests and alpine communities throughout the western U.S. and Canada. Currently, whitebark pine is proposed as threatened under the Endangered Species Act by the U.S. Fish and Wildlife Service due to the extensive population declines across its range. We consider the Greater Yellowstone Interagency Whitebark Pine Monitoring Program responsible for monitoring the health of whitebark pine populations in the Greater Yellowstone Ecosystem (GYE) since 2004. The GYE, home to two of the most iconic U.S. National Parks, has experienced significant declines in whitebark pine communities due to white pine blister rust infection (Cronartium ribicola; blister rust), mountain pine beetle (Dendroctonus ponderosae) outbreaks, wildland fires, and drought. The GYE monitoring effort is a well-established, cross-jurisdictional endeavor spanning three western states, five National Forests, three National Parks, and Bureau of Land Management land in Wyoming. USGS has provided statistical support to the GYE effort since 2011.  Monitoring results are used by land managers to understand the health status of whitebark pine in the GYE and to inform conservation actions not only for whitebark pine but for other species including grizzly bears (Ursus arctos horribilis).

The primary status objective for the monitoring program is to estimate blister rust prevalence (or the proportion of trees infected) in the GYE every four years. We found ignoring observation errors led to lower estimated prevalence of blister rust in the general population. Using model-based approaches, we found that the probability of infection has increased since 2004. However, overall prevalence likely has not changed because of the mountain pine beetle- induced shift towards smaller diameter trees that have a lower probability of infection compared to their larger cohorts. Our assessment underscores the need for continued evaluation and updating of a monitoring program's sampling design and analytical procedures to maintain relevancy.