Status of whitebark pine in the Greater Yellowstone Ecosystem: A step-trend analysis with comparisons from 2004 to 2023

Shanahan, Erin; Daley, Rob; Druin, Lydia; Legg, Kristin

doi:10.36967/2313989

Whitebark pine (Pinus albicaulis) is a high-elevation conifer of the northern Rocky Mountains and the Pacific Northwest. This slow-growing, long-lived conifer influences critical ecosystem functions in subalpine environments, including snow capture, landscape stability, soil amelioration, and overall forest health and resilience. Moreover, its nutritious seeds feed the federally threatened grizzly bear (Ursus arctos), Clark’s nutcracker (Nucifraga columbiana), red squirrel (Tamiasciurus hudsonicus), and other species. However, whitebark pine is declining in high-elevation forests due to several factors: a nonnative pathogen (white pine blister rust, Cronartium ribicola), unprecedented mountain pine beetle (Dendroctonus ponderosae) outbreaks, wildfire, and drought. The ensuing mortality, particularly of large, mature trees, has reduced cone production and compromises this species’ essential ecosystem services. Consequently, in 2023, the U.S. Fish and Wildlife Service listed the whitebark pine as federally threatened. Given these long-standing concerns, the Greater Yellowstone Network has been monitoring whitebark pine as part of the Interagency Whitebark Pine Monitoring Program since 2004. The monitoring program represents a tri-state, cross jurisdictional effort to track whitebark pine population condition and trends on National Park Service, U.S. Forest Service, and Bureau of Land Management lands in the region. This report documents the prevalence of blister rust infection, tree mortality, and recruitment of whitebark pine during the fifth time-step period, from 2020 to 2023 and compares it with prior time-step periods spanning 2008 to 2019. We found that estimates of blister rust infection continue to increase, using both an improved Bayesian modeling approach and the initial design-based ratio estimator. Based on model estimates, 47% of all whitebark pine in the Greater Yellowstone Ecosystem (GYE) are infected with blister rust. Furthermore, 47% of trees observed with cones or cone scars were infected with blister rust. We continued to observe tree size shifting towards smaller diameters following the mid-2000s mountain pine beetle outbreak and subsequent wildfire and disease activity across the ecosystem. This remains a critical concern given that 94% of cone-producing trees have larger diameters. Tree mortality continues to stabilize after peaking from 2008 to 2011, with 226 new trees reported dead during 2020 to 2023. Tree recruitment persists throughout the ecosystem, with 280 more trees growing past the > 1.4 m height threshold for tagging and monitoring—more than the number of new trees tagged over the previous 2016 to 2019 period. Changes in the GYE whitebark pine population are tracked through continued annual monitoring and regular trend reporting

Status of whitebark pine in the Greater Yellowstone Ecosystem: A step-trend analysis with comparisons from 2004 to 2023

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