Status of whitebark pine in the Greater Yellowstone ecosystem: A step-trend analysis with comparisons from 2004 to 2019

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

doi:10.36967/2306875

Whitebark pine (Pinus albicaulis) is a high-elevation conifer of western North America. Its nutritious cones are forage for the federally threatened grizzly bear (Ursus arctos), Clark’s nutcracker (Nucifraga columbiana), red squirrel (Tamiasciurus hudsonicus), and numerous other subalpine species. It is a pioneer species in harsh, exposed subalpine zones, creating microclimates that help other conifers establish, such as subalpine fir (Abies lasiocarpa). However, substantial damage from a nonnative pathogen, white pine blister rust (Cronartium ribicola), unprecedented mountain pine beetle (Dendroctonus ponderosae) outbreaks, fire, and drought, have contributed to the widespread decline of whitebark pine in high-elevation forests throughout its range. The resulting mortality, particularly of large cone producing trees, has led to losses in cone production and declines in the ecosystem services provided by whitebark pine. Consequently, it was listed as a federally threatened species in 2023 by the U.S. Fish and Wildlife Service. Given the long-standing concerns for this high-elevation keystone species, the Greater Yellowstone Network has been monitoring whitebark pine as part of the Interagency Greater Yellowstone Ecosystem Whitebark Pine Monitoring Program since 2004. This program tracks 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 and compares results of the current time-step period 2016 to 2019 with earlier time-step periods since monitoring began. Tree mortality has continued to stabilize after peaking in 2008 to 2011, with 219 new trees dying from 2016 to 2019. Further, we tagged an additional 240 trees in 2016 to 2019 that passed the >1.4 m height threshold for monitoring. This was an increase in the number of new trees tagged over the previous 2012 to 2015 period. Estimates of blister rust infection prevalence are higher than previously reported due to an improved modeling approach, but within each sampling period, they have consistently remained at approximately 30% of all whitebark pines in the Greater Yellowstone Ecosystem infected. Additionally, nearly half of trees observed with cones or cone scars were infected with blister rust. We continued to observe a decline in tree diameter size, with a shift towards smaller diameter trees after heavy mortality due to the mid 2000s beetle outbreak, fire, and disease. This remains a critical concern because over 94% of cone-producing trees are larger-diameter trees. These changes are tracked in detail 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 2019

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