PS 23-68 - Status of threatened whitebark pine in WA National Parks

Abstract: Whitebark pine (Pinus albicaulis), a federally-protected tree species found at high elevations near tree line, has experienced high mortality in in the Cascades and Olympic Mountains of western Washington primarily due to introduced white pine blister rust (Cronartium ribicola). Long-term plot monitoring implemented at Mount Rainier and North Cascades National Parks since 2004 tracks infection and mortality rates due to rust and mountain pine beetle. In recent monitoring (2021), we found significantly increasing rates of rust infection (P < 0.001) over time and significantly lower rates of infection at North Cascades (mean infection rate in 2021 of 76% of plot trees at Mount Rainier and 50% at North Cascades, P < 0.05). Despite differences in infection rates, we found only non-significant increases in mortality over time and no difference between the parks (P < 0.05), although previous work showed higher mortality in Mount Rainier (near 50% 2008), perhaps reflecting strong park differences in historic rates of mortality in the 20^th century. Mountain pine beetle has continued to be a lower threat to species persistence in this region, in contrast to other areas of the species range: beetle incidence was on 1.2% of trees on average at Mount Rainer, lower at North Cascades, and not significantly different between parks (P< 0.05). Increasing vapor pressure was a significant climatic predictor of mortality only at North Cascades, potentially tied to favorable rust infection conditions, but binary infection status was not significantly predicted by local climate factors in either park. This may be due to the overall high incidence of rust across populations at both parks. Given these high rates of infection, continuing mortality, and low evidence for populations protected from disease by environmental factors, these results support management aimed at active recovery in both parks through cone collection, identification of disease-resistant seed sources, and restoration plantings.