White pine blister rust (WPBR) is an exotic fungal pathogen (Cronartium ribicola) first introduced into western North America around 1910. It has caused dramatic population declines in several species of 5-needled pines (“white pines”). Declines in whitebark pine (Pinus albicaulis) have been so severe that in 2012 Canada listed the species as endangered, and the U.S. Fish and Wildlife Service has been considering similar action with a full review due in 2017. White pines typically act as critical foundation species, and their loss can have complex cascading effects throughout forest ecosystems.
WPBR was first discovered in the southern Sierra Nevada Mountains in 1969. However, the pathogen was expected to have a relatively small impact in this region due to the drier climate compared with the Pacific Northwest. In the late 1990’s, researchers established 154 long-term monitoring plots throughout Sequoia and Kings Canyon National Parks (SEKI) using a stratified random sampling design to create a formal baseline assessment of the extent and severity of WPBR infection. Over the past four years (2013-2016), a collaboration between the National Park Service, US Forest Service, US Geological Survey and UC Berkeley allowed for a thorough resurvey of the plots. Crews travelled to remote wilderness to find permanent markers, reconstruct the original plots and assess the white pines for overall health and symptoms of WPBR.
Preliminary results indicate that the incidence of blister rust in SEKI has increased overall, particularly in the western white pine (Pinus monticola) and whitebark pine populations. Interestingly, blister rust remains unconfirmed in foxtail pine (Pinus balfouriana), and has decreased for sugar pine (Pinus lambertiana). Further analyses may reveal whether specific topographical (slope, aspect, drainage) and climatic conditions (temperature and precipitation) help describe the current patterns of spread. Unlike initial assessments predicted, our results suggest that blister rust will continue to threaten high elevation white pines in the Sierra Nevada and may become an increasing threat for the highly vulnerable whitebark pine.