Monitoring whitebark pine stand health in the central Washington Cascades

Whitebark pine (Pinus albicaulis) plays a vital role in colonizing newly disturbed areas, providing shade for other tree species to germinate, and supplying food for a variety of birds and mammals, such as Clark's Nutcrackers (Nucifraga columbiana) and grizzly bears (Ursus arctos horribilis). Decline of whitebark pine populations has been attributed to several factors, including white pine blister rust (Cronartium ribicola), mountain pine beetle (Dendroctonus ponderosae) outbreaks, and fire exclusion. In 2009, the U.S. Forest Service began to install permanent plots in whitebark pine stands in Washington and Oregon as part of a Pacific Northwest restoration strategy to track blister rust and mountain pine beetle mortality. Forest Service crews conducted surveys on these plots that included standard tree inventory measurements and assessments of blister rust, mountain pine beetle, and fire activity. During summer 2020/2021, we remeasured 12 of these plots located in 3 areas of the Okanogan-Wenatchee National Forest (Mission Ridge, Clover Springs, and Lake Ann) and 1 area within the Ahtanum State Forest (Darland Mountain). On average, 15% of trees were newly infected by blister rust, 5% of trees died from blister rust, and 12.6% died from all causes combined in the 11-12 years between surveys. Despite this, the density of live whitebark pine trees experienced a nonsignificant increase due to regeneration, while the density of whitebark pine snags increased significantly and the density of whitebark pine seedlings decreased significantly. The percentages of trees with blister rust infection, seedlings with blister rust infection, and live trees with mountain pine beetle damage were heterogeneous over space and time. Our results help quantify parameters that are central to understanding the population dynamics of whitebark pine in the Pacific Northwest and informing management decisions, but the findings should be interpreted in light of the limited sample size and spatial extent of our data. Regular monitoring of a wider array of permanent whitebark pine plots will be critical to management of this tree species.

Automated summary

Whitebark pine plays a vital role in colonizing disturbed areas and providing food for birds and mammals, but its populations have been declining. Despite the increase in blister rust infection, the density of live whitebark pine trees did not significantly decrease due to regeneration. However, the density of whitebark pine seedlings significantly decreased and the density of whitebark pine snags significantly increased.