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The impact of pests, pathogens, competition, and climate water deficit on whitebark pine populations in the southern Cascades
Whitebark pine (Pinus albicaulis) is vulnerable to a number of threats including an introduced pathogen (Cronartium ribicola), epidemic levels of native mountain pine beetle (Dendroctonus ponderosae), competition, and climate change. To elucidate factors influencing the presence of pests and pathogens, cone production, and canopy kill on whitebark pine, I surveyed trees in 30 50×50 m plots containing whitebark pine in two parks in the southern Cascades, Crater Lake and Lassen National Parks. In each plot, I recorded both tree-level and stand-level data related to tree health, including symptoms of blister rust (the disease caused by C. ribicola) and mountain pine beetle, and reproductive vigor (cone production). In addition, I developed a Thornthwaite-type water balance model to calculate average yearly climate water deficit for each plot. I developed a series of separate generalized linear mixed effects models predicting the incidence of infection, beetle infestation, cone production and percent crown kill for individual trees in both parks separately. In both parks, competition from other species, particularly mountain hemlock (Tsuga mertensiana) negatively impacted cone production. In Crater Lake, water stress is also a good predictor of blister rust infection and cone production. The presence of mountain pine beetle and blister rust increased canopy kill for whitebark pine in both parks, which can cause the slow deterioration of cone-bearing branches. Lastly, I found evidence for a pest-pathogen interaction; mountain pine beetle attack was greater for trees which had experienced previous blister rust infection in Crater Lake. My results show that whitebark pine populations in the southern Cascade range are experiencing moderate levels of blister rust infection compared with other sites across the species range, and that competition from shade-tolerant species is presenting an additional threat in both parks I studied. My work also suggests that while blister rust infections are equivalent in the two parks, mortality is lower in Crater Lake. The difference may be due to lower beetle activity in Lassen than in Crater Lake, and the contrast presents a useful model to better understand how recent climate changes are driving beetle outbreaks.