Response of whitebark pine stands to mountain pine beetle outbreaks in the southern Sierra Nevada, California

Background/Question/Methods

Whitebark pine (Pinus albicaulis) is threatened throughout much of its range in North America due to the combined impacts of mountain pine beetle (MPB; Dendroctonus ponderosae), white pine blister rust (Cronartium ribicola), and climate change. The impacts of MPB have been especially influential in altering the structure, composition, and successional trajectories of whitebark pine stands in northern parts of the range. However, it is not clear whether these impacts are similar in whitebark pine stands of the southern Sierra Nevada, which represents the southern extent of the range of whitebark pine where white pine blister rust is either rare or absent. We inventoried whitebark pine stands in areas of recent and severe MPB-induced mortality and adjacent undisturbed control stands of the southern Sierra Nevada to compare patterns of tree mortality, regeneration, and size class structure. Our whitebark pine inventory consisted of a total of 72 fixed-radius plots (each 0.05 ha) at three study sites, and we recorded tree mortality, tree regeneration, and stand structural and environmental variables in stands lacking any evidence of white pine blister rust.

Results/Conclusions

There was a significant decline in basal area and tree densities of whitebark pine in MPB-impacted stands, with the greatest mortality in larger (>20 cm dbh) whitebark pine trees. At all sites, there were significant declines in mean tree diameter, maximum tree diameter, and the number of tree size classes following MPB attack. Severity of MPB attack was positively related to the mean diameter of whitebark pine trees within stands, suggesting that stands containing larger whitebark pine were more susceptible to MPB attack. Density of young (<3 year) whitebark pine seedling clusters was positively associated with percent dead canopy cover and severity of MPB-attack, suggesting increased whitebark pine regeneration in response to stand impacts by MPB. All three sites showed a relatively stable production of whitebark pine regeneration at least within the past 50 years, with a pulse of new seedlings in the past 2–4 years in MPB-impacted stands. Our results show whitebark pine stands are heavily impacted by MPB outbreaks and suggest low resistance but potentially high resilience to initial attack. Stand structure and composition patterns indicate several important similarities and differences between whitebark pine forests in the southern Sierra Nevada and more northern regions.