Pest and pathogen outbreaks have contributed to unprecedented levels of tree mortality across the Western United States. The combination of the invasive pathogen, white pine blister rust (Cronartium ribicola) and mountain pine beetle (Dendroctonus ponderosae) has caused precipitous declines in populations of white pines (Pinus subgenus Strobus). However, the southern Sierra Nevada have had historically low levels of beetles and blister rust, which has important implications for white pine health. To examine the extent and severity of white pine blister rust in Sequoia and Kings Canyon National Parks, 154 plots were established and surveyed for blister rust between 1995-1999. To measure long-term changes in the blister rust distribution and to measure current levels of beetle activity, we resurveyed these plots from 2013-2017. We predicted that: 1) incidence and severity would increase across all white pines; 2) mountain pine beetle attacks would be highest in sugar pine populations; and 3) mortality rates would be lowest in high elevation white pine populations. To test these hypotheses, crews identified trees with blister rust infections and beetle attacks. Crews also measured white pine recruitment, noted presence of alternate blister rust hosts, and measured tree diameter and height.
Results/Conclusions
During the late 1990’s, blister rust incidence was relatively low and overall, only 26% of plots were infected across the five white pine species. Severity was greatest in lower elevation sugar pines and no blister rust was initially found in foxtail or whitebark pines. Results from the resurvey observed substantial changes in both incidence and severity of white pine blister rust. Incidence dropped to 6.5% for sugar pines but increased by over 155% in western white pines. Blister rust now infects over 1% of whitebark pines. However, no blister rust was confirmed in foxtail pines, leading us to reject hypothesis one. In contrast, we accepted both hypothesis two and three. Mountain pine beetle was prevalent across all white pines, particularly in lower elevation sugar pines (6.5% of individuals had confirmed attacks). While mortality rates were highest in lower elevation white pines, our results also confirmed dramatically high mortality rates in sugar pines, warranting further investigation into the population’s long-term health in the Sierra Nevada. Our findings also suggest that blister rust and beetles are becoming an increasing threat to white pines at higher elevations. These results may contribute to the conservation of white pines, which provide critical ecosystem services throughout the Western United States.