Current knowledge of condition and biophysical setting for Great Basin bristlecone pine (Pinus longaeva) stands is insufficient to accurately assess vulnerability to climate change, contemporary and projected wildfire scenarios, and climate/fire interactions. Objectives include:
During the most recent glacial maximum (22,000-16,000 y.b.p.) Great Basin bristlecone pine (or just bristlecone pine) was a widespread dominant in the Great Basin and on adjacent landscapes of the Interior West. As the climate warmed, bristlecone pine retreated to high elevations where today its natural distribution is mostly restricted to isolated stands on mountain ranges from eastern California to central Utah. Bristlecone pine occurs in near mono-specific and mixed-species stands (especially at lower elevations) although a combination of slow growth rate and low shade tolerance would seem to make it less competitive than other conifers. Although known for old-growth stands at upper tree-line, bristlecone pine has high drought tolerance with some populations documented at or below 7,600 ft. on dry mountains systems.
The major potential threats to bristlecone pine include rapid climate change, white pine blister rust (Cronartium ribicola), mountain pine beetle (Dendroctonus ponderosae), and increases in wildland fire. Climate change will likely interact with other potential threats, especially changing fire regimes, and will likely change the suitability of lower elevation sites due to direct effects on temperature and precipitation. Mountain pine beetle has not been associated with persistence problems for bristlecone pine in the past but could have increased impacts with rising temperatures and increased frequency and severity of drought, especially for lower elevation stands.
Although thin bark and a low branching structure indicate low fire resilience for bristlecone pine, the role of historic fire regimes in dictating past stand characteristics and current distribution is not fully known. Fires in low-density stands near tree-line were likely rare and small due to the scarcity and discontinuity of fuels. On the other hand, fire likely played a more important role in denser, often mixed-species stands. Limited evidence suggests that fire severity for this forest type varied through time and space (mixed-severity fire regime) as climate varied at centennial to millennial time scales. In addition, frequent fire likely played an important role in restricting bristlecone pine at lower elevations pre-1850. To the extent this is true the lengthening of fire-free intervals over the past 100 to 150 years could have facilitated downslope bristlecone pine expansion. However, low fire frequency levels through the same period have resulted in increased loading and homogenization of fuels across landscapes increasing the probability of large, high-severity fires and increasing risks to forest stands with bristlecone pine. The isolation and relative small size of many existing stands of bristlecone pine potentially put them at significant risk of complete extirpation from high severity events. Hence the extent that bristlecone pine is currently at risk, given climate-driven changing fire regime parameters, is dependent upon both individual stand metrics and the landscape context in which stands are located. It is assumed that future risk will increase with projected lengthening of fire season and increases in fire severity associated with changing climate, and in response to ongoing densification and homogenization of fuels across landscapes. However, an accurate assessment of both present and future wildfire vulnerability for bristlecone pine will require a more complete inventory of existing stands and their landscape context.
A comprehensive stand assessment and associated shape-files of bristlecone pine stands will provide agency specialists a foundation upon which conservation and management decisions for this species can be properly structured at local to regional scales. Researchers interested in the species and forest type should also benefit from this work. The fire climate and vulnerability assessments should have specific value in forest and fire planning and, in some cases, prioritization and design of restoration activities. A broad-based assessment of genetic heterogeneity will facilitate conservation strategies for this iconic species.