Limber pine (Pinus flexilis), a wide-ranging western conifer (see map at right), is experiencing widespread mortality and reduced capacity for forest recovery due to combined impacts of several factors: the non-native fungal pathogen Cronartium ribicola, causal agent of the lethal disease white pine blister rust (WPBR), mountain pine beetle, dwarf mistletoe, and climate change in the U.S. and Canada (Cleaver 2014; Smith et al. 2013; Smith et al. 2015). Basal area loss of 40% is projected for limber pine in the U.S. over the next 15 years (Krist et al. 2014).
Limber pine is a keystone and foundation species that maintains ecosystem structure, function, and biodiversity. It defines both the grassland and alpine treelines, grows on exposed sites too harsh for other tree species, facilitates forest succession, aides in slope stabilization, snowpack retention and melt, and its seeds are food for birds, small mammals, and bears. As long-lived and wind defiant trees, limber pines also have aesthetic value and cultural significance.
Limber pine occurs in every western state and province except Washington. It is designated with "special status" on Bureau of Land Management lands in Wyoming, "species of management concern" in several western U.S. National Parks, and "endangered" in Alberta, and the species is recommended for endangered status nationally in Canada.
Naturally-occurring resistance to WPBR exists in North American five-needle pine species, and increasing resistance is a cornerstone of WPBR management (Sniezko et al. 2011). Forest surveys alone cannot predict species vulnerability as they cannot determine if the remaining healthy trees are at risk for disease or if they have heritable genetic resistance to support future populations. The frequency and efficacy of WPBR resistance is a critical factor influencing host population sustainability and therefore vulnerability.
Complete resistance in limber pine varies from 0 to 30% within populations sampled to date (Schoettle et al. 2014). Understanding the range-wide geographic distribution of resistance will provide us the tools to sustain and restore the species.
Researchers are establishing a range-wide common garden with 198 families of white pine and then artificially inoculating seedlings with C. ribicola. Goals are to:
1) Quantify the range-wide frequency and geographic variation of resistance;
2) Quantify geographic variation in growth traits and correlations with resistance;
3) Investigate the role of climatic variables driving geographic variation in both resistance and growth traits; and
4) Identify resistant and susceptible seed trees for use as field monitors of change in rust virulence.
This project will be the first to quantify the range-wide geographic variation in WPBR resistance, growth traits, and their relationships to climate. This knowledge will advance our understanding of disease resistance, growth, and climate interactions in general as well as lead to further genomics research, improve forest health projections, guide management efforts, and provide indicators of rust virulence and efficacy of resistance in limber pine.