Ecology of whitebark pine populations in relation to white pine blister rust infection in subalpine forests of the Lake Tahoe Basin, USA: Implications for restoration

Abstract

For over a century, white pine blister rust (WPBR), caused by the introduced fungal pathogen, Cronartium ribicola J.C. Fisch., has affected white pine (Subgenus Strobus) individuals, populations, and associated forest communities in North America. We surveyed eight populations of whitebark pine (Pinus albicaulis Engelm.) across a range of environmental conditions in subalpine forests of the Lake Tahoe Basin, California and Nevada, to determine how WPBR is influencing host fecundity (i.e., female cone production and recruitment), survival, and current population trends. In this region and throughout its range whitebark pine is an important component of subalpine ecosystems. In high-elevation forests of the Lake Tahoe Basin, the primary negative effect of this rust pathogen on whitebark pine is infection and mortality of cone-bearing branches.Mean incidence of WPBR among whitebark pine populations was 35% and ranged from 1% to 65%. Given the high disease incidence, relationships were found between WPBR infection and reproductive output. Percent of individuals infected per population (R2=0.78), average number of WPBR-infected branches per population (R2=0.71), and severity of stem girdling (R2=0.76) were all negatively related with cone production. A positive relationship was found between disease severity (i.e., extent of stem girdling) and available water capacity and a negative relationship was found between disease severity and%sand content. Environmental conditions, including soil properties, may affect pathogen growth and potentially influence the rate at which green infected trees lose the ability to produce female cones. Demographic trends for 7 of 8 whitebark pine populations were stable, with estimates of geometric growth rate (λ)⩾1.0. Population stability is a function of adequate population sizes, mixed-size structure, high survivorship, and moderate fecundity. Results show that WPBR is adversely affecting fecundity but having little effect on survivorship. Restoration strategies are designed to facilitate whitebark pine recruitment and deploy genetically diverse seedling material and potentially WPBR-resistant phenotypes. Our study highlights a need for long-term demographic data and monitoring of WPBR-infection to better understand population consequences and host responses in the future.