The relative roles of climate and historic moisture availability on the survival of Pinus albicaulis populations in Grand Teton National Park, WY, U.S.A
Whitebark pine (Pinus albicaulis) is a critically threatened North American conifer. In modern times, it has experienced a significant decline in population due to pine beetle infestations, blister rust infections, fire suppression, and climate change. Despite the knowledge that P. albicaulis is severely threatened by multiple stressors, little is known about the historic capacity of this species to handle these stressors. More specifically, it is unknown how P. albicaulis has dealt with past climatic variability, particularly variation in the type of precipitation (rain vs. snow) available for soil moisture, and how differences in quantity of precipitation have influenced the establishment and growth of modern stands. We investigated the role of past climate change and precipitation source on two subalpine P. albicaulis stands along an elevational gradient located in Paintbrush Canyon of Grand Teton National Park. Using increment cores obtained from 20 living P. albicaulis trees at each site, we determined average dates of establishment, annual growth trends, and responses to changes in climatic variables through time. We also isolated alpha-cellulose present in a subset of cores from each site for isotopic analyses (δ18O) to determine the role of historic moisture availability and source on growth trends.
Basal age data show differences in dates of establishment of P. albicaulis between sites. P. albicaulis trees sampled at the lower elevation site showed an average date of establishment of 1750 CE, while P. albicaulis trees sampled at the higher elevation site had an average date of establishment of 1895 CE. Annual ring width data indicate declining annual growth rates of P. albicaulis at both sites towards present since the mid 1900s. An investigation of the relative roles of different climatic variables obtained using PRISM data, including maximum and minimum temperatures and precipitation amounts, suggest that the observed decline in annual growth at both sites is a direct result of increasing growing season temperatures. More specifically, this modern decline observed at both sites can be attributed to increasing minimum June and July temperatures. Preliminary isotopic data suggest differences in historic moisture availability between sites due primarily to elevational differences. These results indicate that current populations of P. albicaulis in the western U.S. are undergoing large ecological changes. Management of subalpine conifer ecosystems should include long-term paleoecological data to inform decisions about potential mitigation and regeneration strategies.