Isotopic heterogeneity in whitebark pine (Pinus albicaulis Engelm.) nuts across geographic, edaphic and climatic gradients in the Northern Rockies (USA)

The overall health and persistence of whitebark pine is of international concern. Extensive tree mortality and loss of vigor due to the non-native pathogen white pine blister rust (Cronartium ribicola A. Dietr.), mountain pine beetle (Dendroctonus ponderosae Hopkins), altered fire regimes, and climate change endanger the existence of whitebark pine, which is an important food source (pine nuts) for several wildlife species. Prior stable isotope analysis revealed variability in intrinsic water-use efficiency and nutritional status; however, small sample sizes prohibited the identification of geo-climatic effects on those isotopes. We summarize carbon isotope discrimination (Δ13C, a measure of intrinsic water-use efficiency) and the natural abundance of δ13C, δ15N and δ34S in whitebark pine nuts in the Northern Rockies. Spatial differences in geography, soil parent material and climate (e.g., summer mean temperature, number of degree-days below 0 °C, mean annual precipitation, Hargreaves climatic moisture deficit, and the frost-free period) contributed to the low to moderate spatial resolution in selected models (R2 values ranged from 0.24 to 0.51). The importance of soil parent material in all models demonstrated whitebark pine exhibits previously unknown edaphic variation. Identification of the mechanistic drivers contributing to spatial heterogeneity in Δ13C provides an opportunity to select seed sources better suited to optimize long-term survival, vigor and cone production. Matching genetic resources to sites projected to support whitebark pine in future climates would ensure species persistence, while safeguarding an important wildlife food. Suitable wildlife habitat for projected warmer, drier climates was characterized as low values of Δ13C (<13.5‰) and was concentrated in the southeastern portion of the Northern Rockies. Whitebark pine nuts at the landscape scale exhibited unique δ34S and δ15N values relative to other wildlife foods, but these isotope values can overlap other plant materials in more localized areas. Future applications to determine the proportion of pine nuts in assimilated diets will need to accommodate spatial heterogeneity, potential temporal variation in δ13C, nutrient concentrations, animal tissue turnover rates, and seasonal availability of other foods. Further studies of the abiotic and biotic factors on C, N, and S cycles and the relationship between photosynthetic and heterotrophic tissue are needed for a better mechanistic understanding of the determinants of the natural abundance of δ13C, Δ13C, δ15N and δ34S.