This award supports a team of researchers who seek to identify site characteristics and tree species in an effort to investigate analytical methods that capture a winter temperature signal from delta18-oxygen isotopic variations in tree-ring cellulose from two sites in central Colorado and Montana Rockies. The science goal is to assess the potential for reconstructing cool-season temperatures for the Rocky Mountain region. The researchers will also evaluate the usefulness of delta18-oxygen isotope chronologies for reconstructing atmospheric circulation patterns that influence temperature variability across North America, such as the Northern Annular Mode and the Pacific North American Pattern.
The broader impacts involve working with Colorado and Montana land- and water-resource managers to use information from past climate and hydrology to inform future planning. The project would support a new postdoctoral scholar with plans for K-12 lessons based on the project.
C.A. Woodhouse, A.Z. Csank, S.W. Leavitt The primary goal of this project was to test and evaluate the feasibility of reconstructing winter temperatures from oxygen isotopes in annual growth rings of trees in the central and northern Rocky Mountains. A secondary goal was to explore the potential linkage between a tree-ring isotope proxy of winter temperatures and long-term reconstructed changes in snowpack and streamflow because of their importance for future water management and terrestrial and ecosystem processes. Understanding climate variability over past centuries is important for putting 20th century and recent climate conditions in a long-term context. Tree-ring records from the Rocky Mountain region have been successfully used to reconstruct summer temperature, winter precipitation and annual streamflow, extending these records centuries into the past . However, we have not been able to reconstruct winter temperature variability. This is important because winter and spring temperatures can influence snowpack, river flow, and water supply. A warm conditions can compound the effect of a low snowpack and exacerbate drought conditions. Recent studies provide evidence for a winter temperature information in the oxygen isotope ( δ18O) derived from tree-ring cellulose, which is mechanistically linked to the strong relationship between air temperature and the proportion of δ18O ratio in precipitation. During winter, the δ18O of falling snow reflects the temperature on days when snow is falling. In the spring the cumulative temperature signal recorded in the snowpack is transferred into the soil water and is then taken up by the trees and incorporated into cellulose. Based on this understanding, we hypothesized that δ18O in trees that rely on winter moisture contains information about winter tempeatures. During the course of our study we sampled trees from six sites in the Colorado River Headwaters region of western Colorado. Sites were selected from three different elevations, low, mid and high, and two different species from each elevation (Douglas-fir and piñon pine). In Montana we obtained samples from two pre-existing collections including high elevation samples of sub-alpine larch and a mid elevation collection from Douglas-fir. We produced 100-year long δ18O chronologies from all of our sites to compare with climate records from the area to test whether δ18O chronologies could be used as a proxy for winter temperature. What we found was that differences between the sites accounted for more of the variability in how monthly and temperature and precipitation were reflected in the isotopic value of tree rings that tree species. Our two sites in Montana both correlated with winter temperatures as we expected, meaning it is possible to use δ18O chronologies from Montana as a proxy of winter temperature. Being able to develop proxy records beyond the period covered by instrumental data of winter temperatures in the northern Rocky Mountains has important implications for improving our understanding of how climate influences snowpack in this region. In Colorado, by contrast, our δ18O chronologies were predominantly correlated with spring/summer temperatures and at two sites with summer precipitation. Although we were unable to develop a proxy for winter temperatures for the Colorado Rockies, we were able to demonstrate that records of tree-ring δ18O have the potential to provide information on spring/summer temperatures and summer precipitation. Proxy information on spring/summer temperatures and summer precipitation could be used to help us better understand hydroclimate variability in the Colorado River Basin by allowing us to discern the role of warm-season climate on streamflow. The tree-ring measurements and site tree-ring chronologies and δ18O chronologies will be made freely available online through the International Tree-Ring Data Bank: www.ncdc.noaa.gov/paleo/treering.html.
