Carbon isotope discrimination in tree ring alpha-cellulose indicates that drought stress cannot explain white spruce growth declines in the eastern Brooks Range, Alaska
Dates
Year
2014
Citation
Brownlee, Annalis H., 2014, Carbon isotope discrimination in tree ring alpha-cellulose indicates that drought stress cannot explain white spruce growth declines in the eastern Brooks Range, Alaska: University of Alaska Anchorage.
Summary
High latitude trees are increasingly showing mixed growth responses to climate warming. Recent studies of white spruce (Picea glauca ) in the Brooks Range of Alaska have revealed that trees in the west have generally shown positive growth responses, while trees in the central and eastern Brooks Range have shown mixed and negative growth responses, respectively. An earlier study suggested that the growing season climate of the eastern Brooks Range is warmer and drier than in the west. On this basis, growth declines in the eastern Brooks Range were attributed to drought stress, caused by rising temperature and increasing evaporative demand. I investigated the hypothesis that drought stress can explain white spruce growth declines in [...]
Summary
High latitude trees are increasingly showing mixed growth responses to climate warming. Recent studies of white spruce (Picea glauca ) in the Brooks Range of Alaska have revealed that trees in the west have generally shown positive growth responses, while trees in the central and eastern Brooks Range have shown mixed and negative growth responses, respectively. An earlier study suggested that the growing season climate of the eastern Brooks Range is warmer and drier than in the west. On this basis, growth declines in the eastern Brooks Range were attributed to drought stress, caused by rising temperature and increasing evaporative demand. I investigated the hypothesis that drought stress can explain white spruce growth declines in the central and eastern Brooks Range by measuring the carbon isotope composition (δ13 C) of tree-ring alpha-cellulose. I collected increment cores from trees at four sites near the Arctic treeline, spanning a west-to-east gradient in the Brooks Range. I measured ring widths and calculated carbon isotope discrimination (Δ13 C), needle intercellular CO2 concentration (Ci ), and intrinsic water-use efficiency (iWUE) from δ13 C values. Trees at my sites exhibited divergent growth responses to recent climate warming, consistent with other studies. The western sites generally showed positive growth responses to rising air temperature, while the central site showed a more intermediate response, and the eastern site showed a declining growth response. These divergent growth trajectories could not, however, be explained by changes in Δ 13 C. While iWUE increased at all sites over the past century, rising atmospheric CO2 concentration has more than compensated for reduced stomatal conductance, such that more substrate (Ci ) is now available for photosynthesis than during the early 1900s. My results, therefore, suggest that drought induced stomatal closure is probably not an important constraint on the growth of white spruce under contemporary conditions near the Arctic treeline in Alaska.
