Russ Monson, Univesity of Arizona

"Tree Rings and Stable Isotopes – The Current Western US Megadrought and What the Trees Are Telling Us About It"

Dr. Monson studies how climate change in the Western US is influencing the carbon and water cycles in mountain forests. He also studies how the emissions of organic compounds from forests across the globe influence atmospheric chemistry, particularly with regard to the the lifetime of compounds that influence climate.

Abstract: The western US is locked in a severe and rare climate anomaly – the driest megadrought in the past 500 years. A megadrought is defined as a sustained dry anomaly lasting two decades or more. We have studied multi-decadal time-series of ring widths, and the carbon/oxygen stable-isotope composition of cellulose, in the tree rings of Ponderosa pine trees at 15 sites in the southwestern US to reconstruct the emergence of the current megadrought and its impacts on tree growth. We used the time-series to test the results of past studies in which multi-year drought 'legacies' were hypothesized, causing trees to exhibit prolonged physiological effects during drought recoveries. This has been discussed previously as revealing a tree's physiological 'memory' of past drought stress. We found that inclusion of tree-ring data from the current megadrought caused a statistical skew in past interpretations. Our results showed when the megadrought data is omitted from the time-series analysis, physiological memories lasting, at most, 1-2 years are observed, rather than the 5-7 years of past studies. Thus, a primary cause of the previously-reported multi-year legacies is a shift in the climate system itself, an exogenous factor, that may be more important than endogenous physiological memory.

Open to the public (contact gonyerk@reed.edu for the Zoom link & password)