Downstream decreases in water availability, tree height, canopy volume and growth rate in cottonwood forest along the Green River, southwestern USA
Richard Thaxton1*, Mike Scott2, John Kemper3, Sara Rathburn4, Sabrina Butzke5, Jonathan Friedman6
1Department of Earth and Spatial Sciences, University of Idaho, Moscow, ID, USA
2Sante Fe, NM, USA
3Department of Geography and Geosciences, University of Vermont, Burlington, VT, USA
4Department of Geosciences, Colorado State University, Fort Collins, CO, USA
5Contractor to the US Geological Survey, Fort Collins, CO, USA
6US Geological Survey, Fort Collins, CO, USA
Increased streamflow diversion coupled with flow reductions caused by climate warming and prolonged drought conditions are increasing hydrologic stress in Fremont cottonwood (Populus fremontii) forests across the southwestern United States. The spatial variability of this decline, however, is large and poorly understood. Along the Yampa and Green Rivers moisture stress and flow diversions increase downstream. To investigate effects of this gradient on cottonwoods we measured the diameter, percent live canopy, and height of randomly-selected trees at 3 sites: Deerlodge Park on the Yampa River (DLP), Island Park on the upper Green (ILP), and Canyonlands National Park on the lower Green (CAN). From these same trees we took increment cores to understand differences in tree growth in each forest over time. To characterize the hydrologic and meteorological changes at each site, we compared observed flow to natural flow (the flow that would have occurred in the absence of flow diversion). Cottonwoods at CAN were shorter and had lower percent live canopy and growth rate than similarly aged trees upstream. Furthermore, cottonwoods at CAN had more strongly negative trends of growth, height, and live canopy percentage with increasing height above the water surface and age. In addition, the correlation between tree growth and vapor pressure deficit shows a much stronger negative shift since 1999 at CAN than at the other sites. All of these differences suggest higher drought stress at CAN, which we attribute to the combined effects of peak flow declines from Flaming Gorge Reservoir, greater flow diversion, and the higher and increasing vapor pressure deficit at CAN. Further research is needed to understand, anticipate and perhaps mitigate past, present and future effects of drought stress in these iconic forests.