Effects of Summer and Winter Drought on Cottonwood Growth, Wind River Wyoming
Jonathan Friedman1*, and Richard Thaxton2
1U.S. Geological Survey, Fort Collins, CO, USA; friedmanj@usgs.gov
2Colorado State University, Fort Collins, CO, USA; rthaxton@rams.colostate.edu
Increasing temperature and demand for water are intensifying drought stress in riparian ecosystems, but mechanisms of resulting tree death are unclear. We related tree-ring width of plains cottonwood (Populus deltoides subsp. monilifera) and narrowleaf cottonwood, (Populus angustifolia) to inter- and intra-annual flow variation in the Wind River, Wind River Indian Reservation, Wyoming. Between Riverton and Boysen Reservoir (Downstream Site, elevation 1459 m) we collected cores and slabs from 56 live trees and 44 dead trees. Downstream of Diversion Dam (Upstream Site, elevation 1698 m) we collected cores from 63 live trees. We cross-dated sanded slabs and cores using skeleton plots and the program COFECHA, and used the program RCSSIGFREE to adjust for tree age and to develop a chronology representing annual tree growth for each site. At the Downstream Site, cottonwood growth was limited by growing season water availability as indicated by a correlation of 0.60 between annual growth and log of average flow in May-September for years 1942-2016. The drought of 2000-2003 produced the lowest growing-season flow and the lowest growth rate since 1942, apparently resulting in widespread tree mortality. At the Upstream Site growing-season flow and cottonwood growth were not correlated and the chronology indicated strongly declining growth in the 1950s and 1960s followed by a sudden and sustained increase in growth starting around 1974. These changes in growth at the Upstream Site are consistent with winter (December-March) flows that decreased through the 1950s and 1960s but then increased in the early 1970s when winter diversions to the Wyoming Canal ceased. We hypothesize that winter low flows in the 1950s and 1960s decreased growth at the Upstream Site by desiccating fine roots. In conclusion growth and survival of cottonwoods along the Wind River can be strongly affected by flow variation both during and outside the growing season.