Ponderosa pine trees are one of the most ubiquitous conifer species in western North America, extending from southern British Columbia all the way down to northern Mexico. In the American Southwest, winter snowmelt and summer rains play a critical role in these forests' survival, but the 23-year megadrought may have these semi-arid trees at the end of their rope, according to new research by University of Arizona scientists. "Forests in the Southwest are no strangers to droughts but have largely been able to cope with periods of drought throughout history," said Brandon Strange, who recently graduated from UArizona with a doctorate in natural resources from the College of Agriculture, Life and Environmental Sciences. "However, the current megadrought is the most severe since the year 800 CE."
The research team – which also included Russell K. Monson of the UArizona Laboratory of Tree-Ring Research, Paul Szejner from the Natural Resources Institute Finland and Jim Ehleringer from the University of Utah – compared carbon isotope analyses from 17 different ponderosa pine populations across the Four Corners, from two time periods: pre-megadrought (1960-2000) and during the megadrought (2000-2017). "What we're seeing is that despite their attempts to limit how much water they lose by reducing stomatal conductance, they simply can't; it's too dry and too hot," Strange said. "The likelihood of hydraulic failure increases with increasing heat waves and aridity. Without more information, we can't really say but it's not looking good."