Ecophysiology of riparian cottonwood and willow before, during, and after two years of soil water removal

Riparian cottonwood/willow forest assemblages are highly valued in the southwestern United States for their wildlife habitat, biodiversity, and watershed protection. Yet these forests are under considerable threat from climate change impacts on water resources and land-use activities to support human enterprise. Stream diversions, groundwater pumping, and extended drought have resulted in the decline of cottonwood/willow forests along many riparian corridors in the Southwest and, in many cases, the replacement of these forests with less desirable invasive shrubs and trees. Nevertheless, ecophysiological responses of cottonwood and willow, along with associated ecohydrological feedbacks of soil water depletion, are not well understood. Ecophysiological processes of mature Fremont cottonwood and coyote willow stands were examined over four consecutive growing seasons (2004-2007) near Salt Lake City, Utah, USA. The tree stands occurred near the inlet of a reservoir that was drained in the spring of 2005 and remained empty until mid-summer of 2006, effectively removing the primary water source for most of two growing seasons. Stem sap flux density (Js) in cottonwood was highly correlated with volumetric soil moisture (h) in the upper 60 cm and decreased sevenfold as soil moisture dropped from 12% to 7% after the reservoir was drained. Conversely, Js in willow was marginally correlated with h and decreased by only 25% during the same period. Opposite patterns emerged during the following growing season: willow had a lower whole-plant conductance (kt) in June and higher leaf carbon isotope ratios (d13C) than cottonwood in August, whereas kt and d13C were otherwise similar between species. Water relations in both species recovered quickly from soil water depletion, with the exception that sapwood area to stem area (As:Ast) was significantly lower in both species after the 2007 growing season compared to 2004. Results suggest that cottonwood has a greater sensitivity to interannual reductions in water availability, while willow is more sensitive to longer periods of soil water depletion. These data shed light on the linkage between soil water deficits and ecophysiological processes of threatened riparian forests given potential land-use and long-term drought impacts on freshwater resources.