Fremont cottonwood (Populus fremontii) is a foundational species in riparian habitats of the southwestern United States. Tall and widespread, the trees and their yellow-green foliage provide structure and a range of ecological services: Wide canopies keep soil and water temperatures cool while providing habitat for migrating and cavity-nesting birds; Leaf litter provides nutrients for insects and microbes; Trunks are felled by beaver, who construct complex wetlands that are hotspots of biodiversity in otherwise arid landscapes.
Cottonwoods are not a long-lived tree, but regenerate rapidly according to the ever-shifting sediments along desert rivers. Their fluffy seeds germinate on bare alluvium deposited by periodic floods, and cannot grow any further from the water than their roots can reach.
Along the Green River, Utah, where water flow is closely regulated by dams, flooding no longer happens according to a natural frequency or intensity. Lower peak flows mean lower flood intensity, decreasing the area available for seedling establishment. Human water diversions for agriculture and settlements – on top of increasing temperatures and prolonged drought – continue to reduce overall water volume. Projections indicate that due to climate change, this trend is likely to continue across the Southwest.
This combination of factors, known as hydrologic stress (i.e., not enough water), is an increasing threat to cottonwoods in the Southwest. In the short term, trees can adapt by reducing their growth rate, but hydrologic stress generally results in increased mortality or increased susceptibility to future drought. Scientists often measure hydrologic stress in plants using vapor pressure deficit (VPD), the relationship between air temperature and relative humidity. As VPD increases, plants must draw more water from their roots.
A recent study by Richard Thaxton (University of Idaho) and colleagues sought to examine the relationship of temperature and water availability to Fremont cottonwood growth and survival. They studied cottonwood trees at three locations in the upper Colorado River basin: the Yampa River at Dinosaur National Monument, the Green River near its confluence with the Yampa, and farther downstream the Green River at Canyonlands National Park. The third (southernmost) site is the hottest and receives the least precipitation (greatest VPD). At each site, trees were randomly selected for sampling, which included measurements of diameter, height, and percent live canopy. Tree ring cores were also extracted to examine growth rates over time.
The researchers found that both canopy cover and tree height were significantly lower at Canyonlands than the upstream sites. They also grew more slowly than trees at the other sites. They conclude that riparian forests at Canyonlands are experiencing chronic hydrologic stress due to both higher temperatures and water diversions. Riparian cottonwood forests at Canyonlands, they write, are “declining in association with decreasing mean flows in the Green River and increasing drought.” Old trees higher above the water level are most threatened.
This research supports previous work that predicted that the current range of Fremont cottonwood is likely to constrict due to climate-induced habitat loss. Presuming continued water stress, riparian forests are likely to become thinner, restricted to a narrower margin along the river where depth to groundwater is shallower. More research is needed, the authors note, on the history and prevalence of dead trees along the river (they studied only live or recently dead trees), and how other factors such as stand density, understory composition, and the spread of invasive species are related to changes in river hydrology.
This work has several implications for the Escalante River, where for 15 years GSEP has helped coordinate and lead efforts to remove Russian olive, a woody invasive species. Russian olive grows in dense stands that shade out cottonwood seedlings, constrict the waterway, and reduce hydrologic connectivity of the floodplain. Following treatment and the return to a more natural hydrology, native woody species such as cottonwoods and willows passively regenerate. The Escalante is free-flowing (without any dams or major diversions), running some 90 miles from its headwaters to its confluence with the Colorado River at Lake Powell, an elevation gradient of roughly 7,500 feet. Cottonwoods becoming reestablished in the lower reaches, where temperatures are greater, are likely experiencing greater hydrologic stress, potentially complicating restoration efforts there.
Kevin Berend, Conservation Programs Manager
[Image credit: Alex Moliski via Unsplash]
