Sustain Our Nation's Forests and Grasslands

Current and future conditions in arid land riparian ecosystems

The riparian vegetation along the upper Gila River in southwestern New Mexico has high species richness of woody plants and extremely high densities of nesting birds including the Federally endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) and the Federally threatened Yellow-Billed Cuckoo (Coccyzus americanus). Forest Service photo by D. M. Smith/

Arid land riparian ecosystems are limited, the climate is changing and further hydrological change is likely in the American Southwest. To protect riparian ecosystems and organisms, scientists and land managers need to understand how disturbance processes and stressors such as fire, drought and non-native plant invasions affect them. Riparian vegetation is critically important as foraging, resting, migrating and breeding habitat to birds and other animal species in the southwestern United States.

To address these issues, Rocky Mountain Research Station Grasslands, Shrublands, and Desert Ecosystems Science program manager Deborah Finch and research associate D. Max Smith reviewed the ecohydrology of southwestern streams along the Middle Rio Grande to describe the effects of hydrological changes, wildfire and invasions on plant communities and riparian-nesting birds. They examined climate change projections and output from population models to gauge the future of arid land riparian ecosystems in an increasingly arid Southwest.

High flows as seen here on the Animas River in northwestern New Mexico are important for providing germination sites and recharging groundwater aquifers, thereby promoting reproduction and survival of woody riparian vegetation. Forest Service photo by D. M. Smith.

The structurally diverse, species-rich vegetation along many southwestern streams supports high densities of territories and nest sites for a variety of birds, including several species of high conservation priority. Survival and reproduction of woody riparian plants may depend on periodic floods and droughts. Woody species, such as Fremont cottonwood (Populus fremontii) and Arizona sycamore (Platanus wrightii), among others, provide birds with nesting sites and foraging opportunities that may be absent or rare in adjacent plant communities. In addition, as in other regions, human activity has heavily altered rivers and streams of the American Southwest, resulting in significant changes to disturbance regimes.

Finch and Smith found that their model supports the contention that Middle Rio Grande cottonwood forests are in decline and will be replaced by other woody species by the end of this century. Nonnative woody species, such as Russian olive and saltcedar, are present throughout the study area and will likely increase in abundance as cottonwood declines. Replacement of cottonwood by these nonnatives will change the structure of the Middle Rio Grande riparian forest by increasing the density of low-stature vegetation and decreasing canopy height. Hydrological models, incorporating greenhouse gas emission scenarios, project that these and other changes will worsen with climate change. You can read about their findings in the latest RMRS General Technical Report.