Colorado Plateau Extreme Drought in Grassland Experiment (EDGE)

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In drylands, short-term extreme droughts can have profound ecosystem effects, depending on the timing (seasonality) of drought and the sensitivities of the dominant plants and plant functional types. Past work suggests that cool season drought may disproportionately impact regionally important grass and shrub species. In this study, we are examining the impacts of extreme seasonal drought on grassland communities of the Colorado Plateau. This experiment was established in 2015 at one large site near Canyonlands National Park in southeastern Utah. We are focusing on the impacts of extreme (a 66% reduction in precipitation) warm and cool season drought on grass and mixed grass-shrub communities using large, moveable passive rainfall reduction shelters. This ongoing experiment is part of a larger network imposing extreme drought in grasslands globally (http://edge.biology.colostate.edu/). 

Background & Importance

A curved-roofed structure that excludes most of the precipitation from landing on the dryland plant community below.
EDGE rainout shelter in a dryland ecosystem. (Credit: Michael Duniway, Southwest Biological Science Center. Public domain.)

Drylands cover 41% of the terrestrial land surface and are one of the most vulnerable regions to changes precipitation and temperature. These arid and semiarid ecosystems are extremely resource limited, most notably for water, and thus any changes in water availability can lead to large ecological shifts. Like many drylands, the southwestern U.S. is experiencing warming and changes in precipitation. The early (2001-2010) was the warmest decade in in the instrumental record and models predict continued warming for this region. While periods of drought are a climatic feature of the southwest, recent “hot droughts” have had profound impacts on many southwest ecosystems.  Grassland on the Colorado Plateau provide a myriad of important ecosystem services, including forage for livestock, habitat for wildlife, and conservation of soil resources. Thus, gaining new insights into drought vulnerabilities of Colorado Plateau grasslands is needed to mitigate deleterious impacts of future droughts. 

General Methods

We are imposing extreme seasonal drought by excluding 66% of the natural precipitation using shelters that cover both grasses and shrubs. We have three precipitation treatments: 1) drought during the summer/fall growing season (May – October), 2) drought during the winter/spring cool season (November – April) and 3) ambient. We have applied each of these treatments to two different plant communities: 1) perennial grass community without shrubs and 2) mixed shrub-grass communities. Rainfall manipulations treatments will be imposed for four years (2015 – 2018). Experimental plots are 4 x 4 m and hydrologically isolated.  We passively alter rainfall reaching the plots by using a version of a hoop house rainfall reduction shelter (6 m x 9 m and 3 m tall). There are 6 replicates of each treatment (3 precipitation x 2 plant community; total of 36 plots). 

Treatment effects on plant cover, composition, and physiology, as well as micrometeorological conditions are being measured.  Plant cover, composition, and production are measured annually in spring and fall. Net photosynthesis and plant water status are measured monthly during the growing season. Plant phenology is measured by weekly. We have a complete meteorological station at the study site (data are available here) as well as micrometeorological stations measuring the effect of shelters on potential evapotranspiration. Soil moisture and temperature at two depths in each experimental plot is recorded by data logger.

Dryland plant community beneath rainout shelter
Collecting data on dryland plant community beneath EDGE rainout shelter. (Credit: David Hoover, USGS. Public domain.)

We have numerous side studies taking place, including detailed root ecohydrologic investigations, in-depth studies on shrub ecology (Ephedra species), and others.

Future Directions

This is a relatively short-term, intensive experiment that we expect to provide valuable insights into the vulnerability of grassland ecosystems to extreme seasonal drought.  Drought experiments of this scale are relatively rare—especially on the Colorado Plateau.  We expect this study to provide valuable information on mitigating potential impacts of extreme drought on dryland ecosystems of the southwest US.

Field technicians taking root data on dryland plant species
Field technicians collecting root data on dryland plant species. (Credit: David Hoover, USGS. Public domain.)