Chronic Drought Impacts on Colorado Plateau ecosystems (Rain-Out Experiment)

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In drylands, chronic reductions in water availability (press-drought) through reduced precipitation and increased temperatures may have profound ecosystem effects, depending on the sensitivities of the dominant plants and plant functional types. In this study, we are examining the impacts of moderate, but long-term chronic drought using a network of 40 drought shelters on the Colorado Plateau. This experiment was established in 2010 over a wide range of plant communities and soil types. We are focusing on four key plant functional types (including cool season (C3) and warm season (C4) plants) that are expected to differ in their drought sensitivities: C3 grasses, C4 grasses, C3 shrubs and C4 shrubs. After four years of the experimental treatments, we observed high mortality of the C3 grasses due to the combination of the experimental press-drought and a naturally occurring pulse-drought. In addition, there were moderate reductions in the C4 grasses and shrubs, while the C3 shrubs remained unchanged.

Background & Importance

Passive rainfall reduction shelter in pinon-juniper savanna near Canyonlands National Park, USA (August 2010).

Passive rainfall reduction shelter in pinon-juniper savanna near Canyonlands National Park, Utah (August 2010). (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.  Ecosystems of 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 ecosystems is needed to mitigate deleterious impacts of future droughts.

General Methods

In 2010 (pretreatment year), forty sites were established across the Colorado Plateau in southeastern Utah, ranging in plant communities, parent materials and soil textures common to the region. Within each site, two 150 x 200 cm plots were established over native target species, and then randomly assigned to either control or drought treatments. From 2011 to 2014, control plots received ambient rainfall while drought plots received 35% rainfall reductions year-round using passive rainfall removal shelters. Shelters were 230 x 310 cm wide to provide a large buffer around the plots, with slanted roofs (10° slope, ~1 m tall at the short end) composed of V-shaped plexiglass strips that served as troughs to intercept rainfall. These strips were connected to a gutter system that directed water away from plot. Vinyl flashing was installed vertically around the plot (20 cm outside of plot, but beneath the shelter), to a depth of 30 cm (to prevent belowground flow into plots), with 10 cm remaining above ground (to prevent overland flow into plots) to hydrologically isolate the plots.

A suite of ecosystem response variables are being monitored at each site, including plant and soil cover, plant mortality, plant physiological status, soil water availability, and soil and plant tissue fertility.

Important Results

In 2012, this study site, as well as the majority of the US, experienced one of the worst droughts since the 1930’s Dust Bowl. Based on long-term records, in 2012 the control and drought plots received precipitation below the 5th and 1st percentile, respectively. Therefore, the additive effects of the press- and pulse-drought created a hundred-year drought (in the drought treatment) instead of a twenty-year drought in the pulse-drought only (in the control treatment). 

Rainfall reduction shelter covered in snow in a low elevation blackbrush community near Moab, UT

Rainfall reduction shelter covered in snow in a low elevation blackbrush community near Moab, Utah (March 2012). (Public domain.)

Our results as of 2015 show that the overall press-drought experiment negatively impacted cover of cool season grasses, warm grasses and warm season shrubs, but not cool season shrubs. However, the press-drought affected both grass types starting in 2011, whereas the extreme pulse-drought (2012) had a noticeable greater negative impact on cool than warm season­ grasses. The two shrub types responded very differently to the treatment: the cool season shrubs were not detectably impacted by either drought type, while warm season shrubs had a strong response to the 2012 pulse-drought and this response persisted for the remainder of the experiment. When we examined total mortality (2010 to 2014), cool season grasses were the only plant functional group with significant main effects of the drought, with losses in control and drought treatments (33 and 53% mortality, respectively), as well as differences between treatments.

Further examination of individual level responses for three species were consistent with the patterns observed at the community scale.  The dominant warm season grass (Pleuraphis jamesii) was sensitive to drought across all measured variables, while the cool season shrubs (Coleogyne ramosissima and Ephedra viridis) had little to no responses to the experimental press-drought at any level.

Taken together, these findings suggest that shrubs are avoiding drought, possibly by utilizing moisture at deeper soil layers, while the grasses are limited to shallower layers and must endure the drought conditions. Give this differential sensitivity to drought, a future with less precipitation and higher temperatures may increase the dominance of shrubs on the Colorado Plateau as grasses succumb to drought.

Future Directions

This is a long-term experiment that we expect to continue indefinitely.  Long-term, moderate drought experiments such as this are relatively rare.  We expect this study to continue to provide valuable information on the impacts of moderate drying on dryland ecosystems of the southwest US and globally.

Passive rainfall reduction shelter in a low elevation grassland near Canyonlands National Park, Utah

Passive rainfall reduction shelter in a low elevation grassland near Canyonlands National Park, Utah (August 2014). (Public domain.)