Plant responses to drought on the Colorado Plateau

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This article is part of the Spring 2016 issue of the Earth Science Matters Newsletter

Drylands cover 41% of the Earth’s terrestrial land surface and are one of the most vulnerable regions to climate change. These ecosystems are already water-limited, and most dryland plant communities live near their environmental limits. Therefore, relatively small changes in water availability may lead to large ecological change. Currently, it is unknown whether these ecosystems can keep pace with modeled rates of climate change.

Over the next century, climate models project an increase in two types of drought: chronic but subtle “press-droughts” and shorter-term but extreme “pulse-droughts.” Alone or combined, both types of droughts have the potential to push plants beyond key thresholds, leading to reduced growth or even mortality. Identifying which plants are most sensitive to drought is paramount to predicting the future of these regions.

The Colorado Plateau is a dryland ecosystem in the southwestern U.S. that models suggest could experience press-drought conditions by 2100 due to a 20% decrease in annual precipitation and a 6 °C increase in temperature. To examine the ecosystem consequences of such conditions, scientists funded by the USGS Climate Research & Development Program established a network of 40 rainout shelter sites in southeastern Utah. The study sites span a wide range in plant communities, soil types, and geologic substrates common to this region. Each site consisted of a shelter with clear v-shaped gutters to reduce precipitation by 35% to simulate the combined effects of decreased precipitation and increased temperature and a control plot receiving normal rainfall. The scientists examined the drought sensitivities of four plant functional types that differed in life form (grass vs. shrub) and photosynthetic pathway (C3 vs. C4) over a four-year period. 

map of study sites and image of rainout shelter for drought studies

Left: Map of study sites, represented by black dots. Within the Colorado Plateau (gray shaded area on inset map), sites were selected to represent a wide range of plant communities, soil types and geologic substrates. Right: Example of the rainout shelters utilized in the study. The shelters impose year-round drought by reducing precipitation 35%. Each shelter is paired with a control plot (not shown) with similar vegetation but receiving normal rainfall. (adapted from Fig 1. in Hoover et al., 2015)

(Credit: Adam Kind, USGS. Public domain.)

During the course of the press-drought experiment, a natural pulse-drought occurred, providing the researchers with an opportunity to examine the additive effects of these two drought types. The results revealed that, in this region, C3 grasses are the most sensitive plant functional type to drought, C3 shrubs are the most resistant, and C4 grasses and shrubs have intermediate drought sensitivities. C3 grasses were predicted to be the most vulnerable plant functional type to drought. However, the discovery that C4 shrubs were more sensitive than C3 shrubs was unexpected, given the higher water use efficiency of the C4 photosynthetic pathway. Furthermore, the additive effects of press- and pulse-droughts caused high mortality of the C3 grasses.

The results from this study have important ecological and economic ramifications for the Colorado Plateau. First, C3 grasses are a primary food source for domestic livestock and many native herbivores. A loss of this plant functional type may have large impacts on local food webs as well as regional livestock operations. Second, the unexpected vulnerability of the C4 shrubs suggests that plant functional types defined by life form and photosynthetic pathway may not sufficiently predict drought sensitivities. Finally, this study highlights the importance of the additive effects of press- and pulse-drought, particularly when species are living near their environmental limits.

The paper, Pulse-drought atop press-drought: unexpected plant responses and implications for dryland ecosystems, was published in Oecologia. It is available at https://link.springer.com/article/10.1007/s00442-015-3414-3

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