Drought sensitivities of dominant plant functional types in the Colorado Plateau

Abstract:

Drylands of the Southwestern US are predicted to experience greater water limitations with climate change due to changes in precipitation and increased warming. Certain plants may be living at or near their tolerance thresholds in these ecosystems and thus subtle changes in water availability may have dramatic effects on their performances. We imposed a four-year experiment in the Colorado Plateau to assess the vulnerability of this dryland ecosystem to chronic, but subtle drought using 40 sites varying in plant communities, parent materials and soil textures. Within a site, two plots were selected with matching cover of target species, which were randomly assigned to either control (ambient precipitation) or drought (35% reduction) treatments. Drought treatments were imposed year-round from 2011 through 2014. Over the course of the experiment, we examined plant cover changes and mortality of four dominant plant functional types (PFT’s): C₃ grasses, C₄ grasses, C₃ shrubs and C₄ shrubs. We hypothesized that overall, grasses would be more sensitive to drought than shrubs, and that within these two groups, plants with C₃ photosynthesis would be more sensitive than plants with C₄ photosynthesis.

During three of the four years, precipitation inputs were either near average (50^th percentile, control) or dry (25^th percentile, drought). However in 2012, both treatments experienced extremely dry growing season precipitation with the control and drought below the 5^th and 1^st percentiles, respectively. We observed three general responses to drought in this experiment: 1. change in cover with mortality (C₃ grasses), 2. change in cover without mortality (C₄ grasses and C₄ shrubs) and 3. no change in cover or mortality (C₃ shrubs). The dramatic responses of the C₃ grasses suggest that this PFT is very sensitive to drought and it is living at or near its tolerance threshold in this region. While the C₄ grasses also experienced cover changes, they did not experience widespread mortality and thus have higher tolerance to drought than the C₃ grasses. Finally, contrary to our hypothesis, C₃ shrubs were more drought tolerant than C₄ shrubs. These results suggest that subtle changes in water availability may differentially impact key plant functional types and potentially alter the structure and function of this ecosystem.