Ecophysiological responses of three dominant species to experimental drought on the Colorado Plateau

Abstract:

Water limitations in dryland ecosystems are predicted to intensify with climate change due to the combination of decreased precipitation and increased warming. Plants in these ecosystems may be living at or near their tolerance limits, and thus subtle changes in water availability may have dramatic effects on their performance. To examine the impacts of subtle, but chronic reductions in water availability, we established a network of 40 rainfall removal shelters across a range of plant communities, soil types and elevations in the Colorado Plateau. Each site consisted of a control plot receiving ambient precipitation paired with a drought plot that received a 35% precipitation reduction. After three years, we observed a range of ecosystem-level responses to the treatments by key plant functional types. The experimental drought had dramatic effects on the C₃ grasses (mortality and cover changes), but the treatment effects were relatively minor for the C₄ grasses (cover change only) and C₃ shrubs (no treatment effects on cover or mortality). We investigated the mechanisms behind the relative drought tolerances of the latter two plant functional types by measuring the ecophysiological responses of three dominant species on the Colorado Plateau: Pleuraphis jamensii (C₄ grass), Coleogyne ramosissima (C₃ shrub) and Ephedra viridis (C₃ shrub).

During the 2014 growing season, we measured mid-day leaf water potential and net photosynthesis monthly for these dominant species under the control and drought treatments (n=5). We analyzed the effects of treatment, month and their interaction on these measurements using a mixed effects model for each species separately. Overall, P. jamensii was the most sensitive to drought of the three focal species as evidenced by significant effects of drought on both leaf water potential and net photosynthesis (30% reduction). Neither of the C₃ shrubs had significant treatment effects on either ecophysiolgoical variable. These results provide mechanistic evidence behind the ecosystem-level effects; the drought treatments are causing stress in C₄ grasses but not C₃ shrubs. These results suggest that subtle but chronic changes in water availability may alter the structure and function of the Colorado Plateau ecosystem by differentially impacting key plant functional types.