by Julie Kray & Dana Blumenthal

Average temperatures across the western United States have warmed 1-2 ^oF over the past 50 years, and hotter times are on the way¹. With climate change, there is a real concern that many dry places in the West will see more severe droughts², due in part to higher temperatures accelerating evaporation and transpiration, and in part to changes in precipitation.

Less appreciated, however, is the fact that increased atmospheric carbon dioxide (CO₂) can partially close stomatal pores in plant leaves, reducing transpirational water loss³. This increases plant water use efficiency, leaving more water in the soil. But what happens to soil water when these changes in climate and CO₂ occur simultaneously?  Although presently unknown, the answer to this question will ultimately determine whether plants have enough water to grow.

Understanding the combined effect of higher CO₂ & temperature on water will be particularly important in semi-arid grasslands that currently produce much of the nation’s livestock and provide habitat for many native creatures.

That’s why scientists from CSU’s NREL (Bill Parton, Matt Wallenstein), USDA Agricultural Research Service (Jack Morgan, Dana Blumenthal, Dan LeCain), and the University of Wyoming (Elise Pendall, David Williams) put their heads together to learn how predicted changes might shape the future of Western grasslands. First, they fired up Parton’s ecosystem model, DAYCENT, using data from earlier studies, to predict climate change responses of grassland ecosystems in southern Wyoming⁴. Then they created an experiment to test the model predictions in the field: the Prairie Heating and CO₂ Enrichment experiment (PHACE), near Cheyenne, WY. At PHACE, climatic conditions predicted to occur late this century are created over present-day mixed-grass prairie: CO₂ concentration in the air is increased from 390 (current ambient) to 600 parts-per-million, while temperature is raised 2.5 ^oF by day and 5.5 ^oF by night.

Both model and experiment showed that warmer temperatures indeed dried out the soil, but higher CO₂ increased soil water content much more than expected.  Surprisingly, the two effects compensated for each other almost exactly, so that soil water content at the higher temperature and CO₂ levels equaled soil water under current conditions⁵. Higher CO₂ also increased plant growth, particularly in dry years when associated water savings are most important to plant growth, leading to no net decreases in growth with CO₂ and warming together.

While CO₂ compensated for warming effects on water and plant growth, it would be unlikely to compensate for reductions in precipitation as well. In regions where precipitation is predicted to decrease along with higher temperatures, such as the southwestern U.S., climate change is still likely to increase drought². Overall, however, the early results from PHACE appear to bode well for semi-arid grasslands… a rare bit of good news from climate change science!

View a video about the PHACE experiment here:

[vimeo http://www.vimeo.com/43865348 w=500&h=281]

Video production credit: Fireside Production.

EcoPress will feature more stories and discoveries from the PHACE study in the coming months, so stay tuned!

1. http://www.globalchange.gov/publications/reports/scientific-assessments/us-impacts/full-report/national-climate-change
2. Seager, R. & Vecchi, G.A. 2010. Greenhouse warming and the 21^st century hydroclimate of southwestern North America. Proc. Natl Acad. Sci. USA 107: 21277-21282.
3. Morgan, J.A. et al. 2004. Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO₂. Oecologia 140: 11-25.
4. Parton, W.J., Morgan, J.A., Wang, G., Del Grosso, S. 2007. Projected ecosystem impact of the Prairie Heating and CO₂ Enrichment experiment. New Phytologist 174: 823-834.
5. Morgan, J.A. et al. 2011. C₄ grasses prosper as carbon dioxide eliminates dessication in warmed semi-arid grassland. Nature 476: 202-206.