Grasslands and dry rangelands cover over 30% of the earth’s terrestrial surface and increased population growth is limiting the natural soil water supply in these areas. Changes in soil water supply depend on precipitation, temperature, CO2 concentration, and various soil properties. Most of the world’s livestock depend on this supply of grasslands to eat and survive, and climate change, including increased temperatures and CO2 concentrations, may affect grass productivity. As CO2 concentration increases, stomates need not open as long, allowing plants to retain more water and increase water-use efficiency. However, this increased efficiency may be due to increase overall biomass of the canopy level that results from an increase in CO2. Most grasslands contain both C4 and C3 photosynthetic categories of plants and in the Prairie Heating and CO2 Enrichment (PHACE) experiment reported here (2011), Morgan et al. examined changes in plant productivity and soil water content in response to increases in CO2. They concluded that increased CO2 concentrations offset deleterious effects of increased temperature, maintaining soil water content at levels that occur today and increasing productivity in C4 plants .—Taylor Jones
Morgan, J., LeCain, D., Pendall, E., Blumenthal, D., Kimball, B., Carrillo, Y., Williams, D., Heisler-White, J., Dijkstra, F., West., M., 2011. C4 grasses prosper as carbon dioxide eliminates desiccation in warm semi-arid grassland. Nature 476, 10274–10279.
Jack A. Morgan and colleagues created the PHACE experiment to evaluate the responses of native mixed-grass prairie to one year of increased CO2 exposure and to a three year period of combined CO2 exposure and increased temperatures. In 2006, the ambient CO2 concentration was measured at the ambient level of 385 p.p.m.v. It was experimentally increased to 600 p.p.m.v. In 2007–2009, the temperature was increased by 1.3/3.0o C (day/night) in the canopy. The free-air CO2 enrichment (FACE) was used to alter air composition and the T-FACE system was used to alter the temperature. The authors found that increased temperature and increased CO2 concentration had opposite effects on soil water content (SWC). As CO2 increased, SWC also increased by 17.3%, as predicted by the increased water efficiency due to more closed stomata. However, as the temperature increased, SWC decreased by 13.1%. There was no difference between the control (15.5%) and the combined increased temperature and increased CO2 plot, suggesting that the water conservation effects of increased CO2 cancel out the drying effects of warmer temperatures.
Using the same experimental design, the authors investigated the effects of increased CO2 concentration and increased temperature on above-ground biomass (AGB) and below-ground biomass (BGB), both estimates of a plant’s productivity and growth. As expected, the prairie exposed to increased levels of CO2 increased the amount of above-ground biomass by an average 33% over the first three years, supporting the benefits of CO2 enrichment on plant productivity and growth. The same positive effect of increased CO2 concentration appeared for the growth in roots comprising the BGB. To continue the study of the effects of increased CO2 on SWC and plant productivity, Morgan et al. examined how these changes in AGB related to the soil matric potential (energy of soil water per unit volume) using the biomass enhancement ratio. A strong negative correlation between the soil matric potential and the biomass enhancement ratio resulted and is likely due to the increased water efficiency under conditions of high CO2 concentrations, showing that increased CO2 concentration will increase productivity when water is limited. Morgan et al. also compared the results of this portion of the experiment with results from other similar areas of the Great Plains.
Through these experiments, the authors distinguished significant differences between C3 and C4grasses. C3 grasses showed 34% more growth during increased CO2conditions, but increases in temperature did not have an effect. C4 grasses showed 28% more growth during increased CO2 conditions but also increased growth during warming, suggesting that C4 grasses could be more productive in future atmospheric conditions of high CO2 and increased temperature. The results also suggest that increased CO2 concentration may counter the effects of extreme dryness due to increased temperatures in the future. The authors also created a model analysis that changed the canopy resistance to water loss (change in stomatal closure) through various temperatures, and recorded the effects on the evapotranspiration rate (the combined rate of evaporation and transpiration of the prairie). The results showed that the temperature effect and the increased CO2 effect almost exactly offset one another. Despite this trend, the authors predict the efficiency advantages of increased CO2 will not be able to offset extreme drought conditions and regions like southwestern North America or the Mediterranean may not benefit from these effects. The results from this experiment only show one example of the benefits of increased CO2 in a semi-arid plant community, but it is clear that under certain circumstances, increased CO2 concentration can increase plant efficiency and water use.