Aug 05 2011 | Posted by SSSandy

Plants may lose less water under global warming than expected, scientists find

Some plants may use less water under global warming, says a University of Sydney scientist involved in a major study published in the journal Nature.

University of Sydney soil scientist Dr Feike Dijkstra was one of a group of researchers who found that higher levels of CO2 in the atmosphere caused leaf stomatal pores in grass species to partially close, lessening the amount of water that can escape.

“Warmer temperatures lead to drier soils as more water is transpired by plants into the atmosphere,” Dr Dijkstra said. “But we found that with rising CO2 levels plants don't have to open their stomatal pores as long to take in the CO2 needed for photosynthesis, and thus conserve more water.”

The study is being carried out by the US Department of Agriculture and university scientists in semi-arid grasslands in Wyoming in the United States. “The nice thing about this experiment is that we looked at the effect of both elevated CO2 levels and higher temperatures, something that hadn't been done before in semi-arid grasslands,” Dr Dijkstra said.

The scientists surrounded plots approximately three metres in diameter with a pipe that released CO2, simulating global warming conditions predicted for the end of the century: 600 parts per million (ppm) of CO2 — compared to today's average 390 ppm. Thermal infrared heaters were used to increase the temperature by between 1.5 and 3 degrees Celsius.

The results showed that CO2 does more to counterbalance warming-induced water loss in soils than previously expected. In fact, simulations of warming and CO2 predicted for later this century demonstrated no net change in soil water, and actually increased levels of plant growth for warm-season grasses.

These findings will be significant for the 30 percent of the earth's land surface that is covered by grasslands, providing much of the feedstock for livestock. A large proportion of this land is in Australia.

The results published today cover the first four years of the eight-year Prairie Heating and CO2 Enrichment (PHACE) experiment on native northern mixed grass rangeland at the High Plains Grasslands Research Station near Cheyenne, Wyoming.

Agricultural Research Service plant physiologist and study leader Jack Morgan said warmer temperatures would have different effects on different grass types. “Only the warm-season grasses had their growth boosted higher by CO2 and warmer temperatures,” Morgan said.

“If this leads to a competitive advantage for warm-season grasses, it may increase the challenges faced by ranchers who desire cool-season grasses for early-season forage.”

Morgan said more research is needed to determine how the water-savings effect applies over the long run, as well as to croplands in semi-arid areas.

“It is important to understand that CO2 only offset the direct effects of warming on soil water in this experiment, and that it is unlikely to offset more severe drought due to combined warming and reduced precipitation projected for many regions of the world,” he said.