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Warmer Does Not Mean Drier

CO2 are pumped into a plot that is also warmed by infrared heaters.

Controlled concentrations of CO2 are pumped into a 3-metre diameter plot that is also warmed by infrared heaters overhead.

By Feike Dijkstra

A warmer climate causes grasslands to dry out faster, but a new study has found that more efficient water use by plants in response to rising CO2 concentrations in the atmosphere could completely offset the drying effect caused by warming.

Global temperatures are on the rise as a result of increased greenhouse gas emissions into the atmosphere. To cope with the warmer temperatures, plants transpire more water to help them cool off. As a result, terrestrial eco­systems use more water and dry out faster.

In arid and semi-arid ecosystems, where rainfall is limited and irregular, a warmer and drier environment may have irreversible effects on plant communities. Existing plants may be displaced by plant species that are better adapted to drier conditions. This could significantly alter the arid and semi-arid woodlands and grasslands that dominate the Australian landscape and that provide an important source of forage to livestock.

Not only are global temperatures rising, but so too is the global atmospheric CO2 concentration. But unlike rising temperatures, a rise in atmospheric CO2 has a water-saving effect on plants. In a recent field experiment my colleagues and I found that the water savings caused by an increase in atmospheric CO2 could completely offset the drying effect caused by warming.

The human population emits roughly 5.5 Gt of carbon as CO2 into the atmosphere each year. If CO2 emissions are not curtailed soon, atmospheric CO2 concentrations will more than double by the end of this century. As a consequence, the Earth will be 3.5–6°C warmer in the latter half of this century than it is today.

Both the increase in temperature and the rise in atmospheric CO2 concentration have important consequences for terrestrial ecosystems, but their combined impact on terrestrial ecosystems remains unknown. Some have suggested that because of the fast pace of climate change compared to changes during past geological timescales, plants and other organisms won’t have time to adapt to the new environmental conditions, and therefore existing plant communities will dramatically change in composition – with possibly faster rates of species extinction.

Increases in temperature and atmospheric CO2 will not only affect plants, but also organisms (mostly microbes) that are responsible for the decay of soil organic matter and release of CO2 back into the atmosphere. Because soils globally store more than twice the amount of carbon in organic matter than can be found in the atmosphere as CO2, even small changes in soil carbon can have profound effects on atmospheric CO2 concentrations.

Arid and semi-arid grasslands are thought to be particularly vulnerable to climate change because these systems already are limited by the availability of water. More than 30% of the Earth’s terrestrial surface is covered by areas dominated by grass, including the vast Central Asian steppes, North American prairies and Australian grasslands. These grasslands provide most of the forage for the world’s domestic livestock, but productivity of these lands is primarily limited by water. An increase in temperature that reduces soil water availability will therefore put more stress on plant productivity and on the livestock and livelihoods these grasslands support.

In a warmer world, plants transpire more water through their leaves because warmer air can hold more water vapour, thereby enhancing evaporation from the leaf. This increase in evaporation from the leaf will also help the plant to cool off. Therefore a consequence of higher temperatures is that plants will take up more water from the soil, so the soils will dry out faster. When soil moisture is not replenished with rain water, the acceleration of soil drying reduces plant growth.

On the other hand, an increase in atmospheric CO2 can reduce plant transpiration. Under higher atmospheric CO2 conditions, plants do not need to keep their leaf pores or stomata open as much to let CO2 into the leaf for photosynthesis. Every time plants open their stomata to let atmospheric CO2 in, they lose water into the atmosphere. Thus, at the leaf level, plants conserve more water in a CO2-enriched environment.

While relationships between temperature and water use or atmospheric CO2 and water use at the leaf level are well understood, much less is known about the effects of temperature and CO2 at the ecosystem scale, partly because these effects are difficult to measure at a large scale. Since the early 1990s, scientists have started designing new types of experiments to get a better understanding of climate change impacts on whole ecosystems. In these experiments climate change is simulated in the field.

For instance, an increase in atmospheric CO2 can be simulated by pumping pure CO2 into plots from pipes surrounding the plot. To maintain the atmospheric CO2 concentration inside the plot, the flow of pure CO2 pumped into the plot is continuously adjusted based on measurements of wind direction and velocity using a computer-controlled system. This technique is known as “free-air CO2-enrichment” (FACE).

Field experiments simulating global warming initially used heating cables buried in the soil to warm the soil, while later experiments used infrared heaters mounted above the plant canopy to warm the plants. These experiments have been conducted all over the world in different types of ecosystems, including peatlands, forests, croplands and grasslands. As a result of these experiments, we now know a lot more about the separate effects of CO2 enrichment and warming on plant productivity, carbon and nutrient cycling. However, we still know very little about their combined effects.

Only a handful of experiments exist where the combined effects of elevated CO2 and warming are studied, and only one of those studies is being done in a natural semi-arid grassland. Our experiment, conducted in Wyoming (USA), is a collaborative effort of research teams from the Agricultural Research Service of the US Department of Agriculture, Colorado State University, University of Wyoming and the University of Sydney. In this experiment we used FACE technology to increase atmospheric CO2 to 600 ppm, and infrared heaters to increase the temperature by 1.5°C during the day and 3°C during the night in 3-metre diameter plots.

These increases in atmospheric CO2 and temperature fall within the predictions of the Intergovernmental Panel on Climate Change for the latter half of this century. This experiment is unique in that both the combined and separate effects of CO2 enrichment and warming are investigated in a natural vegetation.

After 4 years of study, we found that the increase in atmospheric CO2 completely offsets the drying effect of warming. As expected, soil moisture levels decreased in plots that were only subjected to warming. On the other hand, soil moisture increased in plots that were only treated with higher CO2.

Higher CO2 often stimulates plant growth, and therefore could potentially increase transpiration. However, in this case the increase in transpiration due to increased plant biomass was relatively small, and overall the plants saved more water with higher CO2 only.

Because warming and higher CO2 levels produced opposing effects, the combined effects of warming and higher CO2 levels did not alter soil moisture compared with plots kept under ambient conditions.

The water savings due to higher CO2 levels are often not taken into account in climate models, and most models predict a drier environment with climate change. But what we have shown with this field experiment is that global warming does not necessarily result in a drier environment, at least not in semi-arid grasslands. Of course, changes in rainfall due to climate change have large impacts on water availability in these systems, but long-term regional rainfall predictions remain difficult and are attached with a high degree of uncertainty. Nevertheless, the water-saving effect of CO2 enrichment offsetting the drying effect of warming needs to be included in climate models when predicting the susceptibility of semi-arid grasslands to climate change.

Regardless of the compensating effect of increased CO2 on soil moisture in a warmer world, combined warming and CO2 enrichment resulted in an important plant species shift to a greater abundance of C4 grasses. The C4 grasses differ from C3 grasses in the way they fix CO2 from the atmosphere. Overall, C3 grasses photosynthesise less efficiently than C4 grasses. Consequently, C3 grasses are more responsive to an increase in atmospheric CO2, and therefore are believed to grow better in a higher CO2 environment than the C4 grasses.

C4 grasses, on the other hand, are better adapted to hotter conditions, and therefore should perform better in a warmer world.

The semi-arid grassland in Wyoming, as well as many semi-arid grasslands in Australia, consist of a mixture of both types of grasses. Outperformance of one type of grass over the other because of climate change is a real possibility that can completely alter the functioning of these grasslands.

As expected, we found that an increase in temperature alone favoured the C4 grasses in the Wyoming experiment. However, both C3 and C4 grasses performed better in an environment with an increase in CO2 alone, most likely because of improved soil moisture conditions due to higher CO2 levels. However, when higher CO2 and temperature were combined, the C4 grasses prospered over the C3 grasses.

If trends in atmospheric warming and increasing CO2 persist, this may lead to a competitive advantage for the warm-season C4 grasses. This will alter the solar energy absorbed by the vegetation and soil, and consequently how much of this energy is transferred into heating the air and used to evaporate soil moisture.

A greater abundance of the warm-season C4 grasses also has implications for livestock grazing. Growth of warm-season C4 grasses occurs later in the year than for the cool-season C3 grasses. The forage quality of C4 grasses also tends to be lower than for C3 grasses, and the C3 grasses are generally preferred by graziers that can provide for early-season forage. Large areas of Australia are covered by grasslands with C3/C4 grass mixtures, and with an increased abundance of C4 grasses, graziers who depend on these lands may face increasing challenges to feed their livestock.

Rising atmospheric CO2 and temperature not only affect the water budget and plant species composition, but also have other important implications: they affect the amount of carbon sequestered in plants and soil as well as the exchange of greenhouse gases with the atmosphere, including the potent greenhouse gases methane and nitrous oxide.

If carbon sequestration increases and greenhouse gas emissions reduce in response to climate, this might then act as a buffer against global warming. On the other hand, if greenhouse gas emissions increase due to rising atmospheric CO2 and temperature, this could accelerate global warming.

Changes in carbon sequestration and greenhouse gas emission from these grasslands will depend on many different factors, including the availability of soil moisture and nutrients, and the composition of the plant and soil microbial community, all of which can be affected by a rise in atmospheric CO2 and temperature. We are just beginning to grasp what the consequences of climate change are for grassland systems and other types of ecosystems, and how that may feedback to global warming.

A big unknown is how climate change will affect terrestrial ecosystems in the long-term. The effects in the Wyoming field experiment were observed after 4 years of experimental CO2 enrichment and warming, but it is unclear if these effects will persist in the long term.

One of the longest climate change field studies, the Duke FACE experiment conducted by Duke University in a loblolly pine forest plantation in North Carolina, USA, has been operating for 19 years. Important information has been gathered from this experiment since then, but funding agencies are usually not interested in financing these experiments for more than 4–5 years due to high operating and maintenance costs even though substantially more research is required to better understand the long-term ecological costs of climate change.

Feike Dijkstra is a Senior Lecturer in Biogeochemistry in the Faculty of Agriculture, Food and Natural Resources at the University of Sydney. He is a research collaborator of the Prairie Heating and CO2 Enrichment experiment in Wyoming, USA.