A three-year study provides insights into how the productivity of a semi-arid rangeland, containing grasses using different photosynthetic pathways, will change in a warmer world with more atmospheric carbon dioxide. See Letter p.202
The grasses that provide forage for most of the world's livestock use either the C3 or the C4 photosynthetic routes to fix carbon dioxide into carbohydrates. Both routes eventually employ the carbon-fixing enzyme ribulose bisphosphate carboxylase. However, C4 grasses are more efficient photosynthesizers as a result of their distinct morphology and their use of another enzyme, PeP-carboxylase, before this ultimate step of carbon fixation. On page 202 of this issue, Morgan et al.1 describe experiments aimed at assessing the effects on mixed C3/C4 grasslands of raised levels of atmospheric CO2 and higher temperatures, and, crucially, how these conditions might change the plants' water budgets.
As any high-school science student or climate-change sceptic knows, CO2 is food for plants. So the presumption that 'more is better', as we release CO2 at an undiminished rate from fossil-fuel combustion, is partly true. This effect occurs because photosynthesis is stimulated when leaves are exposed to above-ambient levels of CO2. But the rate of increase in photosynthesis with rising CO2 will eventually diminish and approach an asymptote as CO2 levels reach double that of the present day2,3.
How other factors affecting leaf photosynthesis and its conversion of carbohydrates into plant matter respond to elevated CO2 depends on a variety of conditions: for example, whether plants are growing in isolation or in groups4; are woody or herbaceous4,5; are growing as monocultures or as mixed species4,5; are growing under natural or managed conditions4,5,6; utilize the C3 or the C4 photosynthetic pathway3,7; are evaluated after the course of a growing season or after many years4,5,8; or are exposed to a warmer or drier climate3.
Morgan et al.1 disentangle part of this web of potentially interacting factors by presenting convincing evidence that, compared with C3 grasses, C4 grasses growing under semi-arid climatic conditions will prosper in a world with higher CO2 concentrations and warmer temperatures. In principle, this response depends on water — the leaf pores, known as stomata, of C4 plants open less widely than those of C3 plants, so C4 plants tend to lose less water vapour through transpiration.
Morgan and colleagues' take-home message is the following: water savings proffered to C4 grasses from partial stomatal closure, induced by high CO2 concentrations, offset any enhancement in evaporation brought about by coincident warming. They draw this conclusion from a novel, multifactorial experiment that exposed plots of C3 and C4 grasses to combinations of ambient or elevated CO2 and/or temperature for three years. For the single-factor plots, they found that elevated CO2 favours the productivity of C3 grasses and that elevated temperature favours that of C4 grasses, as expected3. However, a combination of elevated CO2 and higher temperature favours increased C4 productivity because water-use efficiency (the ratio between photosynthesis and transpiration) in C4 grasses is greater than in C3 grasses.
Thousands of papers and hundreds of reviews4 have been published on how plants respond to elevated CO2. So why is this particular study noteworthy?
First, Morgan and colleagues1 provide one of the first and best views of how a mixed-grass ecosystem (Fig. 1) growing in a semi-arid climate will respond to future CO2 and climatic conditions. The task of divining this information is not trivial, because the perturbation of an ecosystem engenders a suite of nonlinear, positive and negative feedbacks that operate across an array of timescales4. Second, their data counter the conventional wisdom derived from physiological studies that leaf photosynthesis and water-use efficiency in C4 grasses benefit less from high CO2 than in C3 grasses3. Third, the interactive effects of CO2, temperature and soil moisture on C3 and C4 grasses have until now remained elusive. Without the warming treatment, Morgan and colleagues' results would only have confirmed knowledge gained from previous meta-analyses that compared the responses of C4 and C3 grasses to raised CO2 — both C3 and C4 plants respond positively to enhanced CO2 (ref. 7).
However, users of this new information1 must appreciate the conditional aspects of Morgan and colleagues' results. Short-term studies of elevated CO2, over single to several growing seasons, manipulate the ecosystem's eco-physiological 'knobs', such as stomatal conductance, leaf-area index and the soil–water balance. In such circumstances, elevated CO2 will indeed cause stomata to close partially, restricting leaf transpiration and conserving soil moisture. But enhanced growth increases the population of leaves and so counteracts potential soil-moisture savings — up to a limit.
Whether this net moisture saving is critical to plant productivity depends on how much rain falls during the growing season. In a more humid environment, the water saving through CO2-induced stomatal closure in C4 plants may be less consequential, as shown in 2009 when precipitation at Morgan and colleagues' test site was greater than normal1. If there is severe drought, however, a rise in CO2 may not provide enough water savings to make a difference. When warming is added to the mix, it can counter the leaf's physiological response to elevated CO2. Concurrent warming promotes transpiration by increasing the gradient in humidity between the leaf surface and the atmosphere. Warming also increases rates of leaf and root respiration, thereby diminishing net carbon acquisition and reducing plant growth. Consequently, the answer to the question, 'How will this mixed-grass ecosystem respond to changes in mean CO2?' will depend, in part, on whether or not climatic extremes in the moisture and thermal environments occur and physiological thresholds are crossed.
In contrast to short-term experiments, studies lasting longer than a decade manipulate biogeochemical and ecological knobs such as leaf and root nutrition, species composition and plant competition. Downregulation of photosynthesis typically occurs in long-term studies of elevated CO2, because the nitrogen needed for ribulose bisphosphate carboxylase becomes scarcer as plant biomass expands and sequesters the limited supply of nitrogen available to the plants' roots2,5,8. These longer-term responses are not captured in Morgan and colleagues' three-year study. Hence, additional years of studying this ecosystem may produce alternative outcomes. Moreover, findings that apply to mixed C3/C4 grasslands may not relate to other ecosystems such as forests4.
What do these results1 suggest for the future? At present, C4 grasses tend to inhabit warmer and drier climates than do C3 grasses3,7. If the trends in atmospheric warming and increasing CO2 persist over decades, a gradual shift in species composition may occur, favouring C4 grasses. Any change in the mixture of C3 and C4 grasses will have a biophysical feedback on the climate system by changing how solar energy is absorbed and partitioned into heating the air and evaporating soil moisture9. Such changes in species composition may also affect ecosystem services — for example, the amount and availability of forage for grazing cattle, bison and antelope.
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