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Natural greenhouse

Climate changes are likely to affect the composition of tropical forests

FABIO COLOMBINIJatobá: harmed by too much carbonFABIO COLOMBINI

If concentrations of carbon dioxide (CO2), the main greenhouse gas, keep on rising, the profile of the trees in tropical forests may change significantly over the next few decades. Studies coordinated by Carlos Alberto Martinez, from the University of São Paulo (USP) in Ribeirão Preto, suggest that tree species classified as pioneers – the first to occupy an open area, because they are born and grow rapidly – may become the dominant ones in forests if the levels of gas double or even rise by 50 percent. This competitive advantage has an explanation: even with high levels of carbon dioxide, this type of tree carries out photosynthesis at appropriate levels. Trees that grow more slowly, on the other hand, develop less in environments where CO2 is over a certain level.

Martinez compared the response of four tree species – two pioneers, embaúba (Cecropia pachystachya) and the dragon’s blood tree (Croton urucurana), and two non-pioneers, the pink jequitiba (Cariniana legalis) and guarantã (Essenbeckia leiocarpa) – in scenarios with three concentrations of CO2: 360 parts per million (ppm), a level a little below the current level; 540 ppm, 50 percent greater; and 720 ppm (level predicted for 2070 if emissions do not retreat). Samples of the species were placed in chambers into which CO2 was injected and the level of gas monitored to avoid undesirable fluctuations.

Published in 2008 in the book Photosynthesis: energy from the Sun, the result of the experiment showed that pioneers in any scenario managed to increase photosynthesis. “These are plants that take from 10 to 15 years to reach the adult phase and have what we call a strong drain, the bole, which is capable of absorbing and accumulating extra levels of CO2,” explains Martinez. “This guarantees their fast development.”

In non-pioneers, the response was very different. The plants in this group only take good advantage of CO2 in photosynthesis up to concentrations of 540 ppm. Over this level, and until it reaches 720 ppm, a drop of up to 50 percent in the capacity to use the extra gas was recorded, compared with the control scenario (360 ppm). “The non-pioneers need 50 to 100 years to reach maturity and they live for 100 to 1,000 years. When the tree starts growing the bole of non-pioneers is ill prepared to accumulate this extra CO2,” the researcher says. “There seems to be a saturation limit above which the species is unable to respond further in a positive way and its photosynthesis capacity begins to decline.” Martinez is testing an explanation for the worse performance of the pink jequitibá and the guarantã in an environment rich in carbon dioxide: the possible accumulation of carbohydrates in the chloroplasts, the power plant of vegetable cells in non-pioneer species, might be responsible for the reduction in the levels of the rubisco enzyme, which is fundamental for fixing and assimilating carbon.

However, the presence of too much carbon dioxide is not the only variable to be considered when analyzing the possible impact of climate change on plants. It is also necessary to evaluate other stress factors, such as luminosity, temperature variation and soil nutrients. Therefore, Martinez made his analyses a little more sophisticated. He crossed variables and showed that when grown in soil poor in nutrients at a CO2 concentration of up to 720 ppm, pioneers lose about 40 percent of their capacity to absorb the gas available for photosynthesis. Under these conditions, the reduction in non-pioneers is 60 percent. In other words, even so the first group of trees has an advantage over the second. In a scientific paper to be published in January 2011 in Environmental and Experimental Botany, the researcher from USP shows that pioneers can also tolerate bright light situations better. This is another stress factor that may be exacerbated by climate change.

Realistic test
For Martinez, the different variables involved in the phenomenon of global warming should be evaluated together and not in isolation, so that the real conditions of forests can be replicated as strictly as possible. “If these results are maintained in a natural environment, pioneer plants would be more competitive in a possible future scenario of global warming and CO2 concentrations above 540 ppm” , is the researcher’s analysis. “This seems to be a trend that needs to be confirmed by broader studies involving other plant species and families.” The USP researcher should be studying, in the near future, what happens to plants when besides coming into contact with extra CO2 they are also exposed to a rise in temperature. Martinez is blunt: we need to know the comparative advantages of plants in order to cope with a scenario that will certainly be adverse.

The project
Impact of high concentrations of CO2 on the physiology and initial growth of four Brazilian forest species, in a future climate simulation (nº 2005/54804-7); Type Regular Line of Research Aid; Coordinator Carlos Alberto Martinez y Huaman – FFCLRP/USP; Investment
R$ 13,010.59 (FAPESP)

Scientific article
MARTINEZ, C. A. et al. The effects of elevated CO2 on tropical trees are related to successional status and soil nutritional conditions. In ALLEN, J. F. et al. (org.), Photosynthesis. p. 1.379-82. 2008.