Decline in the productivity of plants is questioned

A study published in Science magazine with Brazilian participation refutes research indicating a decline in the capacity of forests to sequester carbon and attributes the phenomenon to increasing droughts

By Fábio de Castro

Agência FAPESP – A study published by Science magazine in August 2010 warned of the decline in the productivity of plants, which would have been brought about by the droughts occurring over the last decade. The phenomenon, according to the authors, would be a threat to food security and biofuel production.

The warning, however, has just been refuted by a new study conducted by team of Brazilian and American scientists. According to the new study, published in the August 26 edition of Science magazine, the previous study contained a series of modeling errors, in addition to the fact that it takes statistically insignificant trends into consideration.

The 2010 study was conducted by Maosheng Zhao and Steven Running, both of the University of Montana (United States). The new article had participation from researchers at Universidade Estadual de Campinas (Unicamp), Universidade Federal de Viçosa (UFV) and Boston University (United States).

According to Simone Aparecida Vieira of Unicamp’s Environmental Studies and Research Center (Nepam), the study undertaken by Zhao and Running utilized satellite images and mathematical models to investigate terrestrial net primary production (NPP) – or the quantity of carbon effectively fixed on vegetable biomass – and the climatic variability from 2000 to 2009.  

“Based on these models, they concluded that there was a reduction in NPP and attributed this to the drought registered in the Southern Hemisphere in the period. But there are a series of inconsistencies.”

One of the problems is that the annual drop in vegetation productivity - the roughly 0.1%,  or 1% over a decade - that the researchers cited is below the margin of error for this type of measurement. “We evaluated the results of their sensory remote algorithm against our field measured NPP and we concluded that their algorithm has an average error of 28%, enough to undermine confidence in the results,” Vieira told Agência FAPESP.

UFV and Boston University researchers opted to criticize the models utilized in the 2010 study. The Unicamp scientist, who works with field ecology, was invited to contributed data from field work conducted for his doctoral thesis.

The research developed during his doctoral studies through a FAPESP fellowship is being replicated in the Atlantic Rainforest, after his post-doctoral studies linked the BIOTA-FAPESP program.

“The estimates obtained from measuring trees in the field – not only in my study, but in those of other authors – show that in eight cases the quantity of carbon fixing in vegetation was underestimated by the model. In three cases it was overestimated,” he said.

Another problem with the study, according to Vieira, is that in order to support the affirmation that forests grew less, the author cites a 2009 Science magazine article by Oliver Phillips, of the U.K.’s Leeds University, on the effects of the 2005 drought on Amazonia.

“That article, however, did not say that the forest had grown less. It showed that there was high mortality, prompting biomass to diminish. The growth, however, was stable. Several other articles have also shown this. There are certain tree species that are more susceptible to drought, but when they die, this ends up being offset by the growth of other more resistant species, due to factors like lower competition for light and water,” he explains.

This theoretical NPP reduction that would have been induced by the increased frequency of droughts led to the conclusion that this reduction could have a negative impact on foodstuff and biodiesel production in the world.  The inference, according to Vieira, is risky.

“It must be clear that the increased frequency of droughts is a proven fact and we do not deny it. But the way this study was conducted makes it incapable of proving that there was an NPP reduction. We could not, for example, verify any drop in the foliage area, which would be expected if there was, in fact, a drop in primary productivity,” he affirmed.

According to the Nepam researcher, the new study found flaws in the algorithm used by Zhao and Running to establish the NPP model based on satellite images. The satellite images were also analyzed without taking into consideration the distortions caused by clouds and aerosols.

“The model that was utilized cannot capture exactly what is happening because it does not consider the dynamics of soil humidity and the relationship between the varied ecophysiological processes of the system. The trend they described was neither observed in our field data, nor in an algorithm created by my colleagues,” he explained.     

The conclusions of the 2010 study are grave in Viera’s opinion because they presuppose that forests are not removing carbon dioxide from the atmosphere at the levels that had been imagined.

“Tropical forests are very important for maintaining the global carbon cycle. Presupposing that their importance is not that great could weaken the argument in favor of conservation,” he said.

Reinforcement and answers

The same edition of Science contained another article in which Belinda Medlyn of Macquarie University in Australia also questions the 2010 study.

“Zhao and Running described that net primary production (NPP) declined in the last decade and that this reduction was caused by drought. Nevertheless, her results are not direct measurements, but rather are based on the results of models that have a strong temperature correlation,” he notes.

Science also published Zhao and Running’s comments about the questions raised by Brazilian, Australian and United States researchers. According to the two, the results of the 2010 study were not based on direct measurements and can be explained as a logical consequence of the model applied in the analysis.

“Samanta et al. and Medlyn challenge our report of reduced global terrestrial net primary production (NPP) from 2000 through 2009. Our new tests show that other vegetation indices had even stronger negative changes through the decade, and weakening temperature controls on water stress and respiration still did not produce a positive trend in NPP. These analyses strengthen the conclusion of drought-induced reduction in global NPP over the past decade,” the authors of the original study commented.

The articles “Comment on ‘Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009’ ” (doi:10.1126/science.1199048), by Arindam Samanta, Simone A. Vieira and others,  “Comment on ‘Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 to 2009’” (doi:10.1126/science.1199544), by Belinda E. Medlyn, and “Response to Comments on ‘Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009’ ” (doi:10.1126/science.1199169), by Maosheng Zhao and Steven W. Running, can be read by Science subscribers at www.sciencemag.org.