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Changes in the growth and species composition of tropical forests cannot be fully explained by global environmental changes, say researchers.

Recent studies in the Amazon rainforest have suggested that changes such as the increase in atmospheric carbon dioxide (see Carbon emissions 'may alter forest growth patterns') and other factors such as nutrient deposition, temperature, drought frequency and irradiance are increasing the productivity and biomass of forests.

Shifts in the composition of plant species — favouring fast-growing species over slow-growing ones — have also been noted.

But the results of research, carried out to find if this was true of other tropical forests, do not fit this theory.

The study, published in PLoS Biology this month (4 March), found that past human and natural disturbances, as well as changes in resource availability also contribute to forest change. 

Researchers analysed data from tropical regions of Africa, America and Asia, covering more than two million trees and 400 hectares in ten large plots over a 20-year period.

They found that aboveground biomass only increased in seven out of the ten plots  and, at half the sites, slow-growing species increased in dominance over fast-growing plants.

Helene Muller-Landau, from the Smithsonian Tropical Research Institute in Panama and one the authors of the study, says that even though environmental factors such as increasing atmospheric carbon dioxide and other human influences are almost certainly favouring some species over others, these are not the only factors affecting tropical forests at any given site.

"Past and current local human influences, as well as local natural influences, can also lead to increases in some species at the expense of others … The story is more complex," she told SciDev.Net.

Muller-Landau says the discovery that slow-growing species are also thriving "should not be taken as evidence that we can proceed with business as usual".

"There is no question that anthropogenic global change is a problem," she says.

"What it does suggest is that we need to be more aware of differences among tropical forests in their trajectories and in the influences of local and global anthropogenic change".
Link to full paper in PLoS Biology


PLoS Biology doi:10.1371/journal.pbio.0060045 (2008)