Staff photo Rose Lincoln/Harvard News Office

Science & Tech

Warming may not spark tree growth

6 min read

A bright spot in the gloomy global warming picture has been scientists’ predictions that at least some carbon dioxide will be removed from the atmosphere by a burst of growth from tropical forests.

New research from the Arnold Arboretum, however, questions that prediction, finding that trees in two forests on opposite sides of the world have been growing dramatically slower, not faster, as temperatures have risen over the past 20 years.

Kenneth Feeley, a postdoctoral fellow at the Center for Tropical Forest Science, a partnership between the arboretum and the Smithsonian Tropical Research Institute, examined tree growth data from forest plots in Panama and Malaysia.

The 50-hectare plots are part of a remarkable network of 18 scientific forest plots established by the Center for Tropical Forest Science starting in 1981. The center exhaustively studies the trees in each plot, measuring them every five years, creating a database of growth information spanning 3 million individual trees of more than 6,000 different species.

Feeley examined growth data from the two oldest plots, at Barro Colorado Island in Panama and at Pasoh, Malaysia. The results surprised him, both because they showed growth slowing, not accelerating, and because of the magnitude of the slowdown.

“I was surprised at the magnitude and at how widespread it was,” Feeley said. “In Malaysia almost every single species was declining in growth. It is very rare that you find anything in these forests consistent across 800 species.”

The study, conducted with colleagues S. Joseph Wright from the Smithsonian Tropical Research Institute, M.N. Nur Supardi and Abd Rahman Kassim of the Forest Research Institute Malaysia, and Stuart J. Davies of the Smithsonian Tropical Research Institute and the Center for Tropical Forest Science, is expected to be published in an upcoming issue of the journal Ecology Letters.

Though the results contradict earlier research in the Amazon that showed tree growth accelerating over the past several decades, Feeley said he doesn’t believe the Amazonian study is incorrect. Instead, he said, it appears that the Amazonian research can’t be easily generalized to tropical forests everywhere and that the effects of rising global temperatures differ regionally.

Researchers observing increasing rates of growth of Amazonian forests had thought that increasing carbon dioxide levels in the atmosphere acted as a carbon dioxide fertilizer, prompting the trees to grow faster and having the beneficial side effect of locking up more carbon in plant tissues. This process could occur because trees absorb carbon dioxide and release oxygen, the opposite of human beings and other animals. Carbon dioxide is one of the raw materials in photosynthesis, the process that uses energy from the sun to create carbohydrates out of carbon dioxide and water.

Feeley said his study has two important implications. First, climate models should not assume that some of the carbon dioxide released into the atmosphere by burning fossil fuels will be absorbed by trees. And second, more study is essential to understand just what effect climate change will have on tree growth around the world.

A widespread slowdown in the growth of tropical trees could have wide-reaching impacts, given how important forests are economically. In addition to being sources of lumber, paper and other products, forests provide environmental services such as protection for watersheds. Further, they are homes for a vast assortment of plant and animal species, an increasing number of which are endangered.

“All forests are not responding [to climate change] in the same way. I think it’s vital that we look at more sites,” Feeley said.

Arnold Professor Robert Cook, director of the Arnold Arboretum and the Harvard University Herbaria, praised Feeley’s work, saying that with so much data from so many trees over two decades, the results are quite “robust.”

“This has striking implications for any notion that tropical forests, whose biomass dominates a large portion of the Earth, are going to have a positive impact on global warming,” Cook said. “In fact, it may be just the opposite.”

Feeley’s study found that growth rates for the vast majority of trees in the two plots were declining by as much as 50 percent. Further, he said the slowdown is not a sign of an immature forest maturing as the trees reach their full height, because both are mature, undisturbed forests. There is no record of major human disturbances at either forest over the past 500 years. Further, he said, trees in smaller areas that have experienced natural disturbances such as a large tree fall also show slower growth rates.

To get at the root cause of the growth slowdown, Feeley examined climate data at the two sites over the study period. He found that daily minimum temperatures had risen over that time and that the number of rainy days had increased, meaning less sunlight was reaching the trees’ leaves. Feeley said what may be happening is that warmer nighttime temperatures are increasing the trees’ respiration rate, while the diminished sunlight is decreasing the rate of photosynthesis. Together, that means the trees have less energy coming in and more going out, leaving less behind for growth.

Feeley warned, however, that that explanation is not conclusive. Tropical forests are undergoing many changes and other factors could also be at work. One may be the increased growth of woody vines called lianas. Though not currently tallied in the scientific examination of the forest, an increase in liana growth could affect tree growth negatively, as the vines rapidly rise to the forest top and shade out the trees below. Unlike the large, solid trunk of a tree that locks up considerable amounts of carbon as it’s created, Feeley said the vines invest very little energy in creating their own stems and so an increase in liana growth would provide little help in absorbing carbon dioxide from the atmosphere.

“Any increase in liana growth will have a negative impact on the carbon budget of a forest,” Feeley said.

Feeley said he plans to continue this research, either by examining the question on a larger scale or by looking more closely at specific parts of the forests to better understand what’s happening, perhaps by examining trees in different soil types or by examining trees on sloped, dry ground versus those in swampy areas.

“The results here by themselves are striking,” Feeley said. “This highlights the need for more research.”