"This suggests that further warming will have even stronger effects," lead researcher Deborah Clark, a tropical forest ecologist at the University of Missouri at St. Louis, told United Press International.
Like all plants, trees live off sunlight via photosynthesis, the process by which energy from the sun helps to break down water and carbon dioxide to produce sugar and oxygen.
However, like animals, plants also consume oxygen, using the gas to burn sugar for power. This process of respiration spews off carbon dioxide, a potent "greenhouse gas" that traps solar heat and helps warm the planet.
Normally, the balance between carbon dioxide emission and consumption within plants is tipped in favor of consumption, which fuels plant growth. However, the more heat the plant absorbs, the more plant growth declines and carbon dioxide is emitted.
Clark and colleagues reasoned tropical rainforests might be among the first to show increased carbon dioxide emission levels in response to global warming, because they are among the warmest ecosystems on Earth, averaging roughly 80 degrees Fahrenheit annually.
"We became interested in this when our long-term measurements of tree growth had come to cover several years, and we could see that tree growth varied greatly from year to year," Clark said. "We realized that something about yearly differences in weather patterns must be doing this, and we started to focus on what that might be."
The scientists examined the annual growth of six tree species in an old-growth rainforest at the La Selva Biological Station in Costa Rica. They measured the trunk diameter of 164 adult tree species between 1984 and 2000 in an area the size of about 700 football fields.
"To measure some of these trees required carrying two to three ladders cross-country through the forest in order to be able to measure the tree trunk diameter, often 3 to 10 feet above the ground to get above the large buttresses that protrude from many tropical trees," Clark said.
The researchers had no direct way of measuring carbon dioxide emissions from the La Selva rainforest. Instead, they calculated CO2 emissions from tropical lands worldwide during the same time span using data collected by nine climate monitoring stations located from the Arctic to the South Pole, five of which are positioned either in or outlying the tropics.
In findings made public Monday from the Proceedings of the National Academy of Sciences, when Clark and her team matched tree growth with local temperature readings, they found growth often was stunted during the hottest years -- most notably during the record-hot, 1997-1998 El Niño. During warm years, atmospheric gas samples revealed tropical regions as a whole also spewed more CO2 than they absorbed.
"There is now an urgent need for studies to see if what we found at La Selva is occurring generally across tropical forests," Clark said. "If the patterns we have found prove to be general, it would mean that the rate of global warming will be much greater than what has been expected based on human fossil fuel use alone."
She added, "no one knows that the optimum temperature is for photosynthesis of a tropical forest. This is clearly a burning question now."
Ecologist Chris Field, of the Carnegie Institution of Washington in Stanford, Calif., said although the findings are not definitive, "it's an important suggestion and it needs to be explored further."
(Reported by Charles Choi, UPI Science News, in New York)