—————————————————————–
“Some scientists and environmentalists have
suggested that, given the way carbon dioxide
spurs plant growth, tropical forests could in
time come to act as a sink, offsetting some of
the man-made carbon dioxide build-up.

“That optimism will have to be reassessed,
though, if photosynthesis becomes less productive
in the tropics.”

The trends measured by Feeley suggest that entire
tropical regions might become net emitters of
carbon dioxide, rather than storage vessels for
it. “The Amazon basin as a whole could become a
carbon source,” Feeley says.
————————————–

NATURE
news@nature.com – the best science journalism on the web

Published online: 10 August 2007; | doi:10.1038/news070806-13

Rising temperatures “will stunt rainforest growth”
Plants suffering in the heat could make global warming worse.

Michael Hopkin

Global warming could cut the rate at which trees
in tropical rainforests grow by as much as half,
according to more than two decades’ worth of data
from forests in Panama and Malaysia. The effect –
so far largely overlooked by climate modellers –
could severely erode or even remove the ability
of tropical rainforests to remove carbon dioxide
from the air as they grow.

The study shows that rising average temperatures
have reduced growth rates by up to 50% in the two
rainforests, which have both experienced climate
warming above the world average over the past few
decades. The trend is shown by data stretching
back to 1981 collected from hundreds of thousands
of individual trees.

If other rainforests follow suit as world
temperatures rise, important carbon stores such
as the pristine old-growth forests of the Amazon
could conceivably stop storing as much carbon,
says Ken Feeley of Harvard University’s Arnold
Arboretum in Boston, who presented the research
at the annual meeting of the Ecological Society
of America in San Jose, California.

Losing their balance

The amount of carbon that a forest stores depends
on the balance between the rate at which it draws
carbon dioxide from the atmosphere through
photosynthesis and the rate at which it gives
carbon dioxide back through respiration. In
carbon sinks, which are mostly found at high
latitudes, photosynthesis outstrips respiration
and the amount of carbon stored increases. In
general, tropical forests are today thought to
act as stable stores of carbon, with their
photosynthetic input and their respiratory output
more or less in balance.

Some scientists and environmentalists have
suggested that, given the way carbon dioxide
spurs plant growth, tropical forests could in
time come to act as a sink, offsetting some of
the man-made carbon dioxide build-up.

That optimism will have to be reassessed, though,
if photosynthesis becomes less productive in the
tropics. The trends measured by Feeley suggest
that entire tropical regions might become net
emitters of carbon dioxide, rather than storage
vessels for it. “The Amazon basin as a whole
could become a carbon source,” Feeley says.

Feeley and his colleagues analysed data on
climate and tree growth for 50-hectare plots in
each of the two rainforests, at Barro Colorado
Island in Panama, and Pasoh in Malaysia. Both
have witnessed temperature rises of more than 1ºC
over the past 30 years, and both showed dramatic
decreases in rates of tree growth. At Pasoh, as
many as 95% of tree species were affected, Feeley
and his colleagues report. The research has also
been published in the journal Ecology Letters(1).

Sinking feeling

Feeley suspects that the effect occurs because
plant photosynthesis is impaired if the
temperature rises above a certain threshold. The
effect, he adds, has not been included in models
of the global carbon cycle, meaning that
predictions of the future performance of tropical
forests as carbon stores may be unduly optimistic.

That said, he stresses that the effect is far
from proven, and could be due to other factors.
“Under increasing carbon dioxide alone, we know
the growth rate will increase,” he says. “But
there are lots of factors – it’s naí¯ve to think
of any one in isolation.” The study acknowledges
that increased cloudiness – or even a growing
role for lianas – may account for some of the
results.

Yet ultimately, those changes are also related to
climate change, which can be expected to have
effects all over the tropics. “If we’re correct
and the temperature is driving these changes,
this is something we’re going to see in a lot
more places,” Feeley predicts. “It has very
important implications – we may need to look
elsewhere for our excess carbon sink.”

So far, the Amazon rainforest – the world’s
biggest – has not suffered significant climate
warming. But with even the most optimistic
predictions of climate analysts asserting
temperatures are to rise by 2ºC over the coming
century, most rainforests could feel the effect
before too long.

References

1. Feeley, K. J. et al. Ecol. Lett. 10, 461-469 (2007).

Story from news@nature.com:
http://news.nature.com//news/2007/070806/070806-13.html

Nature Publishing Group, publisher of Nature, and
other science journals and reference works
© 2006 Nature Publishing Group

Bookmark the permalink.

Comments are closed