Tropical rainforest | P. Groenendijk
The world's forests provide a huge carbon sink,
around a third of manmade carbon emissions, and
helping to moderate global temperature rise.
A new study argues that the speed of tree growth
in tropical rainforests isn't keeping pace with rising carbon
dioxide levels in the atmosphere, and so it may be "too optimistic"
to expect this buffering effect to keep pace with rising
But another scientist tells us the finding needs
to be examined carefully, and it could be difficulties in taking
measurements in tropical rainforests that are leading to the
Rainforests are an important carbon
As part of the Earth's natural carbon cycle,
vast amounts of carbon dioxide are taken out of the atmosphere and
absorbed by the land each year. Tropical rainforests, the extremely
productive forest ecosystems found gathered around the equator, are
responsible for much of that exchange.
Through photosynthesis, trees convert carbon
dioxide, water and sunlight into the fuel they need to grow,
locking up carbon in their trunks and branches in the
Experiments scientists have carried out in
temperate forests and greenhouses suggest that when
there's more carbon dioxide in the air, trees can grow more quickly
because their photosynthesis rate speeds up. This process is called
'carbon dioxide fertilisation'.
expect that as manmade carbon dioxide
emissions increase, forests will absorb and store more
But a new study, published in Nature
Geoscience, suggests that tropical rainforests
might not be absorbing more carbon as emissions rise. Despite a 35
per cent increase in atmospheric carbon dioxide over the last 150
years, the study suggests that trees in the tropics aren't growing
Studying tree rings
The researchers studied over a thousand trees of
different ages, covering 12 different species and three different
parts of the tropics. They chose areas of old-growth forests in
Thailand, Cameroon and Bolivia that were undisturbed by
deforestation or human settlements.
They analysed tall 'canopy' trees, which are the most common
type in tropical forests and typically reach around 30 metres in
height, and also smaller 'understorey' trees that grow to around 10
Examining tree rings can show how quickly a tree
has grown from year to year. As a tree grows, it puts on extra
layers of wood around its trunk, creating a new ring each year. The
more a tree has grows in a year, the more wood it adds, and the
wider the tree ring is.
Tree rings. Source: Creative Commons 2.5: Arnoldius