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Climate Change Congress: Climate change likely to reduce tropical rainforest carbon storage
But David Hilbert of CSIRO, Australia, has found that the
Australian rainforest has showed a consistent trend of lower tree mass in
warmer climates. Hilbert and colleagues studied 17 sites in north-east Australia
for up to 35 years. There was no trend over time, but both the growth rate and
the mortality rate increased with temperature. (Recruitment rate - the growth
of new trees - was independent of temperature but increased with increasing
mortality). As the mortality rate increased, the basal area - the
cross-sectional area at a height of 1.3 m of all trees larger than 10 cm in diameter,
and an indicator of the amount of carbon stored - decreased.
Hilbert says that the ecosystem feedbacks in global
climate models are based on short term processes such as carbon fixation by
photosynthesis or decomposition, whereas in the longer term stocks of carbon
are controlled by tree demographic processes. “Despite higher tree growth
rates and higher turnover of biomass, rainforests in warmer climates stores
less carbon because of the higher mortality rate,” he added.
The team estimates that tropical rainforests will lose 14
Mg of carbon stored per hectare per degree of climate warming. So that means a
total loss of storage in the world’s rainforests of 24.5 Pg of carbon per
degree of warming - equivalent to 2.5 times 2007’s carbon emissions. If warming
proceeds at 0.05 degrees per year (the maximum IPCC prediction), that would
give a storage loss of 1.2 Pg of carbon per year. Scarily, that’s greater than
the amount we assume that the rainforests remove from the atmosphere each year
today.
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