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Climate Change Congress: Climate change likely to reduce tropical rainforest carbon storage

By Liz Kalaugher

Tropical rainforest currently removes about one petagramme of carbon from the atmosphere each year, a figure equivalent to around one-tenth of 2007 emissions. And most global climate models project that the rainforest’s net storage of carbon will continue or even increase as a result of carbon dioxide fertilization.

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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