Drought kills mature trees faster

Drought is a forest-killer. The death of large tree has a particularly dramatic impact on the ecosystem and also affects the climate, since trees play such a vital part in the carbon cycle. Scientists have long suspected that the death of big trees has a bigger effect on global warming then small trees dying, but now the theory has been proved once and for all.

New research by the Los Alamos National Laboratory reveals how drought affects mature trees much more than smaller new trees, with less growth and less chance of long term survival.

A team of researchers studied the world’s forests, everything from semi-arid woodlands to steamy tropical  rainforests and cloud forests, to find out how a tree’s size affects its drought response. In total they studied a massive 40 drought events in 38 different forest locations, systematically reviewing the worldwide ratio of tree size to tree death for the first time.

Why do big trees matter so much?

  • Living trees soak up and store CO2 but dying trees release it. Instead of being a place where carbon is safely sequestered, a damaged forest can become a frighteningly efficient carbon source
  • Big trees also release more water than small ones, cooling the land and supporting the formation of clouds. Once this fine natural balance is disturbed, the whole thing starts to fall apart
  • Mature trees also play a vital role in supporting biodiversity, creating environments all sorts of essential and fascinating animals and plants depend
  • Big trees are more vulnerable since it’s more difficult for them to find and transport the water and nutrients they need to their leaves. Worse still, tall trees with crowns high up in the rainforest canopy are exposed to higher solar radiation
  • In every seasonal tropical forests the team looked at, the growth rate of under-story trees increased when there was less water available, especially so when their bigger relatives lost leaves when stressed by drought

The effects on climate change?

In a final salvo the researchers pointed out that the worldwide consequences of damaged forest ecosystems, less biodiversity and a spiralling carbon cycle will be “great”.

We have to assume there’s a tipping point, a time at which it’s too late to reverse things, when climate change runs away with us. Let’s hope we manage to sidestep that, or the human race will be in a whole world of trouble.

Article by The Rainforest Foundation.



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