Science - January 8, 2009

RAINFOREST GAPS ARE NOT ‘CONTAGIOUS’

Leaning trees on the edges of open patches in tropical rainforest do not fall any sooner than trees in the middle of the forest. With this conclusion, published in last month’s Ecology, Wageningen forest ecologists cast doubt on the finding based on earlier American research, that open patches are ‘contagious’.

A gap in the rainforerst.
Forest ecologists Dr. Patrick Jansen, Dr. Peter van der Meer en Professor Frans Bongers monitored six thousand trees in the interior of French Guyana for five years. They studied the open patches in twelve hectares of forest and looked at where trees fell. It was true that most of them fell near open patches, but this was because twenty percent of the forest consisted of open patches, so there was always one nearby. The individual trees right at the edge of open patches actually had a smaller chance of falling.

The researchers found that the best predictor for falling trees was the diameter of the tree: the older and thicker, the bigger the chance. Wind was not a decisive factor for the trees bordering the usually small gaps in the forest. Such trees often lean towards the light. In theory, this makes them vulnerable. But the extra light they get enables them to grow faster, and that may outweigh the risks incurred by leaning – at least, this is the explanation offered by the forest ecologists.

‘Our research suggests that existing gaps do not cause new gaps in the forest’, says Jansen, ‘but we researched a virgin forest with natural and relatively small open patches. Forest harvesting often creates large open patches in the rainforest, and trees on the edge of these patches probably are more vulnerable.’
Together with American colleagues, the Wageningen ecologists want to do further research in the Amazon. They want to use satellite photos to establish whether more new gaps grow up in the forest around larger open patches where trees have been felled.

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