According to conventional wisdom, open spaces (canopy disturbances) in the leaf canopies of tropical rainforests are ‘contagious’; trees around existing open spaces are more likely to fall over. However, this is not true according to forest ecologists from Wageningen University. In an article in Ecology, they reported that trees around open spaces are actually less likely to fall over, and they also cause relatively small holes in the leaf canopy.
Wageningen forest ecologists spent five years studying the fate of almost 6000 trees in the interior of French Guyana. They mapped out all the open spaces in twelve hectares of forest. Their aim was to determine if trees around open spaces had a greater chance of falling over. They also wanted to see if falling trees around open spaces created more damage – and therefore larger holes in the leaf canopy – than those that fell further away from open spaces.
Initially, it indeed appeared that the open spaces were ‘contagious’. Most of the trees that fell were in the vicinity of an open space. But that was simply because a relatively large proportion of the forest area is located near open spaces. Open spaces make up 20% of the forest, and many of the trees therefore had an open space nearby. The idea that many holes appear in the leaf canopy around open spaces is therefore deceptive.
Large diameter trees
Viewed individually, trees that grow beside open spaces have a slightly smaller chance of falling over. The best predictor of falling over is the tree’s diameter. The thicker (and older) a tree is, the greater the chance of it spontaneously falling over, regardless of where it is growing. Large trees also cause much more damage when they fall and make larger openings in the leaf canopy. However, since there are fewer trees with large diameters than small diameters in a forest, fewer large diameter trees would fall in a storm.
The forest ecologists think that natural open spaces in virgin rain forests are too small in scale to cause trees to be blown down by the wind. But this can change if larger spaces are created, for example though large-scale tree harvesting or road building. The researchers suggest that further research be conducted on this aspect.
American researchers previously found that trees growing beside open spaces had crooked crowns due to their branches growing towards the light, and that trees with crooked crowns were more likely to fall over. According to the Wageningen researchers, however, this does not automatically mean that trees growing beside open spaces are also more likely to fall over. They think that the growth advantages that arise from the extra light counteract the risks of crooked growth.
With these results, the canopy disturbance theory is back at its starting point. In the 1950s, researchers argued that every stand of forest experienced a predictable cycle of growth, aging, collapse and regrowth. According to the Wageningen researchers, this cycle explains the locations of the open spaces in French Guyana. This means that seedlings therefore have the greatest chance of suddenly being in an open space if they grow under an old tree with a large diameter.