Some trees may 'social distance' to avoid disease

Photograph by Ian Teh, National Geographic
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Borneo camphor trees (Dryobalanops aromatica) demonstrating crown shyness in the Forest Research Institute Malaysia. The phenomenon occurs in some tree species when spaces appear in the canopy to prevent branches from touching, forming channel-like gaps.

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On a warm day in March 1982, biologist Francis “Jack” Putz strayed into a knot of black mangrove trees seeking relief from the afternoon heat. Drowsy from his midday meal and hours of fieldwork in Costa Rica’s Guanacaste National Park, Putz decided to lie down for a short siesta.

As he gazed skyward, the wind stirred the tops of the mangroves above him, causing the limbs of neighboring trees to claw at each other and snap off some of their outermost leaves and branches. Putz noticed that this reciprocal pruning had left tracks of empty space running through the canopy.

This network of treetop chasms, called crown shyness, has been documented in forests around the world. From the mangroves of Costa Rica to the towering Borneo camphor trees of Malaysia, gaps in the greenery abound. But scientists still don’t fully understand why the tops of trees so often refuse to touch.

Beneath the mangroves 40 years ago, teetering on the verge of a post-lunch snooze, Putz reasoned that trees need personal space, too—a critical step toward unraveling the roots of the branches’ bashful behavior.

“I often make great discoveries at naptime,” he says.

Today, a growing body of work continues to support the early observations of Putz and his colleagues. Wind, it seems, plays a crucial role in helping many trees maintain their distance. The boundaries carved by bouts between branches may improve the plants’ access to resources, such as light. Gaps in the treetops might even curb the spread...

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