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