They also can be raised gradually to take measurements from the ground up to the top of the canopy. And they are cheap. Not roughness but rugosity, corrugation. A young forest can be very dark, compact, because its growth has been efficient. As the canopy gets taller, the leaves spread out and more light comes in.
Some trees die, leaving gaps.
New trees grow in, mostly species that thrive in dark places, and gradually the forest becomes more complex, bumpier, with more variations. A laser technique for measuring the bumpiness of canopies also came out of SERC's studies here in Edgewater, one of many SERC contributions to the science of canopies. These studies are becoming steadily more important as people try to manage such things as atmospheric gases and species diversity. If a city manager's concern is how to take up greenhouse gases like carbon dioxide, for example, I can advise him to conserve younger forests, which have a higher uptake of CO2. If his concern is species diversity, however, I would recommend conserving older forests.