National Geographic : 1997 Jan
Canopy communities Many old-growth trees are so tall thatthe epiphyte communities on them vary with height, much as forests themselves vary along a mountain slope. Transplanting lichens to branches, Steve Sillett (left) investigates habitat require ments inthe H.J. Andrews Experi mental Forest in Oregon. Research suggests that rich communities develop only after a forest is several centuries old. Can opies of mature forests become, like rumpled terrain, more varied hosts than younger stands, thus promoting variety and richness in epiphyte species. Sillett and others find that nitrogen-fixing lichens, which most heavily colonize the oldest forests, are especially important ecologi cally. When wet, such lichens leak Tree Giants of North America excess nitrogen, which may be ab sorbed by other epiphytes or the tree itself. As old-growth forests of the Pacific Northwest are logged and replaced by young trees, nitrogen-fixing lichens are on the decline-and with them a critical source of forest nutrition. Canopy insects have special niches. Tim Schowalter of Oregon State University in Corvallis focuses on a caterpillar of the silver-spotted tiger moth (above left), which increases light to the forest floor by eating needles. Arachnids, includ ing the tiny crab spider (right), are numerous, with available insect prey flying up from the ground or wafting inon breezes. Future stud ies will measure the role of such "tourist" insect species. Tree reproduction is likewise difficult to study from the ground. Consider a single branch of a noble fir (top). It has all the following on it at the same time: Large, green first-year female cones; large, gray second-year female cones; small terminal leaf buds; and the slender remnants of female cones.