According to paragraph 3, one reason that researchers in tropical forests must often rely on a tree's bark for species identification is that A. distinct species of tree often have similar kinds of lichens and moss growing on their trunks B. the leaves of many different tropical tree species are very similar to each other in appearance C. many rainforest trees have neither epiphytes nor lianas D. the bark of the trees is less affected by changes in light and moisture than leaves are