main idea A. In tropical forests different trees of the same species flower at different times so they are not able to pollinate one another. B. Flowering in tropical trees is often unpredictable and when it occurs, the flowers themselves tend to be too high up to be clearly visible. C. Many tropical tree species have leaves that look very similar from the ground, and because the trees often grow close together, correctly matching a leaf with a particular tree can be difficult. D. The leaves and bark of a single rainforest tree are often very different at different heights due to the need to adapt to different conditions, which adds to the difficulty of species identification. E. Because trees in rain forests are spaced closely together, it is easy to tell whether neighboring plants are the same species, but that is not much help in determining which species they are. F. Often, a rain-forest tree has to be identified by its bark, but trees of like species are widely scattered, the list of possible species is very long, and many have similar-looking bark.