Identifying tree species in tropical rain forests may be harder than you think. Plant species identification can be difficult for all kinds of reasons even identification of trees, which are big and conspicuous. For example, for some willow trees, both leaves and flowers may be needed for identification, but the two may not be present at the same time. Yet whatever problems may confront us in temperate climates, we can be sure that the tropics will pose far worse difficulties. In tropical rain forests, the flowers of a given tree species are typically not in bloom and so cannot be observed. In seasonal rain forests (with a distinct wet season and a distinct dry season), many trees adjust their flowering to the rains, so flowering is to some extent predictable. But much rain forest (as in much of the Amazon region) is nonseasonal, and trees may flower at any time. To be sure, different trees of the same species generally flower simultaneously, for if they did not, they could not pollinate each other. They must be responding to signals from the environment at large, or else (or in addition) they must be communicating with one another. But what those signals are is unknown, at least to us. To the human observer, the flowering seems random. In any case, in a tropical forest (at least in a secondary forest, which is forest that is regrowing after previous harvesting or clearance), the trees grow very close together, and most are remarkably thin, like poles, and grow straight up and disappear into the gloom, twenty meters overhead. Even if there are flowers, you would not necessarily see them. The leaves may not be accommodating either, at least when viewed from the ground. Rain-forest trees all face the same kinds of conditions and have adapted in the same general kinds of way. Rain forests are wet by definition. But in some there is a dry season, and even when there is not, it doesn't rain all the time. Thus the forest floor may be moist, but the topmost leaves of the canopy are far above it and are exposed to the fiercest sun. So the uppermost leaves must resist desiccation (drying out). Yet from time to time, and in due season every day, they must also endure tremendous downpours. Leaves that can cope with such contrasts tend to be thick and leathery (to resist drought), oval in shape, and have a prection at the end known as a drip tip to let surplus rain run off the leaf. Many hundreds of trees from dozens of only distantly related families have leaves of this general type. But even if you can distinguish individual leaves, it is hard to be certain if they belong to the tree you are interested in or to the one next to it or to some epiphyte (a plant that grows on other plants) or liana (vine) slung over its branches. Often, in short, researchers must base their identification of a tree on the bark of its trunk. The trunks of tropical trees are sometimes highly characteristic, being deeply furrowed or twisted, but in most species the bark is simply smooth and gray, dappled with lichen and moss. In a temperate forest you can be fairly sure that any one tree is the same species as the one next to itor, at least, it will be one of a list that is unlikely to exceed more than half a dozen (oak with ash in much of Britain; lodgepole pine with aspen in the northernmost reaches of North America; alder, cotch pine, and spruce in the Baltic; and so on). But in the Amazon in particular, you can be fairly sure that any one tree is not the same species as the one next to it. Often there is a third of a mile between any two trees of the same species, and there can be up to 120 different species of trees in any one acre. The task, often, is to identify an individual tree that may be not much thicker than your arm from the appearance of its bark, out of a total list of several hundred (or thousand) possibilities – which may well include some that have not been described before, so that there is nothing to refer back to.