Forests to a non-specialist seem to be nothing but a collection of trees. The closer they are examined, the more they differ. Furthermore, their difference may be expressed in a great number of ways. Some scientists write "A plant association mainly of trees or other woody vegetation occupying an area of land is a forest”. Others say that a collection of trees becomes a forest only when it has sufficient density and covers a large enough area to develop local climatic and ecological conditions that are different from those outside. There must be some changes in temperature, moisture, light and wind as well as in the character of upper soil layer. With these changes comes different vegetation under the trees and different animal life in the forest. In technical language a specialist: says that a forest "biocenoze" (life community) has been set up. Originally "forest" meant simply wild or uncultivated land. Today the term "forest" is sometimes applied to an economic unit of operating area. A forest of trees of similar age and composition is called a "stand". Every stand has more or less regular upper layer of green crowns, called "the forest canopy" under which there may be an open space except for the stems of the forest trees or more or less occupied with lower canopies. They are called understories and are typical of tropical forests but may be in the forests of the temperature zone too. Very often a single understorey consists of saplings which are coming in to replace the falling stand of veterans above. Where the forest is open or thin there may be ground cover of grass. The nature of this cover is very different. In dense forests the ground under the trees — the forest floor — may have no living vegetation and be covered with dead leaves and branches. The species composition of the forest is one of the most important features. The stand may be composed of a single species making a pure forest or several species are associated to form a mixed forest. Perfectly pure forests over large areas occur not often however. It was observed that species do not do well when planted in pure stands, especially upon usual forest soils. The reason is that they make very heavy demands upon soil plant food (nutrients). Such pure stands may do well in youth, but as they become older, their growth becomes very slow and the trees die. Stands are classified according to age classes of which they are composed. Even-aged stand is one in which all the trees are of one age. Uneven-aged stand, on the other hand, theoretically has trees of every age, from seedlings to old veterans.