The term 'forest units' can be broken down into two parts: 'TREES' and 'units'. TREES are a type of plant that have a woody stem, leaves, and branches. They are found in many different habitats, including forests. Units are a measure of something, such as area or volume. In the context of forests, units are often used to measure the size of a forest or the number of TREES in an area. Therefore, the term 'forest units' refers to the measurement of the size and number of TREES in a forest.