There are three components that make up tree size. They are diameter or circumference of the trunk, height or crown height, and crown spread (canopy cover).
Measurements of forest trees include diameter or circumference and a trunk height. These dimensions give the volume of wood in a tree that then can be converted to lumber.
Urban and landscape trees may be measured in any number of ways. The most common is to measure diameter because of the speed and convenience of collecting data while giving an indication of size. This data can be used to make general conclusions about age of a population. (Species frequency may skew any age conclusions if the tree population includes a significant number of small maturing trees. Also, any direct correlation between size and age for an individual tree cannot be determined.)
Other size measurements may be taken if specialized data is needed. This relates back to the real purpose of doing an inventory. For instance, crown spread and crown height may be measured to give data on total canopy cover and crown volume over the landscape, which can be plugged into formulas that calculate the amount of solar or rainfall interception (heat island effect, storm water impact). Another size measurement combination could be trunk diameter and trunk height that would give information about urban tree biomass (biomass energy potential).
Some tree inventory systems record the actual tree size, while others use a size class. The advantage to record the actual trunk diameter in 2” diameter classes is that detailed information is gathered but can be categorized into size classes. Commonly used size classes are 0-6”, 7-12”, 13-18”, 19-24”, 25-30”, and 31” and up.