Tree size is most often described in terms of three broad height categories. Tree height is defined as the vertical distance between the base and the topmost branch, and can be calculated by triangulation remotely from the ground.
A small tree is one that will reach a maximum height of no more than 25 feet at maturity. At the extreme, is the Dwarf Arctic Willow (Salix herbacea) which in nature grows to a height of only 2-3 inches at maturity.
Medium-sized trees grow to a height of 25 to 40 feet.
Large trees are representative of the broadest category, achieving heights of 40 to over 100 feet. Extreme height is achieved by Redwood trees (Sequoia sempervirens); the champion, growing in California, is named Hyperion and has been measured to stand 380 feet tall.
Trees in the small category include both single and multiple-stemmed species; medium and large trees are most commonly single stemmed. Whatever its potential height, each tree species goes through a life cycle of seedling, sapling and maturity with each cycle extending over an estimated time period, and maximum size (height, girth and canopy width) is reached at maturity when additional growth all but ceases. Generally, in speaking of tree size, we are referring to tree dimensions at maturity. Both the maximum size it attains, and the amount of time required to reach full size, are governed by the tree’s genetic makeup, assuming that growth conditions are ideal in terms of illumination, soil and water, for the tree.
Two other common tree measurements are used to describe trees: girth and crown spread (width). Girth is the measurement around the tree trunk at a point of 4.5 feet above ground and is used to calculate the DBH (diameter at breast height). At that point there is no influence from the flare of the trunk. Small trees have a DBH of up to 14 inches; medium trees 15-19 inches, and large trees 20 inches or more. In the nursery trade, trees can be described in terms of their caliper, which is the also a measure of diameter, but depending upon plant size made at a point 6-12 inches above the ground. Crown spread is a maximum horizontal measure of the tree crown expressed as a diameter. In many species, the crown spread approximates the subsurface network of the tree’s roots. Trees with multiple stems and/or irregular shapes are difficult to characterize in terms of girth and spread.
Selecting trees of appropriate mature size is important in long-term garden planning with growth rate a related issue. A slow-growing tree which matures into a large tree can give many years of enjoyment before becoming too large; also, moderate to severe pruning can maintain a favorite tree at a manageable height for an indefinite period. However, it should be noted that not all trees respond well to pruning to maintain a smaller stature.
Species in the TreeWorld inventory are described as being small, medium or large, and in addition, the caliper measurements are provided.