The concept of “size index” - calculating the physical size of trees
As a tree grows, it becomes taller and its trunk diameter increases. Tree sizes are customarily described by reference to their "height" (expressed in metres) and their "calliper" (trunk or stem diameter, expressed in mm, and measured at 300 mm above ground level). Some species are naturally stocky – thick stemmed and, comparatively short. Some, naturally taller, possess a more slender trunk or stem.
Under the NATSPEC guidelines, the physical bulk of a tree is the product of its height (m) and calliper (mm – measured at 300mm). So:
- A tall, slender tree, say 4 metres high with a calliper of 60mm has a "size index" of 240 (4 x 60) and, using appropriate tables, falls within the size range appropriate for a 200 litre container. It can be described as a "200 litre" tree.
- At the other extreme a different, stockier species might have a height of 3 metres with a calliper of 80mm. It too has a size index of 240 (3 x 80) and is also a nominal or calculated "200 litre" size.
The stockier tree typically has a bigger spread (or wider crown) than the taller more slender one. Although differently shaped, each tree has similar physical bulk.