Forest Productivity Council PolicyDEFINITION AND ESTIMATION OF TOP HEIGHT FOR SITE INDEXExecutive SummarySeveral definitions of top height have been proposed for British Columbia over the past 18 years. The debate has centered on three main issues: choice of plot size, the definition of "suitable" top height trees, and choosing the tallest versus largest diameter trees. The choice of a plot size or sizes has been identified as the key issue. After consideration of the issues, the Technical Advisory Committee (TAC) recommends adoption of the following definition of top height:
This means that the top height tree must be healthy, not have a broken or damaged top, and not have its height growth affected by a competitor. The tree should not be a residual left from previous logging. If the largest diameter tree does not meet these criteria then no top height sample is taken (a "null" plot). The largest diameter tree is selected regardless of species. This definition does not preclude measuring heights for other purposes. The primary aims are to ensure that trees identified as top height trees can be validly used to estimate site index and that site index is estimated in a consistent, unbiased fashion. Although the definition of top height is arbitrary, the recommended definition is considered the most robust among the alternatives. It is simple in concept and inexpensive to implement in the field. In short, it is a definition likely to be measured and applied in an unbiased, consistent fashion. There will be several implications as a result of adopting this definition, including adjustments of models. However, it is felt that these costs will be greatly offset by the long-term benefits of consistent, repeatable, unbiased measurements across all business areas. Last updated: 01/13/99 |