Forest Productivity Council Policy

DEFINITION AND ESTIMATION OF TOP HEIGHT FOR SITE INDEX

3.0 Potential Top Height Definitions

3.1 Option 1 - Top Height 1998: One largest diameter tree per 0.01 ha plot

Top height is the height of the largest diameter tree on a 0.01 ha plot, providing the tree is suitable Suitable trees are trees which provide heights and ages that can be validly used to estimate site index. 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.

The tree height and age must be such that the extant height-age curve can be applied validly. This excludes young and old stands. For instance, most of the currently accepted B.C. Ministry of Forests height-age curves do not work well below 20 years (bh) or above 100 years (bh). Unless the curves can be shown to operate outside these bounds, these restrictions apply.

This definition does not preclude measuring heights for other purposes. The primary aims are to ensure that trees selected as top height trees can be used to validly estimate site index and that site index is estimated in an unambiguous, consistent, unbiased fashion.

3.2 Option 2 - Top Height 1995: one largest tree per 0.01 ha plot (TH95)

This option entails adopting the VRI definition of top height as the Ministry standard. This definition is:

A top height tree is defined as the largest diameter "suitable" tree on a non-empty 0.01 ha plot.

Option 2 is similar to Option 1 except that Option 2 allows the next largest diameter suitable tree (in some cases dead and/or down trees) to be selected if the largest diameter tree is not suitable. Also, Option 2 is not explicitly restricted to valid site index trees.

3.3 Option 3 - Top Height 1997: Four Trees per 0.04 ha Plot (TH97)

This is the majority option proposed by the Technical Top Height Committee (established by the Resources Inventory Branch in 1996).

For a given canopy layer in a stand, top height is defined as the expected value of the height of the four largest suitable* trees by species, on circular plots 0.04 ha in area.

* suitable =	dominant or codominant;
	no significant damage affecting height growth (usually < 5% reduction in height unless the general condition of the stand dictates otherwise);
	should not be remnants of a previous stand except when described as a separate layer.

3.4 Option 4 – Status Quo

This option is simply to maintain the status quo, which involves using different definitions for different applications. These are summarized in Table 2.

Table 2. Current top height definitions used in different applications.

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Last updated: 01/13/99