Issues Involved in Defining Top Height

Forest Productivity Council Policy

In discussions concerning top height definitions, three main issues arise. These are summarized below.

For the key issue of plot size, the choice is either:

To state the top height definition as the average height of the 100 largest trees per ha and allow for a range of plot sizes to be used; or
To embed a plot size requirement into the definition. In this case, arguments have been put forth for both 0.01 ha and 0.04 ha plots.

Proponents of choice (a) put forth the following arguments:

Allows for different applications to use the most appropriate plot size.
If top height is based on the largest tree in a 0.01 ha plot then many more plots would be required for model development due to increased variability. It would also be difficult to screen for inadequate top height trees because there would not be any other trees for comparison on the plot.
Using larger plots for 1, 2, or 3 tree plots can minimize the problem of introducing bias by using different plot sizes.
This choice is compatible with the PSPs used to calibrate growth and yield models.
Does not require sectoring of existing plots larger than a set fixed size.
Using a fixed plot size would eliminate the possibility of determining top height on a large portion of historical data and data from outside sources.
Height-age and site index models currently in use may have to be redeveloped if the definition included a fixed 0.01 ha plot.

Proponents of choice (b) put forth the following arguments:

By embedding a fixed plot size in the definition, all associated problems with bias are eliminated.
Allows for consistency across all uses both administratively and operationally.
Acknowledge that there will be work required to make the transition from the current status quo, but feel that this is worthwhile given the long-term benefits of consistent, repeatable, unbiased measurements.

Proponents of choice (b) with a 0.01 ha plot put forth these additional arguments:

A 0.01 ha plot is operationally efficient for silviculture, inventory and research purposes. In particular, larger plots are simply not practical for silviculture or inventory purposes. While more plots may need to be established as compared to using larger plots the price per plot is lower.
If a single suitable tree is not available on the plot then the plot is considered a null plot. With larger plot sizes there is a common problem of finding fewer suitable top height trees than the target number. In mixed species stands, it is very common for the top height trees to be different species. This leads to the problem of what to do with the measurements from the suitable top height trees that are measured. What part of the population do these trees represent?

Proponents of choice (b) with a 0.04 ha plot put forth this additional argument:

A 0.04 ha plot would minimize the required re-tooling of existing growth and yield, height-age, and site index models.

There is general agreement that a valid top height tree should:

Have no damage affecting height growth.
Not have been suppressed.
Be standing and live with good vigor.
Be a dominant or co-dominant tree (although this qualification is redundant given the above criteria; the definition of dominant/co-dominant is somewhat subjective).
Be able to be accurately measured for breast height age.
Not be a veteran.

The main debate centers on the choice of tree species. The options presented are:

Top height trees should be selected at a minimum from the leading species (i.e., the species that makes up the greatest proportion of stand volume) and preferably from most species present in the plot; or
Top height trees should be the largest suitable trees regardless of species.

Proponents of choice (a) put forth the following argument:

Even for simple modeling, separate ages, heights and site indices are needed for the leading species and preferably, also for the second in each layer in all polygons.

Proponents of choice (b) put forth the following arguments:

Top height trees should be the largest trees regardless of species; otherwise, the estimates of site productivity will be biased.
This does not preclude measuring other heights for other purposes but they should not be called top heights.

The arguments for choosing the largest diameter tree are:

In older stands, particularly on steep coastal sites, choosing the largest diameter trees is easier to do in the field and also easier to audit.
There are larger errors in measuring heights than diameters. This can lead to more incorrect choices of the top height trees if two or more trees are similar in size.
It may be necessary to measure more that one tree height to ensure the tallest is measured, thus adding to cost.

The arguments for choosing the tallest trees are:

Last updated: 01/13/99