Stand Height ©
Stand height measurements are often used in estimating the quality of a site.
Determining stand volume from the summation of individual tree volumes based on measurements of diameter and height of individual trees is a costly process. A less costly and more practical approach is to predict stand volume using established relationships between various stand parameters (e.g. Stand Basal Area and Stand Height) and stand volume.
Stand Height Terminology
The height of even-aged forest crops (plantations) may be described in several ways:
Mean Height Unweighted
This is the arithmetic average height of all the trees in the stand, the international symbol for which is h.-
Sum of all the heights divided by the number of trees in the population
Mean Height Weighted
Weighting the height estimate by basal area, which allows the larger trees to contribute more to the mean, is commonly practised overseas. This expression of mean height, called Lorey's mean height is designated by the international symbol hL.
Sum of tree height multipled by tree basal area for all trees, then divided by the basal area of the stand.
As angle count sampling selects trees proportional to basal area, the mean height of trees included in one or more angle count sweeps gives an estimate of hL. However, if the sampling intensity is small, as is often the case, the estimate may be slightly positively biased because the smaller trees are under-represented.
Lorey's mean height is more stable than the arithmetic mean height, being much less affected by thinning from below. However, it is affected by thinning from above.
Stand mean height is meaningless as a stand characteristic in uneven-aged stands but is of considerable value in even-aged stands, e.g.:
- The mean height of a stand at a given age is an expression of site productivity, height growth being mainly a function of time and the factors of the site: it is little affected by stand density. Nevertheless, stand mean height has limited use as an index of site because it is altered by some silvicultural treatments, e.g. thinning from above or below.
- Stand mean height (h) can be used as an independent variable in stand volume tables.
Just as tree volume = g x h x FF, where FF = form factor,
so stand volume = G x h x Stand FF.
Short-cut Methods of Estimating Stand Mean Height
It is laborious and costly to measure the height of every tree in a stand to derive mean height. This problem can be overcome:
- by an approximation,viz. accept stand mean height as being the mean height of those trees having the arithmetic mean basal area (= quadratic mean dbhob) of the stand. Defined this way, mean height can be derived in two ways:
- Directly - determine the mean height of those trees having a d equivalent to or approximating the quadratic mean dbhob (international symbol, dg).
- Indirectly - compile a stand height curve and read off the height equivalent to dg.
- by sampling, e.g. in the United Kingdom, mean height is taken to be the average height of 10 trees per hectare selected systematically from the whole of the main crop.
The effect of thinning on h- and hL prompted foresters to seek a crop parameter which was relatively unaffected by thinning. The parameters they chose were predominant height, top height and dominant height. These are derived as:
the average total height of a specified number of the tallest (predominant height), or thickest (top height) trees in the stand , or of all or some of the dominants with or without the codominants (dominant height). The international symbol is hdom.
Whether the tallest or thickest trees are used depends on whether the tallest trees are easy to identify from the ground.
Predominant and top height are more practical indices than stand mean height for classifying site over extensive areas of forest because they are little affected by thinning under most thinning regimes.
In Australia, predominant height (incorrectly called top height in some areas) is defined as the arithmetic mean height of the tallest trees in the stand generally at the rate of 40 to 75 ha-1:
- NSW and ACT, 40 ha-1
- Q'ld., 50 ha-1
- South Aust., 75 ha-1
Work by Scott (1972) suggests that 40 ha-1 is quite sufficient for Pinus radiata in the A.C.T.
Assuming 50 trees ha-1 is specified for predominant (top) height assessment, it is common practice to base the estimate on plots of area 0.02 ha, i.e. 1/50 ha. The heights of the two tallest (largest) trees per plot are measured and the height of the tallest (largest) tree is accepted as the predominant (top) height of the plot.
New Zealand definitions
The New Zealanders define predominant height (which they call predominant mean height) as the average height of the tallest tree, free of malformation, in each 0.01 ha plot within the stand. They define top height as the height predicted from a stand height curve (Petterson curve) for a dbh corresponding to the quadratic mean dbhob of the 100 largest diameter trees ha-1 in a stand.
U.K. definitions
Current practice in the U.K. is to determine the 100 trees per hectare of largest d and height 10 of them (selected systematically). From these, a h /d regression is calculated:
h = a + b d + c d ^2
and the height corresponding to the mean diameter of the 100 largest trees is calculated from this.
Comparing height definitions
A relationship exists between stand mean height and stand predominant (top) height. The relationship for P. radiata, compiled by M.J. Hall (APM) from published data. This relationship holds for all sites and ages but experience suggests it should be accepted with reservation. A similar type of relationship is claimed for the Southern Pines in Queensland. In New Zealand, the relationship established between mean top height (MTH) and predominant mean height (PMH) is:
MTH = -0.3533 + 1.0179 PMH (Burkhart and Tennent 1977) .
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| Document URL | http://online.anu.edu.au/Forestry/mensuration/STNDHGT.HTM |
| Editor | Cris Brack © |
| Last Modified Date | Fri, 9 Feb 1996 |