Forest Mensuration. Brack and Wood
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Measuring the height of a single tree © |
In forest mensuration, height is used to describe both the quantity and quality of tree and stand growth:
There are various expressions of tree height:
the vertical distance from the base of the tree to the uppermost point (tip).In forests where leaning trees are common, measuring the linear distance (or slope distance) from base of the tree to tip may be more appropriate. The linear distance will be greater than the vertical distance. However unless the lean is more than about 8 degrees, the difference between the two is less than 1% . Measurement crews and data interpeters must be aware which of these two height definitions is used.
Measurement of total height may be difficult in dense stands of trees with rounded crowns, e.g. tropical rain forest, and in young plantation tree crops before thinning opens up the canopy.
Trees with an excurrent habit have a well defined tip.
One can measure total height more accurately in excurrent species. Most conifers are in this group.
Trees with a deliquescent habit have a poorly defined tip. The heights of trees of deliquescent habit (i.e. umbrageous species) are much more difficult to measure accurately. The convention is to sight to the point in the crown which is vertically above the top of the bole and judged to be at the crown surface.
Many hardwood tree species (e.g. most eucalypts) exhibit an excurrent habit when young, but change to deliquescent habit when they mature.
the distance from the base of the tree to the highest point of the main stem where the diameter is not less than some specified value or where utilisation of the bole is limited by branching or defect.
Merchantable height is therefore a variable quantity which depends on the specifications for the merchantable product.
A hypsometer is an instrument used to calculate the heights of trees by triangulation.
Methods of indirectly measuring the height of standing trees fall into two basic groups:
It should be noted that most of the 'noise' in data bases of tree height occurs due to failure by measure crews to account for tree lean.
There are two general methods of measuring the height of leaning trees, direct and indirect. The direct methods are the same as for a vertical tree (i.e. climbing or using height sticks ) while the indirect methods must be modified. It is imperative that field crews involved in measuring tree heights inspect each tree closely for lean and, if a lean is detected, take appropriate action. In many cases, the lean and associated error with not taking the appropriate action will be very small and can be ignored, but in other cases serious error will occur (Table of errors ).
Precautions are necessary with the indirect methods to minimize error. Extreme error is most likely to occur when the operator stands in the plane of lean (i.e. the tree is leaning directly towards or away from the operator) and does not use the appropriate method. Depending on where the observer stands in the plane of lean, the error will vary from maximum positive to maximum negative.
However, ocular estimates are used in South Australia on stripline assessment of site quality at age 9-10 years where the greatest height assessed is approximately 18 m. The South Australian experience is that trees up to this height can be estimated reliably by eye when done by experienced workers.
It is not uncommon practice in forest inventory to estimate log length in standing trees to the nearest 1-2 metres. Ocular estimates of length of product and height of defects are used in conjunction with measurements and models in some inventories, e.g. MARVL (Deadman, M.W. and Goulding, C.J. 1978. N.Z. Journal of Forestry Science 8(2): 225-39). These estimates are usually satisfactory to 25 m height, but beyond this, significant error may be introduced and the results are unreliable.
When estimating the height of a tree by eye, always place vertically at the base of the tree a "calibration" pole of known height (e.g. 2 or 3 metres). Stand well back from the tree when making the height estimate.
The readings from the two positions should agree within the limits of instrumental error - this is an absolute check on instrument and operator error (sighted to correct tip, etc.). Thus, differences of up to 1 m in readings for a 40 m tree are acceptable - precision of instruments under forest conditions is no better than this.
The most common errors include:
http://online.anu.edu.au/Forestry/mensuration/HEIGHT.HTM
Cris.Brack@anu.edu.au
Mon, 6 Feb, 1997