Limiting Curve Method for site index curves ©

Draw two curves based on the upper and lower limits of the data
Interpolate other curves of similar shape passing through specified points on the height axis at STANDARD AGE

Disadvantages

The shape of the family of curves is determined by the extremes of the data, i.e. the data representing the best and poorest sites only.
The trend of the limiting curves will be wrong unless the history of height for age for the best and poorest sites is complete.

Guiding Curve Method

Plot at various ages the average height over a range of sites against age and draw a master curve. A good fit to this master curve is often given by the equation below (also see Husch, Miller and Beers, 1971, p. 356-7).
H - 1.3 = ae(b+c(1/A)) where H is predominant height (m) e is base of natural logarithms A is age at breast height a,b and c are constants.
Build up a family of curves from the master curve by proportion.

Advantage

All data contribute to the shape of the master curve.

Disadvantage

If all sites are not represented equally at all ages, the average height at the various ages will be biased. Therefore the guiding curve and family of curves will be biased in both position and shape. Possibly, nothing can be done about it because stands representative of all sites at all ages may not occur. Frequently in plantation establishment, the best sites are planted first.

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Document URL	http://online.anu.edu.au/Forestry/mensuration/LIM_CRVE.HTM
Editor	Cris Brack ©
Last Modified Date	Fri, 9 Feb 1996