Learn how to use the site index curve method to estimate SI.
Site Index Curves
The site index curve method predicts SI from height and age measurements
taken on carefully selected sample trees. Site index curves are available
for the following species:
- Coastal Fd, Hw, Cw, Ba, Ss, Dr, Act
- Interior Pl, Sw, Fd, Bl, Cw, Py, Lw, At, Ep, Hw, Pw.
Site index curves and tables for B.C. species are provided in Appendix
8 Site index curves and tables for B.C.
The site index curve method is best suited to even-aged, pure species,
middle-aged stands that are healthy (Figure 7.1).
Usually, a stand is not suited to the Site Index Curve method if:
- it is uneven aged
- the sample trees have been overtopped by trees or brush
- it has received partial cutting removing the larger trees
- many of the sample trees are seriously damaged or diseased
- the sample trees are old (age class 8 and 9)
- the sample trees are excessively large or rotten at DBH.
Figure 7.1. Stands suited
to site index curves include many 30- to 140-year-old stands.
When correctly used, the site index curve method produces an accurate
estimate of SI. However, the required heights and ages can be costly
and difficult to accurately measure. Sampling is very similar to the
growth intercept method. To use the site index curve method:
- pre-stratify the opening
- in each stratum,
- select a site index species
- collect height and age data on suitable top height sample trees
- compute the average SI.
Spread plots across the stratum. In a 5.64 m radius plot, identify
the largest DBH tree of the SI species. Measure it for height and age
if it meets the following criteria:
- undamaged stem with vigorous, uniform annual height growth above
- dominant or codominant crown class and not overtopped by other trees
- vigorous and uniform ring width from pith to bark.
Age is determined at breast height by counting rings on an increment
core taken at 1.3 m above ground on the high side (see Appendix 2).
Total tree height is usually determined with a suunto and distance tape
(see Appendix 1).
For each sample tree, read the corresponding SI from the appropriate
SI curve or table.
With the SI curve, the X axis shows the breast height age and the Y
axis shows the top height for a given species (Figures 7.2A).
The tables have breast height age down the side, top height along the
top, and site index in the body of the table (Figure 7.2B).
Average the SI determined for each sample tree to obtain the average
SI for the stratum.
Figure 7.2. Site index curves for coastal Douglas-fir:
A) graph format, and B) table format. Click on the above thumbnails
to see full-screen versions of the graph and/or table.
Figure 7.3 shows the
data that was used to produce site index curves for interior Douglas-fir.
Note the following:
- The top height growth curves are relatively smooth. Top height
is a stable, predictable quantity in free growing, undamaged stands.
It is partly for this reason that it has become the most common index
of site quality in the world.
- Few plots are over 100 years old. Therefore, the resulting site
index curves should not be expected to perform well when applied to
stands over 100 years old.
- None of the sampled stands appear to have suffered top breakage
or suppression. Stands with these conditions are not sampled for site
index curve constructiontherefore, site index curves should
not be applied to stands with top height tree damage or suppression.
Figure 7.3 Top Height over age in 68 interior
Thrower, J.S. and J.W. Goudie. 1992. Estimating dominant height and
site index of even-aged interior Douglas-fir in British Columbia. West.
J. of Appl. For. 7(1):20-25