CONSTRUCTION OF AN INDIVIDUAL TREE TOTAL VOLUME PREDICTION
MODEL FOR Pinus nigra var. maritima (Ait.) Melville (CORSICAN PINE) IN
S.M.C.U.P. Subasinghe1, T.A.R. Jenkins2 and G.J. Mayhead2
1 Department of Forestry and Environmental Science, University
of Sri Jayewardenepura. 2 School of Agricultural and Forest Sciences,
University of Wales Bangor, United Kingdom.
A total volume prediction model for individual trees was constructed using
total height and diameter at breast height (dbh) as explanatory variables
for intermediate and neutral thinning types using the data obtained from
the British Forestry Commission.
The key question used was vi = gi * hi * fi where v, g, h, and f represent
the total volume, basal area, total height and form factor of an individual
tree. Form factor is the most difficult variable to measure because, it
is highly correlated with species, age, site, variations, stand density,
crown growth and competition from the neighbouring trees.
Assuming basal area and total height of the particular tree will indicate
the competition and age, direct functions were not constructed for competition.
However, it was tested indirectly using crown parameters, stand density
(N) and total basal area (G). Top height related functions were used to
represent the site quality. The final equation tested was v = f (g, h,
s, G, N, crown growth).
For each thinning type, 75% of plots were used for model construction
and the remaining 25% for the validation of the constructed models. Data
were divided by thinning type and then by age in order to fit the models
to one year at a time.
Crown parameters, site parameters, total number of trees or total basal
area were not statistically significant when fitted and the standard residual
distribution indicated no improvement. Basal area and total height were
the significant variables. Finally, basal area was replaced by dbh. For
the final model at each age R2 was between 0.972-0.999 and standard residuals
were distributed without showing any particular pattern. Quantitative
tests indicated negligible bias and very high modelling efficiency for
all ages. Lack of fit test indicated the model was adequate.
There was an attempt to construct parameter prediction models with age
which was not successful because the estimated parameters for all ages
were distributed around 0.5. Finally an averaged value of the parameter
which replaced the form factor was selected for all ages for both thinning
types. The final model is given below.
v = 0.5040 * h * (p dbh2 / 40 000)
Department of Forestry and Environmental Science,
of Sri Jayewardenepura,Sri Lanka. 1995-2000. All rights reserved.