|dc.description.abstract||A simplified process-based model simulating growth and water use in forest
plantations was utilised to predict the growth of Pinus elliottii in South African
forest plantations. The model is called 3-PG (Physiological Principles in Predicting
Growth) and predicted the growth of trees by simulating physiological processes
that determine growth and water use, and the way trees are affected by the
physical conditions to which they are subjected, and with which they react.
Pinus elliottii growth data recorded in 301 sample stands around South Africa were
sourced from forestry companies. A selection procedure reduced the number of
stands to 44, where 32 were used to parameterise 3-PG and 12 were reserved for
testing the final model parameters. This was accomplished by matching model
output to observed data. All stand simulations were initialised at age four years
and continued to the maximum age of recorded growth.
A provisional set of parameter values provided a good fit to most stands and minor
adjustments of the specific leaf area (σ), which was assigned a value of 5 m2.kg-1,
were made, bringing about an improved fit. The predictions of mean DBH, Height,
and TPH were relatively good, achieving R2 of 0.8036, 0.8975, and 0.661
respectively, while predictions of stem volumes were worse (R2 =0.5922, n=32).
The 3-PG model over-predicted DBH in 20 stands, while modelled volume
predictions improved substantially in thinned stands (R2 =0.8582, n=14) compared
to unthinned stands (R2 =0.3456, n=18). The height predictions were generally
good producing an R2 =0.8975.
The final set of 3-PG parameter values was then validated against growth data
from the 12 independent stands. The predictions of mean DBH, Height, and TPH
were relatively good, achieving R2 of 0.8467, 0.7649, and 0.9916 respectively,
while predictions of stem volumes were worse (R2 =0.5766, n=12).
The results of this study demonstrated the potential for 3-PG to respond to many
growth factors and to predict growth and water use by trees with encouraging
realism. Patterns of changing leaf area index (L) over time, responses to drought,
and annual evaporation patterns all look realistic. Consequently, 3-PG is judged to
have potential as a strategic forestry tool.||en