Abstract:
One traditional problem in forest management is to
find the optimal stand level management regime. Four
important silvicultural practices including precommercial
thinning, commercial thinning, fertilization and regeneration
harvest are considered jointly in this study. The
partial analyses, i.e. considering some of the silvicultural
practices, are also discussed.
The inability to account for diameter acceleration in
the two-dimensional dynamic programming technique is overcome
by using a three-dimensional dynamic programming network
with biometric relationships from DFIT. The continuous
growth is fitted into a discrete dynamic programming
network by using the "neighborhood" concept. The descriptors
used are stand age, number of trees and basal area.
The effect of the size of the state space of dynamic
programming is discussed and a basal area interval between four to 20 square feet is suggested when the tree interval
used is 15.
Commercial thinning is considered every ten years and
captures anticipated merchantable mortality. Precommercial
thinning is considered at age ten. Different intensities
of precommercial thinning can be considered jointly with
other silvicultural practices. Three levels of fertilization,
i.e. 400 pounds, 200 pounds and zero pounds of nitrogen
per acre, are applied every ten years after commercial
thinning. An extra dimension representing different levels
of fertilization is eliminated by computations and using
the neighborhood concept. The forward recursive relation
of dynamic programming finds the best management regime for
different rotations as the solution progresses.
Precommercial thinning accelerates diameter growth and
will affect later commercial thinning entries. Commercial
thinning lengthens rotation and fertilization increases
site capacity and raises optimal stocking level.
The solution technique developed also finds the
optimal management for different initial stand conditions.
Plantation is solved by assuming that it is equivalent to a
stand precommercially thinned at age two.
The impact of individual silvicultural practices and
their interactive effects are derived. Under the revenue
and cost assumptions used, it is found that fertilization
has the highest economic impact, commercial thinning is the second and precommercial thinning is the last, when silvicultural
practices are considered individually. The
highest total effect of two silvicultural practices is
precommercial thinning and commercial thinning. Commercial
thinning and fertilization is the second and precommercial thinning and fertilization is the last. Precommercial
thinning and commercial thinning has the highest interactive
effect, commercial thinning and fertilization is the
second and precommercial thinning and fertilization is the
last which is negative. The interactive effect of precommercial
thinning, commercial thinning and fertilization is
positive, that is to say, when these practices are applied
together, the total effect is larger than the sum of individual
effects.
The techniques developed and discussed give practical
answers to questions of stand level optimization with
complex cost, revenue and growth model silvicultural
interactions.
