Abstract:
The objective of this study was to assess the effects of
forest management intensity on long-term productivity of Pacific
Northwest Douglas-fir forests. The components of management intensity
included rotation length, timber utilization standard (whole
tree or bole only), method of slash treatment (remove/burn or leave)
and fertilization practice (urea nitrogen fertilization or red alder
crop rotations).
A computer simulation model of forest nitrogen cycling and
growth was developed. Long-term forest productivity was indicated
by trends in the following variables over time: forest floor and
total soil nitrogen; nitrogen in the Douglas-fir and understory
vegetation; nitrogen losses from vegetation removal and slash
treatment; and Douglas-fir timber volumes (both standing volume and
volume removed by harvesting).
A range of 15 management prescriptions were simulated for a
360-year period. The results indicated that the development of the
Douglas-fir stand caused a steady decline in total soil nitrogen.
Shorter rotation lengths, 50-60 years, produced more rapid depletions
of soil nitrogen than longer, 120-year rotations. Whole tree
harvesting with 60-year rotations, slash removal and no fertilization
caused a 130 percent increase in the amount of soil nitrogen
required over the 360 years, compared to harvesting boles only. The
addition of urea fertilizer increased wood and bark volumes by 15
percent, while decreasing the soil nitrogen requirements of whole
tree harvesting by 14 percent. The use of 15 and 40-year alder rotations
caused 11 and 12 percent increases, respectively, in subsequent
Douglas-fir volumes, while decreasing total soil nitrogen
requirements by 60 to 72 percent compared to urea fertilization.
Slash removal practices resulted in a 23 percent increase in
the average soil nitrogen requirement per 60-year rotation, in combination
with whole tree harvesting and no fertilization. Harvesting
of boles only lessened this effect of slash removal on soil
nitrogen requirements.
The research results indicate that forest managers and decision
makers can no longer make the unqualified assumption that
growth rates will be maintained or increased as management intensity increases. The simulated levels of soil nitrogen depletion after
360 years of management show that the assumed growth rates would
not be maintained over this long a time period.
