Graduate Thesis Or Dissertation
 

A model of forest nitrogen cycling to assess the effects of management intensity on long-term productivity in Douglas-fir forests of the Pacific Northwest

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  • 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.
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