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

The objective of this study was to assess the effects offorest management intensity on long-term productivity of PacificNorthwest Douglas-fir forests. The components of management inten-sity included rotation length, timber utilization standard (wholetree or bole only), method of slash treatment (remove/burn or leave)and fertilization practice (urea nitrogen fertilization or red aldercrop rotations).
A computer simulation model of forest nitrogen cycling andgrowth was developed. Long-term forest productivity was indicatedby trends in the following variables over time:forest floor andtotal soil nitrogen; nitrogen in the Douglas-fir and understoryvegetation; nitrogen losses from vegetation removal and slash
treatment; and Douglas-fir timber volumes (both standing volume andvolume removed by harvesting).
A range of 15 management prescriptions were simulated for a360-year period. The results indicated that the development of theDouglas-fir stand caused a steady decline in total soil nitrogen.Shorter rotation lengths, 50-60 years, produced more rapid deple-tions of soil nitrogen than longer, 120-year rotations. Whole treeharvesting with 60-year rotations, slash removal and no fertiliza-tion caused a 130 percent increase in the amount of soil nitrogenrequired over the 360 years, compared to harvesting boles only. Theaddition of urea fertilizer increased wood and bark volumes by 15percent, while decreasing the soil nitrogen requirements of wholetree harvesting by 14 percent. The use of 15 and 40-year alder ro-tations caused 11 and 12 percent increases, respectively, in subse-quent Douglas-fir volumes, while decreasing total soil nitrogenrequirements by 60 to 72 percent compared to urea fertilization.
Slash removal practices resulted in a 23 percent increase inthe average soil nitrogen requirement per 60-year rotation, in com-bination with whole tree harvesting and no fertilization. Harvest-ing of boles only lessened this effect of slash removal on soilnitrogen requirements.
The research results indicate that forest managers and deci-sion makers can no longer make the unqualified assumption thatgrowth rates will be maintained or increased as management intensity
increases. The simulated levels of soil nitrogen depletion after360 years of management show that the assumed growth rates wouldnot be maintained over this long a time period.