SOME SIMULATION ESTIMATES OF MEAN ANNUAL INCREMENT OF DOUGLAS-FIR - RESULTS, LIMITATIONS AND IMPLICATIONS FOR MANAGEMENT

Patterns of development of mean annual increment in relation to age pr edicted by the widely used DFSIM, SPS, TASS, and ORGANON simulators we re examined. Although predictions differ considerably among simulators for portions of the range of sites, ages, and treatments, comparisons indicated that (1) culmination is relatively late, (2) the curve is r elatively flat in the vicinity of culmination, and (3) systematic thin ning tends to delay culmination. Harvest ages of 40 to 50 years reduce volume production relative to potential by amounts ranging from moder ate to large according to site, treatment regime, and simulator. Withi n unknown upper limits, moderate extension of rotations to minimize co nflicts among timber production and environmental, aesthetic, and wild life values would not materially reduce long-term volume production an d might increase value production.