. TRIM timber projections : an evaluation based on forest inventory measurements. Forests and forestry Mensuration Mathematical models; Forest surveys North Carolina. 5 15 25 35 45 55 65 75 85 Stand age (years) Figure 9—Average natural pine inventory volumes in North Carolina from the 1974 and 1984 FIA measurements and the average-site yield curves used for inventory projections. Lines connecting the 1974 and 1984 inventory volumes represent a 10-year "trajectory" of average volume. This upward trend in the inventory volume represents a "snapshot" of the average volume per
. TRIM timber projections : an evaluation based on forest inventory measurements. Forests and forestry Mensuration Mathematical models; Forest surveys North Carolina. 5 15 25 35 45 55 65 75 85 Stand age (years) Figure 9—Average natural pine inventory volumes in North Carolina from the 1974 and 1984 FIA measurements and the average-site yield curves used for inventory projections. Lines connecting the 1974 and 1984 inventory volumes represent a 10-year "trajectory" of average volume. This upward trend in the inventory volume represents a "snapshot" of the average volume per acre for only a 10-year period. This upward trend in volume cannot be expected to continue for two reasons. First, as was discussed, stand volume is assumed to approach the normal stocking level asymptotically. On undisturbed acres, this increase in stocking is expected to level off at some volume for which there are only a few stands. The second reason goes back to what Davis and Johnson (1987) presented and the data supported. These stands are subject to periodic volume- reducing disturbances. At some point in the future, the average volume per acre could be expected to decline in one or several age classes. Conclusions The analysis presented here was an effort to validate TRIM inventory and growth projections by comparing them to field measurements. The objective was to model natural pine in North Carolina between 1974 and 1984 and to examine the model's projected inventory and growth under two methods of inventory aggregation. The measured growth, harvest, and area data were used in an effort to simulate, as closely as possible, what took place over the 10-year remeasurement period. The results indicate that fully stocked empirical yield tables developed from cross- sectional inventory volume do not adequately project stand growth. Individual stands are subject to disturbances that affect their volume trajectories. The empirical yield curves flatten off and produce little
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