. TRIM timber projections : an evaluation based on forest inventory measurements. Forests and forestry Mensuration Mathematical models; Forest surveys North Carolina. 7. 3 O 1 - °20 30 40 50 Age (years) Figure 8—Volume trajectory and accumulation of growth in loblolly pine stands subject to multiple thinnings; adapted from Clutter (1963). The data indicate that fully stocked stands represented 45 percent of all natural pine acreage statewide and 61 percent of all natural pine acreage in survey unit 3. Between 1974 and 1984, about percent of all natural pine acreage was annually disturbed b
. TRIM timber projections : an evaluation based on forest inventory measurements. Forests and forestry Mensuration Mathematical models; Forest surveys North Carolina. 7. 3 O 1 - °20 30 40 50 Age (years) Figure 8—Volume trajectory and accumulation of growth in loblolly pine stands subject to multiple thinnings; adapted from Clutter (1963). The data indicate that fully stocked stands represented 45 percent of all natural pine acreage statewide and 61 percent of all natural pine acreage in survey unit 3. Between 1974 and 1984, about percent of all natural pine acreage was annually disturbed by partial cutting or natural phenomena. If independence is assumed, potentially 23 percent of the acreage in natural pine lost volume from disturbance in 10 years. Because of this, probably few acres in the older age classes have been free of disturbance, which would support Davis and Johnson's (1987) theory. A curve through a cross section of the current volume in older stands would not predict the future growth rate because disturbed stands lower the average volume which lowers the curve. The effects of disturbance are included in fully stocked empirical yield tables and these effects biased the growth predictions. Net Change Figure 9 shows the statewide average volume per acre of natural pine in North Carolina from the 1974 and 1984 FIA data and the average site yield curves. The average volume per acre increased in almost all age classes between 1974 and 1984. The solid lines between inventories represent average 10-year "growth tra- jectories" made by the 1974 inventory. Reasons for an upward net change could include both growth on growing stock and the harvest of stands with less than average volume. If this 10-year trajectory in inventory volume is compared with the yield tables in fig- ure 9, why growth yields were more successful than empirical yields becomes ob- vious. When the approach-to-normal function is invoked in TRIM, plotted inventory projections
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