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Allowable Harvest
The allowable harvest for a particular forestland ownership is generally defined as the volume of renewable wood products that may be harvested during a given period (typically annually) on a continual basis at a given intensity of management, and is typically structured to achieve a balance between growth (increment) and harvest. Calculation of allowable harvest facilitates allocation of personnel time within an organization and provides useful information to local and regional wood products industries on expected wood product volumes entering the marketplace. Calculation of allowable harvest requires knowledge of present wood products volumes and knowledge of volume growth rates.
For DFW lands, standing volume and growth rates are being determined from about 1,800 forest inventory sample plots on 72,000 acres that will allow DFW to utilize Woodstock™ software to make allowable harvest calculations for state wildlife lands during calendar year 2007 (as of January 1, 2007, about 1,200 plots had been completed). The Woodstock™ forest modeling software helps to determine sustainable wood supply levels based on current volumes and growth rates, and facilitates construction of long-term models for sustainable management of wood supply, habitat, biodiversity, and other forest values.
In 2002, DFW compiled some rudimentary allowable harvest projections based on forest inventory data from 18,000 acres of state wildlife lands collected by University of Massachusetts students during the summer periods from 1986-1998. Inventory data were entered into NED/SIPS V. 1.11, and NE-Twigs, a growth and yield simulator within NED/SIPS V. 1.11, was used to project net board foot yields from each forest type over a 100-year time frame in 5-year periods starting from the year 1992 (average of inventory year 1986-1998). The Inventory Resource Allocation Model (IRAM) (Gould 1981) was applied to derive the 100-year non-declining even-flow (NDEF) harvest schedule (area and volume cut by decade) for major forest types, and to determine the harvest schedule obtained by applying strict Area Control to meet DFW's landscape composition goals for successional forest stages that are achieved through forest harvesting operations.
The IRAM effort assumed that about 65,000 acres of upland forest would be actively managed on DFW lands, and that 2/3 of all regeneration harvests would employ even-aged silviculture on a 100-year rotation, while 1/3 would employ uneven-aged silviculture on an approximately 120-year rotation. The IRAM effort further assumed that half of all even-aged regeneration harvests would involve two-cut shelterwood harvests with preparatory and removal cuts spaced 10 years apart, and that half of all even-aged regeneration harvests would involve aggregate retention cuts (ARCs), which are modified clearcuts featuring clustered retention of overstory trees. Lastly, the IRAM effort assumed that uneven-aged regeneration harvests would feature a 30-year cutting cycle with group selection cutting on 25% of the stand during each entry. NDEF harvest schedules indicated that ≥ 8 MMBF could be harvested annually from DFW lands. Presently, DFW harvests at a fraction of this rate (typically ≤ 1 MMBF annually) due to staffing limitations.
Gould, E. 1981. IRAM: A simple model for forest resource allocation. The Harvard Forest, Petersham, MA.