The allowable (or sustainable) 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 or potential wood product volumes entering the marketplace. Calculation of allowable harvest requires knowledge of present wood products volumes and knowledge of volume growth rates.
In order to achieve established landscape composition goals for successional forest habitats, DFW conducted a comprehensive Forest Inventory of state wildlife lands from 2004-2007, then developed a process in 2008 to estimate the maximum sustainable harvest of wood products for state wildlife lands using both area-based and volume-based methods. The DFW Estimate of Sustainable Harvest document describes this process in detail and determined that DFW could sustainably harvest nearly 8.4 million board feet (mmbf) of timber annually, plus >8,000 cords of firewood and >16,600 cords of pulpwood. Projected annual harvests using even-aged regeneration (at 100 year rotation age) to meet DFWs landscape goal for young forest habitat on an area basis (about 500 acres annually out of >104,000 total forest acres) yields less than 3.1 mmbf of timber annually (36.6% of sawtimber growth), plus 5,436 cords of firewood (65.3% of firewood growth) and 5,571 cords of pulpwood (33.4% of pulpwood growth). DFW target harvest levels for establishing young forest habitat through even-aged silviculture are well within sustainable levels. Sustainable harvest in this context means that the annual removal of woody biomass (e.g., timber, firewood, pulpwood) during management operations does not exceed net annual growth (net growth equals total growth minus mortality). Determination of allowable harvest ensures that DFW management practices are sustainable on a statewide basis and across different forest types.
Prior to completing a comprehensive statewide forest inventory of DFW lands in 2007 and the sustainable harvest process described above in 2008, 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.