Approving an Annual Allowable Cut from a Low-Intensity Inventory
Terai district. A community forest of 250 ha of Sal (Shorea robusta) with scattered Terminalia and Lagerstroemia has submitted a five-year operational plan proposing an annual allowable cut (AAC) of 2,100 cubic metres of round timber. Supporting inventory: 60 circular plots of 500 square metres laid on a 250 m x 250 m grid, giving an estimated standing volume of 62,500 cubic metres with a 95% confidence interval of plus or minus 14%. Growth is based on 12 permanent sample plots (PSPs) measured twice, giving a net annual increment of 3.1 cubic metres per hectare per year with a confidence interval of plus or minus 25%. Harvest records for the past three years show 2,300; 2,600; and 2,450 cubic metres removed, including illegal fellings. Local demand is about 3,000 cubic metres per year, and the CFUG argues that 'the forest looks dense, so there is enough'. The DFO has 45 days to approve, revise or reject the plan.
1. Calculate the sustainable harvest implied by the data and test the proposed AAC against it. Show your reasoning.
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Step 1 - increment in absolute terms. 3.1 cubic metres per hectare per year x 250 ha = 775 cubic metres per year. Applying the stated plus or minus 25% gives a plausible range of roughly 580–970 cubic metres per year.
Step 2 - compare with the proposal. The proposed AAC of 2,100 cubic metres is 2.7 times the central increment and more than twice the upper confidence limit. It cannot be sustained under any reading of these data.
Step 3 - compare with actual removals. Average recorded removal is about 2,450 cubic metres per year - roughly three times increment. The growing stock is being mined. The CFUG's 'looks dense' argument describes basal area, not growth; a dense stand can still be losing volume.
Step 4 - defensible yield. Harvest no more than 60–70% of net increment to allow for mortality, logging damage and a reserve: about 450–540 cubic metres per year, with 10% withheld until the PSPs are remeasured. The gap between that and demand (3,000) must be addressed explicitly, or the AAC will simply be ignored in practice.
2. What decision would you take on the plan, and what conditions would you attach?
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Revise, do not approve. Conditions:
- Reduce the AAC to about 500 cubic metres per year, species- and diameter-limited (no felling below 40 cm DBH except genuine thinning), with annual coupes mapped on the ground.
- Tie harvesting to silvicultural need: improvement thinning in overstocked patches, removal of defective and diseased stems, retention of 8–10 seed trees and about 5 standing dead trees per hectare for cavity-nesting wildlife.
- Require DFO technical marking and a timber account book, with monthly reconciliation of receipts, stumps and depot stock; the CFUG must report illegal felling to the DFO within 48 hours.
- Make approval conditional on PSP remeasurement in year 2 and a mid-term review in year 3.
- Commission a household fuelwood and timber consumption survey, because household demand, not the sale ledger, drives most illegal cutting in Terai CFs.
- Note the precision problem: at 14% sampling error, roughly 120 plots would be needed for 10% precision, so this round's volume cannot by itself justify a large cut.
3. How would you redesign the inventory and growth measurement for the next plan?
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Stratify by canopy cover and site class using recent satellite imagery, and allocate plots proportional to stratum area and variability rather than on a uniform grid.
Use at least 100–150 plots, with nested design: 500 square metres for trees of 10 cm DBH and above, 100 square metres for 10–30 cm, and 25 square metres for regeneration. Locate plots randomly or systematically with a random start, and monument them permanently so they serve as PSPs.
Record DBH, height on a subsample, species, crown class and defect; count stumps and cut stumps to estimate removals independently; apply local or regional volume and biomass equations.
For growth, use 30–50 PSPs remeasured every two years, with increment cores or dendrometer bands, and compute gross and net increment separately by subtracting mortality and damage.
Report standard errors and confidence limits for every estimate, and base the AAC decision on the lower confidence bound rather than the mean - a conservative rule that prevents over-cutting when data are thin.