Divide a per-hectare coppice yield by 2.471 to get tonnes per acre: the parent article’s 3-15 tonnes a hectare a year becomes 1.2-6.1 tonnes an acre. A tonne of wood is not a fixed amount of fuel, though: depending on whether the tonnes are green, air-dried or oven-dry, one reported tonne is worth about 8.0, 14.3 or 17.9 GJ, so measure your own coupes on a stated basis before planning.
Part of Coppicing & Fuel Management
Converting coppice yields to per-acre figures, the moisture-basis ambiguity that makes published tonnages hard to compare, and how to measure a real yield.
Units
1 hectare = 2.471 acres, so tonnes per acre = tonnes per hectare ÷ 2.471. Tonnes here are metric (1,000 kg).
The Parent’s Yields Per Acre
| Species | Rotation | t/ha/yr (parent) | t/acre/yr |
|---|---|---|---|
| Willow | 5 years | 8-15 | 8 ÷ 2.471 = 3.2 to 15 ÷ 2.471 = 6.1 |
| Hazel | 8 years | 3-5 | 1.2 to 2.0 |
| Ash | 12 years | 4-7 | 1.6 to 2.8 |
| Oak | 20 years | 3-5 | 1.2 to 2.0 |
The Basis Problem
The parent does not say whether its tonnes are green, air-dried or oven-dry, and the three differ greatly. Take 1 tonne of oven-dry wood (dry matter only) and the net-heat formula from BTU Content Compared to Wood (net = 18.5 × (1 − w) − 2.44 × w, MJ/kg, with w the wet-basis water fraction):
- At 50% moisture (freshly cut): mass = 1 ÷ 0.5 = 2.0 t; net heat 8.0 GJ/t, so 16 GJ in total.
- At 20% (air-dried): mass = 1 ÷ 0.8 = 1.25 t; net heat 14.3 GJ/t, so 1.25 × 14.3 = 17.9 GJ.
- Oven-dry: 1 t at 18.5 GJ/t = 18.5 GJ, but nobody burns oven-dry wood.
The same real wood can be quoted as 2.0 t green or 1.25 t air-dried or 1.0 t oven-dry, and a yield table that does not say which is unusable for fuel planning. Per tonne of what was reported, the heat spans 8.0 to 17.9 GJ, a factor of 2.2 (17.9 ÷ 8.0), though per hectare of real wood the energy is nearly constant (about 16 to 18 GJ per oven-dry tonne).
Reconciling The Parent’s Example
The parent’s 2 ha wood has coupes giving 4-6 t each every year: 2 t/ha/yr to 3 t/ha/yr, or 2 ÷ 2.471 = 0.8 to 3 ÷ 2.471 = 1.2 t per acre per year. That sits below the parent’s own hazel (3-5) and ash (4-7) figures. Both cannot hold for the same wood; one is wrong or they describe different sites. Until you measure, the conservative planning figure is the lower one.
Worked Example: Measuring Your Own Yield
Inputs: a coupe of 0.2 ha cut at the end of a 10-year rotation gives a stack weighing 5 t when air-dried (assumed for the example).
- Yield = mass ÷ (area × years) = 5 ÷ (0.2 × 10) = 2.5 t/ha/yr.
- Per acre: 2.5 ÷ 2.471 = 1.0 t/acre/yr.
- Energy: 2.5 × 14.3 = 35.8 GJ/ha/yr, or 35.8 ÷ 2.471 = 14.5 GJ/acre/yr.
To make it a measurement, weigh the stack, take a sample to find moisture (Seasoning & Drying), and write the basis beside the number.
Common Mistakes
- Mixing hectares and acres. Divide by 2.471 to go from hectares to acres.
- Comparing species yields from different bases. Convert to one basis first.
- Using planting densities as mature stool counts. Some stools die.
Safety
Weighing heavy stacks
Weighing, stacking and carrying tonnes of wood invite crush and strain injuries: weigh sample stacks in parts, use a levered or hung scale properly rated for the load, and keep clear of any stack as it is built.
What’s Next
- Learn next: Acreage Requirements — turning yield into land
- Combine with: Cutting Cycles — what a cycle means per stool