In many cases, one of the primary objectives of forestry is to provide a sustained yield of timber. The yield of a woodland is determined by its area, the species planted, site conditions, rotation length and silvicultural system. In the UK yield is usually described in terms of “yield class”; this is a measurement of increment (the amount of solid stem wood added to an area of woodland) in cubic meters per hectare per year (m3/ha/yr) expressed in intervals of 2 (ie 4, 6, 8 etc.)
Unfortunately, trees don’t grow in an easy to measure shape, or at a constant rate over their lifespan. Ideally to estimate the yield class you need to measure the standing volume of a woodland (the amount of timber per ha) on more than one occasion, to establish the rate of growth. However, the yield class can be estimated for a species by measuring the top height of the stand of trees and using this measurement and the age of the tree to look up the yield class on a table. These tables are currently being revised by the Forestry Commission, and we expect them to be published soon. Please note that 1) these tables are only available for commercially grown species (mainly conifers) and 2) They only include stem wood, and may underestimate material available from trees with substantial branches. Once you know the Yield Class, you will then be able to predict the standing volume in later years and determine what the yield of the woodland is likely to be in the future.
You can asess the standing volume of timber by using the Estimating woodfuel potential - Measurement protocol devised by Forest Research
Once you know the current standing volume of timber and yield class, it is relatively straightforward to determine how much timber will result from a felling (based on the silvicultural system.)
To maintain a sustainable yield from a block of woodland, you either need to make sure that you are felling less than the annual increment (in the case of uneven aged stands and coppice) or that you are replanting or allowing the site to regenerate effectively after an even aged management felling or clear fell.
Remember that fuel is only one of a number of different product streams to come from forestry. You should find that higher quality of timber will fetch a better price when sold into different markets (such as building timber, furniture making etc.)
This can be an extremely complicated subject and you may need professional support for this stage of woodland management. If you need further information, the Forestry Commission publish the Forest Mensuration Handbook which covers the whole subject of timber measurement in more detail. The current edition was published in 2006, but the 1975 edition can be downloaded as a pdf