Using a database to make simple projections

Yield Class

Foresters use the term 'yield' to refer to the productive capacity of a forest. Yield classes are measured in cubic metres per hectare per year and they are divided into steps of two cubic metres per hectare. A compartment that is said to belong to yield class 12, for example,  has a mean annual timber increment of 12 m ³ per hectare. Different species have very different yields. First rotation monocultures of exotic conifers are capable of achieving a yield class 22 but many deciduous hardwoods may only achieve a yield class 4 or 6.

Yield class may be measured by monitoring the growth rates of trees in a stand over a period of several years. However, it may also be estimated much more quickly by measuring the dominant height of a sample of trees in the stand. Trees will be taller than the average for their age on productive sites. Although tree height can be tricky to measure, especially in a dense stand, it is a much better predictor of yield than any other parameter. Tables, giving the estimated yield per age class at normal stocking, are published by the Forestry Commission.

It is very important for a forester to know the yield of a stand of trees. It enables predictions to be made about rotation length and judgement of how well a particular species is suited to a site.

You will need:

1. A copy of the Little Wittenham compartment map (you can print a copy of compartment_map.TAB in MapInfo);
2. A clinometer;
3. A 30m tape;
4. A notebook and pencil;
5. Forestry Commission Booklet No. 34 - Forest Management Tables (metric) 1971;
6. Forestry Commission Booklet No. 39 - Forest Mensuration Handbook 1975.
7. Warm waterproof clothing and boots;
8. A field first aid kit and mobile phone;

Before you start work, divide the class up into groups of three. Allocate an equal number of compartments to each group. You need only measure yield class in planted stands that have one or two species. You may want to make a master map showing which group is responsible for which compartments.

In the field:

1. Agree a rendezvous time and location with the fieldwork leader and the rest of the group. Do not start work if there is a risk of high wind or heavy rain. Make sure that the field first aid kit and mobile phone are kept in a place where they are accessible to all groups.
2. In each compartment you need to select five 0.02 ha (radius 8 m) sample plots, at random. Avoid sites that are within five metres of the edge of the plot. Alternative sampling procedures can be found in Forestry Commission Booklet No. 39.
3. Measure the height of the 2 tallest trees in each plot. You do not need to measure the smaller trees. Full instructions on how to measure tree height using a clinometer can be found in Philip, M.S. (1994) Measuring trees and forests . Second Edition, CAB International, Wallingford UK.
4. Calculate the average height of the tallest 10 trees (the dominant height) and use this to estimate the yield class for the stand using the appropriate yield class curves for the species in Forestry Commission Booklet No. 34.
5. If your stand contains more than one species you will need to estimate the yield class for each species separately.

You need to be able to see both the base and the top of a tree in order to be able to measure its height with a clinometer