Calculating a stand's yield class
The concept of yield class is often mentioned by foresters but not always understood by the layman. The growth of a tree may be measured in terms of height, diameter, volume or weight, but volume is the most meaningful for management purposes.
In an even-aged stand, the cumulative volume production divided by the age of the stand is referred to as the mean annual increment (MAI).
However, the growth curve of a tree is not a straight line. During the early years, growth is vigorous, but growth reaches a maximum and then declines with age.
The point at which the MAI curve reaches its maximum is the maximum average rate of volume increment that the stand can achieve and this number is the yield class. Therefore, a stand with a maximum MAI of 20 cubic metres per hectare has a yield class of 20.
Fortunately, there is a close correlation between cumulative volume production and the top height of a stand, so yield class is relatively easy to measure. In fact, most crops in Ireland never reach their full potential yield class as almost invariably they are harvested before the age of maximum MAI.
However, yield class remains an important item in the management tool box, not least because the second and subsequent thinnings should aim to remove 70pc of the yield class.
Thus, on a five-year thinning cycle, for a plantation of yield class 20, the aim should be to harvest 70 cubic metres per hectare (YC20 x 5 years x 70pc = 70 cubic metres).