Farm Ireland

Saturday 26 May 2018

Technologies making crucial tree inventory easier than ever

Measuring timber volume of a tree by getting its diameter at breast height (DBH)
Measuring timber volume of a tree by getting its diameter at breast height (DBH)
Aerial image of forestry planatation

William Merivale

Last December the second National Forest Inventory was published, providing invaluable information about Ireland's forest estate at the macro level.

One of the most telling statistics is that 56pc of the entire estate is still less than 20 years old. That said, every year many more thousands of hectares of private forest enter the harvesting stage, making an inventory at the micro level a vital component of sound forest management.

The amount of available knowledge has a direct bearing on the quality of management, so knowing the extent of the assets owned by and available to the owner or manager is fundamental to a successful business.

The best time to start a woodland inventory is straight after the first thinning, when it is possible to 'see the wood for the trees', so to speak. This snapshot must be updated regularly to take stock of the growth in volume, ideally following each harvest.

Over time, the emphasis with many of our uniformly-aged and predominantly spruce plantations will be to convert them to more uneven-aged forests with improved species diversity. This is where inventories will become an even more important tool.

How to do an inventory

An inventory is best set out in the form of a computer spreadsheet. Depending on the size and age structure of the woodland it should be separated into compartments or plots, and sub-compartments if necessary, as shown in Table 1.

An additional notes column should be included if any pertinent points relevant to the compartment in question. For example, other minor species, open areas, or a description of biodiversity, archaeological or other features need to be noted.

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To get as accurate a picture as possible it is important to take sample plots throughout the plantation.

The number necessary will vary, but as a guide for areas of 2-10ha the recommendation is for eight plots in uniform crops and 12 where there is greater variability.

Of course it is also important to choose plots at random and which are evenly spaced throughout the plantation.

The information gathered not only gives an indication of what the woodland contains at a particular point in time but is essential in planning a thinning, and more specifically as a tool to help with thinning control. Second and subsequent thinnings need to be carefully controlled if the crop is to realise its full potential and value.

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), but the growth curve of a tree is not a straight line.

During the early years growth is vigorous before it peaks and begins to decline with increasing age. The point at which the MAI curve reaches its maximum is the maximum average rate of volume increment which 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 since they are invariably harvested before the age of maximum MAI.

However, yield class remains an important issue, not least because 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 70m3/ha (YC20 x 5 years x 70pc).

Technological developments

For much of the last 100 years forest measurement techniques have changed very little, and to take accurate measurements from a sufficient number of sample plots is inevitably labour intensive. Typically, the tools of the trade consist of a girthing tape to measure diameter at breast height, a hypsometer to measure tree height and, more recently, electronic calipers connected to a portable computer which speeds up the recording process.

Some time ago I wrote about the Cork-based forest technology company called Treemetrics. They have been developing a completely new system of forest measurement that is now gaining international recognition for its considerable accuracy.

Using sophisticated laser scanning equipment, the company can measure both the volume of timber in a stand and also assess tree form, taper and stem straightness before a saw goes anywhere near it. Previously, these features could only be established after the tree was cut.

William Merivale is national secretary of PEFC Ireland and a forestry consultant based in Co Cork. Email:

Irish Independent