Jim Hilton pens a column on forestry each week for the Tribune.

Jim Hilton pens a column on forestry each week for the Tribune.

FORESTRY INK: Forest inventory basics

Do you know how to measure a tree’s height?

A friend was showing me his offer letter from the Ministry of Forests (MOF) for his first job as a professional forester in the early 1970s.

Along with details of his salary and promotion options it gave specific details as to the location and short summary of his job in the inventory section of the MOF also called the Forest Service.

Forest inventory was considered an essential part of every forester’s career so most new recruits were required to take at least a few years of field work before going on to other activities.

Basic forest inventory is also useful for anyone with a small forested area so is covered in the government publication entitled “Managing your Woodlot: A non professional’s guide to small scale forestry in British Columbia.

As noted in the manual information on the forest (assets of the business) are essential for the proper short and long-term business plan. The manual goes on to describe the basics of collecting forest inventory information like aerial photos, forest inventory maps and some basic field equipment like a good compass, 50 to 75 metre nylon tape, diameter tape, Suunto clinometer, increment borer ( with reamer) various field books, paint and flagging tape etc.

It is not my intention to provide readers with an introductory field inventory course but give some background on what the various items mentioned above are used for. The nylon tape, compass and map (or photo) are used by a two-person crew to find their way to a predetermined point (plot) to gather tree species, height, diameter and age data of selected trees.

A hip chain which uses rolls of very fine thread allows one person to measure their way to the plot. The clinometer is used to measure the slope to adjust for slope changes on the route to the plot and also for measuring the height of the trees. Inventory crews were issued a Relascope which measured slope as well as diameter of trees rather than using a prism.

I won’t go into detail between the difference of an inventory field survey versus a cruise plot approach only to say the cruise plot approach involves many more plots on a predetermined grid which provides a more accurate determination of the timber volume.

Most people may have experience on counting the rings of a tree stump or a section of a log with each ring representing one year of growth. Sometimes a magnifying glass is useful to count the tight rings (small faint closely packed) on the very slow growing trees.

Most people are not likely familiar with an increment borer which is used to age a tree without cutting it down. Think of a very hard hollow steel tube with a threaded end which allows it to be drilled into the tree. As it is hand cranked into the tree the sharp cutting point creates a wood core (about the size of a drinking straw) which can be extracted with reamer allowing the user to count the rings along the length of the core.

Some of the techniques used by the foresters may also be useful for any homeowner with large trees that may be deemed a problem.

For example if you don’t have a Suunto for measuring a tree’s height but want to know if a certain tree may cause considerable damage to your greenhouse if it happens to blow down in the wind there is simple way to get a fairly accurate measure of its height.

Cut out a piece of cardboard (six inches square) and either fold it along the diagonal or cut it. Hold the triangle up to you eye and sight along the diagonal backing up until you see the top of the problem tree.

If the ground is reasonably flat and you are holding the bottom of the triangle horizontally, the 45 degree angle of the diagonal forms a equilateral triangle so if you add the height from your eye to the ground your distance from the tree will be equal to its height. Hopefully the greenhouse is behind you and not between you and the tree.

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