Environmental Engineering Reference
In-Depth Information
distance between the two arms. Diameter at breast height can also be measured with
a Biltmore stick. Although not as accurate as a diameter tape, it is quicker to use.
Diameter at breast height is measured by holding the stick at a set distance, usually
64 cm, from the eye, and at breast height. The left side of the stick is lined up with
the left side of the tree. The number on the stick that lines up with the right side of
the tree is the approximate dbh.
7.4.2.4 Tree Age Determination
In some cases it is of interest to determine the annual rate of woody biomass
production. This requires an estimate of tree age. Many tree species increase
trunk diameter by producing a single layer of wood each year between the previous
year's growth and the bark. In a horizontal cross section cut through the trunk of a
tree, these growth bands appear as concentric rings, referred to as growth rings, tree
rings, or annual rings. Each ring represents 1 year of growth, so that the tree's age
can be determined by counting the rings. The least invasive way to see the tree rings
is with an increment borer which takes a small (5 mm diameter) straw-like radial
core sample from the tree. An increment borer consists of three parts: handle, steel
shaft (core tube or auger), and extractor. Increment borers come in different sizes;
the length of the shaft should be at least 75 % the diameter of the tree you are
boring. Tree rings should be counted near the base of the tree. For consistency, the
boring is commonly taken at breast height (1.4 m). To extract a tree core, the screw
tip of the shaft is pressed against the tree and the handle is turned clockwise until the
screw bit reaches the center of the trunk. This action forces the core of wood into
the tube. The core is extracted by first breaking the core with a counterclockwise
one-half turn of the handle. The extractor is then fully slipped through the tube.
The core will be removed with the extractor.
There are situations that reduce the accuracy of using ring counts to determine
tree age (Avery and Burkhart
2002
). One year's growth includes both spring wood
(rapid-growing, lighter colored wood) and summer wood (slower-growing, dark
colored wood). The method is most reliable when there is a sharp contrast between
spring wood and summer wood and for fast-growing coniferous species in northern
temperate zones. In tropical or southern temperate zones, tree growth generally
does not produce distinctive rings. Some deciduous species produce limited con-
trast between spring wood and summer wood. Adverse growing conditions, such as
drought, result in very narrow rings that are difficult to distinguish. Conversely, a
period of favorable growing conditions following a drought can result in
false rings
which represent small growth spurts.
7.4.2.5 Coarse Root Biomass
Destructive sampling of coarse roots (
10 mm diameter) is labor intensive and
requires heavy equipment. As such it is not conducive to most wetland
investigations. We recommend an alternative approach to estimating coarse root
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