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Stand measurements
Stocking
Rate
The tree density or stocking rate of a forest
is described as the number of trees per hectare. This can
be easily calculated for each plot as follows:
Stocking rate (stems/ha) = Trees in plot /
Plot area (ha)
Tree Diameters, Tree form and Pruned
height
The diameter of every tree in the sample plot can be measured
using the methods outlined in Diameter. While measuring diameter,
the form of the tree should be inspected and a form factor
(see Form ) recorded along with other points of interest.
It is also worth recording the pruned height of each tree.
One simple way of doing this involves using a graduated extension
pole, such as those used for pole pruning, and measuring the
pruned height to the closest 0.5m.
Stand Height
Measuring the height of trees can be difficult and time consuming.
Fortunately, the height of the tallest trees in a plantation
or native forest are usually quite uniform and therefore,
rather than measuring the height of all trees in the sample
plot, it is common to select a sub-sample. In most cases a
number of the fattest trees (largest DBH) of good form are
measured for height, this is called the "Mean Dominant
Height". The Mean Dominant Height can be estimated by
measuring the heights of the 4 fattest trees of good form
in each plot then averaging the height of the 3 tallest trees
to calculate the Mean Dominant Height (See Tree Height). Where
there is more than one species or age class, it will be necessary
to determine a Mean Dominant Height for each.
Stand Basal Area
Stand Basal Area (SBA) is simply the cross-sectional area
of all trees at breast height per hectare of forest or plantation
(m2/ha). Stand Basal Area can be used to estimate stand volume
and is a useful measure of the degree of competition in the
stand. It is often quoted when planning thinning prescriptions.
Forest Volume
Total forest volume can be calculated from the plot measurements:
Standing Forest Volume (m3/ha) = Plot Volume
(m3)/Plot Area (ha)
Stand basal area measurements can be used to
calculate tree and butt log volumes in the same way that tree
volumes are calculated. In plantations where all the trees
are assumed to be cone shaped and quite uniform, a quick estimate
of total volume can be made from the Stand Basal Area and
Dominant Tree Height (DHt):
Standing Volume (m3/ha) = SBA x DHt/3
Where SBA = Stand Basal Area (m2/ha) and DHt
= Dominant Tree Height (m)
In some areas, and for some species, volume
tables and volume equations are available. Growers in traditional
plantation growing regions can contact the larger plantation
growers for details. For those in southern Australia, the
Farm Forestry Toolbox http://www.privateforests.tas.gov.au/downloadindex.htm,
has an electronic tool that may be used to approximate tree
volumes.
Mean Annual Increment (MAI) & Current Annual
Increment (CAI)
The Mean Annual Increment is simply the average
volume production per year for a forest of known age:
Mean Annual Increment m3/ha/yr (MAI) = Volume
of stand (m3/ha)/Age of stand (yrs)
Current Annual Increment (CAI) is the increase
in volume at a particular age and is determined by annual
measurements of standing volume.
Example:
Current Annual Increment (CAI) at age 2 (m3/ha/yr)
= (Volume at age 3) - (Volume at age 2)
In dense plantations, the CAI will increase
rapidly in the early years, up until competition for light,
moisture or nutrients causes CAI to reach its peak. The decline
in CAI can be more rapid than the early rise. In a mature
native forest, the CAI is often close to zero, meaning there
is no change in the total wood volume on the site from year
to year. For some trees to grow others must die.
When the CAI drops to the point that it is the
lower than the MAI, then MAI will also fall because the increase
in the next year will be less than the average. MAI is a much
used (and often abused) forestry measurement for tree growth,
but whenever a MAI or CAI figure is quoted, the age of the
forest must also be known. This is because MAI changes with
time. So for example, a plantation that has grown at 20 m3/ha/yr
over 10 years might have 200m3/ha, but if grown for another
10 years it will not necessarily achieve 400m3/ha.
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