Establishing sample plots
For forests larger than 3 hectares, the cost
involved in measuring every tree becomes prohibitive. By carefully
measuring trees within a representative sample of the area
it is possible to greatly reduce the time and costs of measuring
without losing accuracy. When measuring to assess the volume
per hectare, sampling usually involves establishing a number
of plots within the forest. Only those trees located
within the plots are measured. Based on the size and distribution
of trees it is possible to estimate the stocking rates (stems/hectare),
species mix, average tree dimensions, stand basal area and
timber volumes. Adequate sampling can also provide an indication
of the variability of these attributes across the site and
can be used to assess the level of precision (the sampling
error).
Stratifying the forest into uniform areas based
upon species, age, management history, soil types or aspect
prior to sampling can improve the precision and may reduce
the costs of measurement. Plantations may be divided on the
basis of soil types, aspect or the presence of different understorey
species. In the same way a group of smaller areas of similar
age might be combined into a single unit for the purpose of
measurement.
Determining the number, size and shape of
sample plots
When assessing timber volumes, the ideal number
of trees per plot will usually be between 15 and 25. Where
there are a number of species or age classes it may be necessary
to increase the size of the plot so as to include a representative
sample of each. If the stocking rates are known then the size
of the plots can be predetermined. For example, if the stocking
rate was expected to be 500 stems per hectare then a plot
of 0.04 hectares would be required to include 20 trees.
The best shape for the plots depends on the
distribution of the trees and the presence of any gradients,
such as slope. In young plantations it is often easier to
establish rectangular plots that span a number of planting
rows. In native forests or older stands where the trees are
more irregularly spaced, large circular plots may be easier
to lay out. On steep ground the plot dimensions must take
account of slope.
The number of plots required depends on the
variability in the parameters of interest. A minimum of five
plots in any one area of uniform forest is recommended with
the number of plots increasing with variability in growth
or character. A pilot survey may be necessary to determine
the level of variability in forest. It may also be possible
to test the level of precision by measuring a number of plots
to determine whether more are required. To avoid bias in the
location of plots it is common to systematically mark out
the location of the plots on a map prior to measurement. This
can be done using a simple grid based on the number of plots
required. In most cases plots are located away from the edge
of the plantation to avoid the edge effect. See Additional
Information for information on sampling procedures and methods
of determining the precision of measurements.
Precautions to take in locating and laying
out plots:
- On sloping ground all distance measurements
should be horizontal.
- In large plantations, plots should not be
located on the edge or take in abnormal features (such as
dams).
- If trees are on the edge of the sample plot,
they are counted as "in" if the centre of the
stem is in the plot.
- All information collected should be checked
before leaving the plot.
Measuring small forests and belts
Where the forest area has a large proportion
of trees on an edge(s) e.g. for belts or riparian plantings,
it may be difficult to determine the area of land that is
actually occupied or utilized by the trees. Measurements of
areas based on stocking rates, basal areas and volumes are
often impractical. Rather than establishing fixed area plots
like this it may be preferable to sample (randomly or systematically)
a number of individual trees throughout a forest. The dimensions
of each tree could be measured. To assess the stocking rate
and basal areas the distance to other trees and their size(s)
could be measured.
Recording or permanently marking the plot
location
All plots should be either permanently marked
(e.g. with steel pegs) or carefully marked on a map. Reference
points are located at the centre of circular plots and the
north east corner of rectangular plots. Being able to return
to the same trees for later measurement is very useful for
assessing tree growth and planning silvicultural operations.
Species that do not shed their bark, such as pine, can be
numbered or marked with paint. Permanent sample points are
ideal for measuring changes in forest growth over time and
may be required for assessment of growth rates, response to
management interventions such as pruning or thinning.
A plot sheet may be used to document the information
collected. It may also be useful for adding comments about
the plot (e.g. undergrowth, slope etc) or individual trees
(e.g. presence of disease or other abnormalities) that may
help interpret the information collected. The location of
the plot, any unusual features and the date and names of those
involved in the measuring should also be noted. See Additional
Information for sample plot sheets.
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