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Tree and Forest Measurement / Establishing sample plots
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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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