In order to obtain growth data on a representative scale for the NECF landholdings, 22 690 plC?ts, each covering a circular area of 250 m2, were measured. They were distributed over 539 commercial compartments totalling 11 344,93 ha, representing a sample intensity of 5%.
5.1 Sampling
The following particulars of each compartment were determined in a systematic sampling system, shown in Figure 6 and described by Von Gadow and Bredenkamp (1991):
• The average stem diameter at breast height (DBH) overbark
Diameter measures are necessary to determine the volume of felled trees and logs. The diameter of standing trees, conventionally measured at a height of 1,30 m above ground level, is known as diameter at breast height, (DBH). The height of 1,30 m was selected for reasons of convenience and formally adopted by the International Union of Forestry Research Organisation (IUFRO). The mean diameter of a sample plot and stand is calculated as the mean quadratic diameter, which is the diameter corresponding with the basal area of the mean tree. The reason being that the use of the arithmetic mean diameter underestimates the mean volume of the trees within a plot (Bredenkamp and van Laar, 1993).
• The average height (HT) of the trees
A Suunto hypsometer, an instrument based on elementary trigonometric principles, was used to measure standing tree height. This requires the measurement of the horizontal distance between the operator and the tree, with the aid of an optical rangefinder, as well as the angles between the horizontal and the top and the base of the tree respectively. The angle between the operator and the top of the tree should be less than 45 degrees since this minimises parallax. No trees with excessive lean and deformation or dead tops were measured. The mean height of the stand was defined as the estimated height of the tree with the mean basal area. It was obtained by fitting a regression equation to a sample of 30 height measurements and corresponding diameters in each compartment. Because of non-linearity of the relationship, both variables were transformed with log (height) as response variable and the reciprocal of DBH as predictor. A regression analysis was used to estimate the height corresponding to the root mean square DBH. This was reflected as mean height. Furthermore, the dominant height was defined as the regression height of the 20% thickest trees that were recorded in each compartment (Bredenkamp and van Laar, 1993).
• The average number of stems per hectare (SPHA)
The stems per hectare for each plot were counted as shown in Figure 7.
The average number of stems per hectare for each compartment were calculated based, on all measured plots.
A sampling intensity of 5% was considered as sufficiently representative and cost- effective as reflected in current Mondi Forests Policy.
5.2 The Circular Plot Method
Sampling can be done in various ways. For the purposes of this study the circular sample plot method was selected. The following considerations were taken into account:
• Method is applicable across species, regimes and other site parameters;
• It deals effectively with variable spacing;
• The experience in Mondi Forest has shown the method to produce superior results to earlier methodologies (K. Chiswell, B. Esler, B. Pienaar, pers.
comm., Mondi Forests, 2000).
The -following equipment was used:
• DBH Callipers
• Hypsometer with range finder template
• Measuring tape (100 m)
• Psion data logger
• Compartment list and estate maps
• Compass
Establishment of Sampling Plots
The relevant estate maps were produced for selecting compartments based on tree age and species. Thereafter, the directional tree rows were defined and the
placement of the plots planned accordingly. If no suiTable tree rows existed, the cruise direction and placement of plots were determined by using a compass.
The first plot was placed 30 meters in from the edge of the compartment. The range finder stick was placed at the centre point of the plot. The radius of 8,92 m was defined by using a range finder to demarcate a plot. The area of the circle plot was therefore calculated using the conventional circle area formula.
Area
=
nr2=
3,14 x 8,922=
250 m2Plots were spaced at 50 m intervals in a row and at 100 m intervals between rows (Figure 6), measuring 500 m2 per hectare, resulting in a 5% sampling intensity.
One member of the team defined the perimeter of the plots with the range finder, while the second member measured the DBH of each tree falling within the plot (Figure 7). The third member of the team captured the data with a psion data logger. The person recording the readings on the data logger verbally repeated all measurements to ensure that they were correctly recorded. DBH/HT pairs were taken per plot in order to establish a DBH/HT regression for each compartment.
Figure 6. Schematic diagram of plot layout.
o
100 m
o
o
Figure 7. Schematic diagram of a plot with trees.
Tree In
Tree Out
o
o
o
Tree Out
o
Radius
o
R92o
o
These data were also used to calculate an average height by using the average DBH as previously described. A minimum of 30 DBH/height (HT) pairs were measured per compartment, thus for example if a compartment was 5 ha in size the minimum DBH/HT pairs per plot would be:
30/(5x2)
=
3 DBH/HT pairs30 DBH/HT pairs covering a range of diameters are generally regarded as a guideline to obtain a statistically viable DBH/HT regression.
5.3 Error factors
On slopes, DBH measurements were always taken a 90° angle to the slope. The DBH callipers were always held at right angles to the longitudinal axis of the tree stem. The reading was taken while the instrument was pressed against the tree.
If the sectional area were not circular, two diameter readings were taken, at right angles to each other and the average recorded.
Hypsometer height readings were taken as per the standard methodologies with the necessary considerations and precautions for slope and distance.