Tract and plot configuration

Tract and plot configuration

a) Tract of 1km x 1km has been chosen as the standard size to lay out a cluster of 4 independent sample plots. The tracts serve to generate forest type/land use maps from aerial photographs and very high resolution satellite data when available in the country and hence to generate areas. They are also used to collect various variables on social and forest attributes. Aerial photography taking is a costly operation even on a sample basis and recent ones are very often unavailable in the countries. Areas of forest types and land use classes can also be generated from the sample plots as shown below.

b. Cluster layout: The cluster sampling is statistically sound design and frequently used in forest inventories. It is employed to lower the cost of the survey. In this methodology, each cluster is composed of 4 plots placed within a tract of 1km2. The plots are strips of 20m x 250m. They are located at 250m apart from each other as shown in figure 3 below. Since the sampling is non-stratified, each plot will have one or more classes (forest, other wooded land, other land, water), depending on the heterogeneity of the site in terms of forest types/land use classes.

In order to minimise the effects from systematic or periodic variations within the surveyed populations, the 4 plots will be placed in perpendicular orientations. This is also proven practical in the fields as the field crews, during the survey, progress from the first plot to the last one without walk backwards for measurements. The way back to the road is easier to find.

Figure 2: Tract with delineation of forest types and land use classes

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c. Plot configuration

Plots have dimensions of 20 x 250m (horizontal distances)

Plot structure and measurements:

The plot is designed to cross the maximum possible of variations within and between the classes, but long strips are discarded for practical reasons. Each plot will include nested plots in three locations. The first is centred at 5 m from the beginning of the plot, the second ones at 125m and the last at 245m. Two levels of nested plots are taken. The first consists of sample areas of 20m x 10m (200m2) for measurements of trees with DBH within the range of 10 to 30cm. The second level consists of circular sample areas of 3.99m radius or 50m2 each. These small size nested plots are foreseen to records saplings of the tree species, soil information and topography (See list of variables). The location of the nested plots is shown in figure 3. In non-forest areas, the nested plots will not be materialised, and the measurements will be carried out on all tree of DBH of 10cm and above.

Figure 3: Plot design

d. Tree location plan

For each permanent plot, a schema with tree location will be prepared for control and future measurements in the monitoring process. Limits of each forest type and other land use class will be identified on the ground and accurately drawn within the plot schema. Horizontal distances along X-axis and Y-axis will be measured and recorded for each tree in all forest types and land use classes falling within the plot. Distance measurements may be done along the slope and corrected to horizontal using the slope correction factors. In future surveys, changes in the land use or within the forest conditions in the plots will be mapped.

Figure 4: Tree and forest type/land use classes distribution with a plot

Remote sensing:

Use of remote sensing techniques to classify vegetation and land use is optional: If up-to-date air photos or high-resolution satellite scenes are available for the site, forest types and land use classes can be delineated to produce site map using the remote sensing technique. The areas generated from air photo interpretation will be used for area estimation. The maps serve also to lay out the sample plots and as reference base for future monitoring work.