Two types of sample plots could be adopted for land-based projects, namely permanent sample plots and temporary sample plots: the type of vegetation determines which of the two is to be adopted.
Permanent plots are used mainly for measuring changes in carbon stocks in perennial vegetation where, for example, the trees may have to be measured over a number of years. This approach is suitable for most of the land-based projects involving tree carbon pools:
• Forests and plantations
Permanent sample plots are generally regarded as statistically more efficient in estimating changes in forest carbon stocks compared to temporary plots because typically there is high covariance between observations taken at successive sampling events in temporary plots (Avery and Burkhart 1983). The disadvantage of permanent plots is that their location can be known and they could be treated differently (e.g. application of fertilizer and irrigation to increase the rate of carbon stock accumulation). Further, these plots might be damaged or destroyed by fire or other disturbances during the project period. This disadvantage could be overcome by ensuring that silvicultural practices are identical for the whole area under a given activity during the period of monitoring and verification: if the plot is damaged, say because of fire, new sample plots can be chosen, with identical soil and plant growth patterns.
Temporary plots The location of temporary plots could vary from year to year or over a number of years. In temporary plots, the measurements are made for a given year and biomass is calculated only for that year. Next year, biomass is estimated from a different plot. Such approach is suitable for projects involving annual vegetation:
• Estimation of grass production in grassland reclamation and savannah projects
• Estimation of production of herbaceous vegetation in forests and plantations.
The advantages of temporary plots are that they may be established more cost-effectively to estimate the carbon stocks of relevant pools and that the sampling would not be affected by disturbances. The main disadvantage of the temporary plots is related to precision in estimating change in forest carbon stocks (IPCC 2003). In temporary plot approach, individual trees are not tracked (Clark et al. 2001) and the covariance cannot be estimated, which makes it difficult to attain the targeted precision level without measuring a large number of plots. Thus, the cost advantage of using temporary plots may be lost by the need to establish more temporary plots to achieve the targeted precision.
Thus, the permanent plot approach could be adopted for forests, plantations, agroforestry and other perennial vegetation systems whereas the temporary plot approach could be adopted for annual vegetation systems such as grassland and cropland.
The shape of a plot has implications for accuracy and ease of measurement. The standard plot shapes used in vegetation studies are rectangles and squares, although strips and circles are also used.
Rectangular or square plots Establishing rectangular or square plots involves measuring out the length or the breadth and using the diagonal to ensure a true right angle at each of the four corners. A square or a rectangle is the most commonly adopted shape for plots for estimating biomass in most vegetation types including forests, plantations, agroforestry, shelterbelt, grassland and cropland because of the following factors:
• Easy to lay out and ensure square corners
• Suitable for young and mature forests as well as for non-tree vegetation such as grassland and cropland
• Suitable for large plots (e.g. 50 x 50 m or 100 x 100 m) or small plots (e.g. 1 x 1 mor5 x 5 m)
• Easy to establish corner points for drawing the boundary for periodical visits and long-term monitoring
• Easy to record GPS readings and to locate the sample plots in later years for monitoring.
Circular plot Circular plots could be adopted for vegetation types such as trees, herbs and grasses. It is easier to draw circular plots of small dimension in the field. However, a circular plot is not popular because of the following reasons:
• Difficult to mark a circular plot in forests and plantations where large trees exist
• Difficult to verify the boundary line and area of the plot
• Difficult to mark the boundary line for periodic visits and not suitable for long-term monitoring
• May not be suitable for agroforestry, shelterbelt and avenue trees
• Not very efficient (Loetsch et al. 1973), since, as the perimeter increases, so will the numbers of trees on the edge of the plot
Strip plots Strip plots or belt transects are long and narrow rectangular plots, normally used for studying rare populations. A strip plot is not usually adopted for biomass studies due to the following reasons:
• Difficult to mark long strips in the field, especially if there are large trees.
• Size of the project area may be a limitation. For example, a sample plot of 2,000 m2 requires a strip 200 m long, and several such sample strips may be required.
• Difficult to draw boundary lines for the long narrow strips for periodic monitoring.
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