Appendix 5a

The procedure for finding the number of sample plots to give a 95 percent confidence level and a 10 percent confidence limit for a stratum of a same-age plantation was as follows:

The DBH measurements of all trees >5cm, in six 10m2 randomized plots in the plantation was recorded. The allometric equation 1.8967 + 2.3698 (lnDBH) (source: Snowdon et al., 2000: Table 1.14) was applied to the DBH measurements to find the biomass in trees and plots and the biomass per hectare for each plot. Carbon ha21 = 0.5 x biomass ha21, as reported in Table A5.1.

The formula applied to find the optimum number of plots to be randomly sampled is:


n = the number of sampling units or plots; E = the desired confidence interval (0.1 for 10 percent interval); t = the sample statistic from the t-distribution for the 95 percent confidence level (set at 2 for an unknown sample size); N = number of sampling units for the stratum, which is area of the stratum divided by the area of the plots; s = standard deviation in stratum (Pearson et al., 2005: 16).

The area of the plantation is 10 hectares. The size of plots is 0.01 hectares. The standard deviation of the carbon per hectares in sample plots (from Table A5.1) is 29.69. Inserting this data in equation (5.1) gives n = 5: that is, 5 plots are

Table A5.1 Carbon per hectare by measurement in six sampled plots

Plot number C t-1 ha

1 90.57

2 89.38

3 56.07

4 132.48

5 52.32

6 99.70

Source: Author's own data.

required to be randomly sampled in this plantation to provide an estimate of the carbon in the whole plantation with a confidence level of 95 percent and with a confidence limit of 10 percent.

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