Verification and Validation of Sampling Intensities in the Pre-Felling Inventory of a Tropical Forest in Peninsular Malaysia

This study was carried out on a randomly chosen 40 ha (800 m x 500 m) forest area within the 50 ha Demography Project of the Forest Research Institute Malaysia (FRIM) in Pasoh Forest Reserve, Negri Sembilan. A total of 30h a out of 40 ha study area were used as a Calibration data set and the 10 ha remaining area as a Validation data set. The percent sample or intensity of sampling was determined at various confidence and error levels, viz., from 80% to 95% confidence with ± 5% to ± 20% error in estimating tree density, basal area and volume. Results obtained from both calibration and validation data sets were used to assess the notion that the statistical reliability of the SMS's Pre-felling inventory (by Forestry Department Peninsular Malaysia) is at 95% confidence and ± 20% error levels. Results showed different intensities of sampling regarding classification into diameter, plot size and species groups to the three parameters measured at different confidence and error levels. At 1% sampling intensity to inventorise density of trees 5 cm - <15 cm dbh in 10 m x 10 m plot, the confidence and error levels of respectively 95% and ± 15% or 90% and ± 10% would be better reliability levels. The 95% and ± 15% levels were also true for estimating basal area and volume. At 5% sampling intensity of trees 15 cm - <30 cm dbh (20 m x 25 m plot), its reliability was at 90% confidence and ± 10% error limits for either tree density, basal area or volume estimation. However, at 10% sampling intensity of trees ≥30 cm dbh (20 m x 50 m plot), the 95% confidence and ± 20% error held true in estimating tree volume. Classification into species groups showed that higher sampling intensity was needed to sample volume of Dipterocarps species compared to tree density, basal area and other species groups at any confidence and error levels. Results also showed that plot 10m x 10m yielded lower sampling intensity for trees <30cm dbh and plot 20 m x 25 m for trees ≥30 cm dbh at any confidence and error levels. The implications of attaching confidence and error levels to the sampling intensity are also discussed.

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