Table 4. Identified plot level biases in the measurement of aboveground forest biomass by community members and professional foresters, with notes on possible effects and severity of these biases.

Topic Bias Effect Potential frequency of occurrence Severity when occurring Remarks
Tree circumference measurement Holding thumb under the measuring tape while measuring circumference Increases circumference measurement Systematic Medium Adds approx. 2 cm depending on size of tree
For trees on slopes, the observer might not maintain the tape perpendicular to the growth direction of the tree at point of measurement (drooping at the back) Increases circumference measurement Systematic Medium Adds up to 4 cm depending on size of tree
Measuring tape sagging on far side of trees too large to reach around Increases circumference measurement Occasional Low Adds up to 2 cm depending on size of tree
Not removing liana from trees Increases circumference measurement Occasional Medium Error proportional to liana size
Measurement too low above buttresses or other deformity Increases circumference measurement Occasional Medium Error proportional to tree size
Estimating (not measuring) large or fluted trees or trees with very high buttresses (> 2 m) because of laziness or need of ladder Increases or decreases circumference measurement Occasional Medium Can severely bias biomass estimate because it affects mainly large trees
Plot demarcation Including trees smaller than 94 cm circumference in the 9–15-m circle, the outer ring of the circular plot Increases the number of trees in the plot Occasional Medium Adds up to 20 Mg biomass per hectare depending on size of tree and wood density
Change in plot size in steep slope Decreases or increases the number of trees accounted for Systematic High Decreases or increases total plot size
Change in plot size due to incorrect plot demarcation Increases or decreases number of trees in plot Occasional Medium
Rejection of large trees on the border of the plot Decreases number of trees in plot Occasional High