Estimating Tree Biomass Using Mean Tree Weight

Estimating tree biomass involves estimating the mean weight of trees according to DBH classes, multiplying it by the number of trees in the respective DBH classes and adding up the totals of all the classes in a given sample plot to obtain the total weight of all the trees in the plot, which is then extrapolated to per hectare by taking into account all the sample plots. Estimating the weight in this manner avoids felling a large number of trees and the associated cost of estimating their weight. The tree biomass method is thus an attractive option, although it is not as accurate as using species-specific biomass equations derived from weighing felled trees. The method involves the following steps:

Step 1: Select the forest or tree plantation stratum, select sample plots and measure the DBH of all the trees in the sample plots Step 2: Using DBH data from field measurements in sample plots, prepare a frequency table using appropriate class interval of DBH for each tree species

° To reduce error, it is desirable to have smaller DBH class intervals (e.g. 5 cm or smaller) depending on the range of the DBH in the sample plots

Step 3: Tabulate the DBH data (Table 17.2 illustrates the format)

Step 4: Extrapolate the total number of trees of the selected species from sample plots to per hectare values to fill that column in the table Step 5: Locate a tree with DBH closest to the mean DBH value in the forest or plantation for each DBH class for the selected tree species Step 6: Harvest the selected tree with the mean DBH and estimate the fresh weight of the mean tree

° If required, separate the harvested tree biomass into commercial stem, twigs and branches and estimate the fresh weight separately for each

Step 7: Estimate the dry weight of the selected tree for each DBH class by selecting a sample of fresh wood for drying in the hot air oven at 105°C, to constant weight; if needed, sample each component separately and add up the total Step 8: Estimate the total weight of all the trees in a hectare in each DBH class, using the dry weight of the tree with mean DBH and the number of trees in that DBH class Step 9 Aggregate the biomass of all trees present in a hectare Step 10. Repeat the steps for each species and aggregate the weight of all the tree species on per hectare basis

° For tree species sparsely represented in the sample plots, use the mean weights derived for other species which has similar tree structure (bole and crown size and shape)

Table 17.2 Estimating above-ground biomass of Syzigium cuminii from DBH values

Mean dry weight

Mean DBH from

of tree with mean

Total biomass, dry

DBH class (cm)

sample plots (cm)

DBH (kg/tree)

No. of trees/ha

weight (kg/ha)

5-10

8.0

15

5

75

10-15

12.5

21

25

525

15-20

18.0

28

175

4,900

20-25

24.0

40

200

8,000

25-30

28.0

55

175

9,625

>30

33.0

70

100

Total 680 30,125

Application The mean tree method is a practical method, which can be used in locations and for tree species for which biomass estimation equations are either not available or cannot be determined.

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