You need to standardize and come out with a factor on the basis of age of plant, girth of plant, height of plant and biomass in per square meter area. Accordingly the factor can be further used for estimation of biomass.
This topic has been the subject of some approximated procedure, but colinearity and allometry can be applied according to international panels, and aerial pictures or ground surveys are already in use, https://cdm.unfccc.int/methodologies/ARmethodologies/tools/ar-am-tool-14-v2.1.0.pdf