Biomass of woody vegetation can be measured either by destructive method or indirect allometric equation-based method.

Destructive method: It involves harvesting all aboveground biomass components of the tree (or shrub) and then oven drying them. The oven dried weight gives estimate of aboveground biomass. This method is the direct and accurate method for biomass estimation, but it is very tedious, time and labour intensive. In many cases it may not be possible to harvest the tree owing to its ecological significance or challenging surrounding environment.

source: EarthWatch Institute

Indirect method: It involves estimating biomass based on calibrated allometric equations to link some structural attributes of the tree to its aboveground biomass; for example, diameter at breast height (DBH), plant height, wood density.

DBH is measured at 1.3 m (0r 4.5 feet) above ground and for all trees greater than 5cm diameter in the sampling plots.  It is measured either using simple measurement tape or caliper. Although this method is easier than destructive sampling, it can be very time consuming and tedious in dense canopies. Furthermore, it is important to use allometric equations that have been calibrated for a given site or at least for similar type of species.

Some key information about woody biomass measurements can be found here.