You can use allometric equations to calculate plant biomass. See: Allometric equations and carbon stocks in tree biomass of Jatropha curcas L. in Senegal’s Peanut Basin, Global Ecology and Conservation, Volume 9, January 2017, Pages 61–69, doi.org/10.1016/j.gecco.2016.11.007
There are many manuals and protocols available that you can find by searching biomass or carbon assessment protocol. However, one straight forward is FC Woodland Carbon Code:Carbon Assessment Protocol that you can find as attachment.