What Is The Formula To Calculate An Increase In Biomass, And Also How Do You Calculate Gross Primary Productivity?

For the calculation of increase in biomass you need to consider a few factors like biomass expansion factor etc. General equation is given by
Aboveground biomass density (t/ha) = VOB * WD * BEF
where:
WD = volume-weighted average wood density (1 of oven-dry biomass per m³ green volume)
BEF = biomass expansion factor (ratio of aboveground oven-dry biomass of trees to oven-dry biomass of inventoried volume)
 
For details
www.fao.org