Phytomass and photosynthesis
&quot;Biomass is defined as live material, necromass as dead, attached material.
Biomass + necromass form the &quot;standing crop&quot; or phytomass.&quot; (from https://link.springer.com/chapter/10.1007%2F978-3-642-68786-0_22)

But even if we were only talking about biomass, there is only a loose correlation between that and photosynthetic activity -  think about single-celled algae, where 'every' cell is photosynthesizing, versus trees, where most cells are non-photosynthesizing, and there may even be times where none are (in trees that loose their foliage in winter).