It increases because the biodiversity of the community in the environment during succession increases.
Explanation:
During primary succession, the nutrients in the rocks are gradually released as pioneer plants help break up the rocks into soil. These nutrients are taken up and more species of plants are able to thrive in the environment including the perennial and biennial plants. These plants provide food and shelter for higher organisms (hence many longer food webs and chains) and this is why biodiversity increases with increased succession until the climax community is reached. Increased biodiversity means increased biomass.
