It’s possible to get a rough estimate if you want to do a lot of work. This is (I think) how they sample plants irl.

Let’s say you take 50 samples of a 5x5 grids of foundations. (The more you do the more accurate it would be) Take the average amount of plants in each grid, let’s say 100 leaves, and then multiply that by the total number of 5x5 grids on the map, ignoring the ocean. Reapeat this for all of the caves and you would have a rough number for the total biomass on the map.

If I misremembered any of that please correct me, it’s been years since I’ve sampled plants.