How much land area do my land-based animals (herbivores) need for food?
I'm trying to work out the kinks of an ecosystem on a planet that is rather similar to Earth, but with some species of animal (both carnivores and herbivores) we aren't familiar with in our world. The particular species I'm working with right now are purely land-based creatures.
I've been able to use for example What is the maximum size of a flying creature? and What efficiencies make a realistic food chain? to come up with reasonable amounts of food needed for each step in the food chain. (In my case, I started with the apex predators and am working my way down the food chain.) I'm also looking to real-world Earth species for inspiration. This is the easy part.
However, the numbers that come out of that is an amount of biomass. Borrowing from the accepted answer to the latter question to illustrate this:
The 2.5 tonnes of mountain goats will, by the 10% rule, need 25 tonnes of plants to support them; the lemmings will need about 5.5 tonnes, and the songbirds will need about 1 tonne of fruits and seeds. Treating the 2 tonnes of insect biomass as roughly 100% herbivores means they'll need 20 tonnes of plants to support them (and everything that depends on them), for a total plant biomass of around 50 tonnes. (This figure does not generally include things like tree trunks, which are not easily consumed by herbivores.)
This leaves me with my problem. Just how much is (for example) 50 tons of the easily digestable plant biomass, in terms of land area?
The answer to that will obviously depend on the specific biome, and have a large fudging factor depending on the local environment. I'm hoping for an answer that gives some kind of conversion figure (like, pulling out of thin air, "approximately one ton per square kilometer") at least for each of tundra, northern latitude forest, mediterranean forest, and jungle. Because I'm shooting for reasonable rather than an absolute truth, something that is within half an order of magnitude or so is probably good enough.
Note that this is science-based, not hard-science, but bonus points for citations.
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