Then the mountain biome I'm working with must not support any type of plant whatsoever. I don't believe it's the temperature, the huge white sand areas in the northwest and the grassy areas of the map (with a few spots of red sand) will grow any outdoor plant, but the muddied siltstone rock will not grow any type of plant. It says no seeds available. I've determined that a roof doesn't help anything grow in that area also.
In worlds where I've been able to acquire sun berries from the elven traders, I've never had trouble planting sun berries, and I'm currently planting whip vines (from elven traders) which according to the raws come from Savage, Dry areas. I can still plant them in any naturally-occuring (outside) soil on my map, though I live in Calm surroundings and based on the brook and the multiple swamps I don't think it's Dry. It's neither Savage nor Dry therefore.
On the off chance that the mountain is a freezing biome its surroundings to the southeast and the northwest are not, then that would explain the difference too. Certainly all the plants as far as a cursory check shows are set 'not freezing', so indeed nothing would grow in a freezing biome if that's to be believed. But my point remains that Whip Vines say they appear in Savage, Dry biomes and yet I'm able to plant them in Calm, seemingly lush surroundings.
Therefore, most maps with natural soil should be able to grow sun berries, assuming that the rules are consistent.