Deserta is neuter plural, so it would have to be desertorum.
That would work, though I'm not sure how strongly the Romans would have associated deserta with sandy desert.
It may be closer to "wilderness".
> dēserta, ōrum, n., desert places, deserts, wastes
Lewis and Short say this.
I suppose that doesn't sound like wild forests or anything.
But it could be cold, northern, rocky areas.
