Forest gardens have been a source of food security in the tropics for at least 10,000 years: while the Neolithic Revolution fostered a turn towards grain-based agricultural subsistence in some parts of the world, in others, agroforestry-style land management flourished.
While forest gardening is a form of land use that is relatively new to the temperate zone, it is certainly possible here: it just has to be tweaked a bit to suit the bioregion in which is it being attempted, introduce species from a wider geographic range, install more infrastructure (greenhouses, etc.), and account for both shorter growing seasons and lower temperatures.
I firmly believe – and this is from experience – that one does not need to move to Costa Rica to make this sort of cultivation a reality, and I would like to spend more time showing the colder side of permacultural design. I started my first edible landscape project when I was 19, and living in zone 3 Edmonton, Alberta, Canada; this was years before I ever heard the word “permaculture.”
I would love to see more temperate-zone forest gardens. If you know of any projects taking place in more marginal climates, please let me know so I can share them! I am particularly interested in USDA zone 8 and lower.