Agroforestry Research Trust
Martin Crawford has grown a food forest - or wood (as it's not a traditional English forest) on a 2 acre site in Dartington, (Devon, England), since 1992. This is more of an experimental and research site, rather than being optimally productive.
His book is entitled 'Creating a Forest Garden. (working with nature to grow edible crops).
Quotes: (copyright for educational purposes).
"A forest garden is a garden modelled on the structure of young natural woodland, utilising plants of direct and indirect benefit to people – often edible plants. It may contain large trees, small trees, shrubs, herbaceous perennials, annuals, root crops and climbers, all planted in such a way as to maximise positive interactions and minimise negative interactions, with fertility maintained largely or wholly by the plants themselves."
I have visited food forests on sites that have been less than an acre or even less than half an acre. I have seen very open food forests with only a couple of trees, to very tree heavy ones. Part of this is dependant on light levels. Here in England, light is poor so any canopy, however thin, can create dense shade under which nothing will grow. (esp if you are on the N. side of a hill. You may see no winter sun.