It seems like swales are comparable to a
greenhouse in at least one way; They moderate temperatures(warmer in winter, cooler in summer)-due to surface angles, slowing wind over the surface, creating microclimates. They also moderate moisture--when really wet, they soak it up, and when it's really dry they release it. For
fruit trees and general
gardening, it will work on any surface, but you'll never know the difference unless you try it. I would suggest planting TWO groups of plants in the same area---1 with a swale or two above or below it, and 1 without the swale. Then you can decide if you want to do the whole area that way.