I am having trouble understanding how planting vertically allows enough light for the plants at the bottom. For example; blackberries need lots of sun but if they are under a fruit tree which is under a larger tree how does it get enough sun?
I dont know either but I do know that some of the best wild blackberries we used to get were out of a pine forest next to the road. This also had horses run here so I guess the manure would have helped.
I do believe there are many plants that just don't need or want a lot of light. If they aren't getting enough, they won't grow well or can be spindly I would assume. I planted some blackberries this summer that are supposed to be hardy to zone 4 (I'm in Alaska on the Kenai Peninsula). I planted them among some Spruce in a rather shady area & we had a very rainy, rather cloudy summer, yet they grew new stems and seem to be doing well.
As long as blackberries can get around 2 hours of sun per day they can thrive, though berry production levels will be less than those that grow in near full sun. Blueberries are another species that does quite well in dappled shade spots, especially if the weather is very hot and humid. Raspberries are very much like blackberries on sunlight needs.