We've been trialling a new polyculture for a few years experimenting with how much mulch we can grow within the beds using a fast growing, nitrogen fixing, biomass producing tree Paulownia tomentosa.
Paulownia tomentosa are planted in the middle of a 1.2 m wide raised bed to provide biomass for mulch and shade during the hottest periods of summer. These are nitrogen fixing plants and fairly deep rooted and do not compete too much with the crops for water.
As you can see below the Paulownia tomentosa (Foxglove Tree) are currently providing shade to our cooler climate crops such as Kohlrabi , Parsley and Broccoli and have provided a nice amount of mulch so far.
We'll coppice them next spring and use the stems for tomato stakes. They will probably put out about 3 or 4 new shoots and we'll thin these down to one stem to provide shade for the next crops.
http://www.balkep.org/
http://balkanecologyproject.blogspot.bg/