Trees that can provide dappled shade for the growing plants.

I'm not a farmer but most vegetables seem to have a life-span of only a few months.

I've heard about alley-cropping, a farming technique in which trees are planted in rows to provide shade for crops during hot dry seasons in parts of Africa. The trees used belong to the legume family so in addition to shading the crops, they add fix nitrogen in the soil. The extra branches are pruned back when it rains.

I'm thinking the two ideas could maybe be blended to create vegetable forests?