In the USA black walnut has been planted with black locust (Robinia pseudocacia L.), 'autumn-olive' (Elaeagnus umbellata Thunb.) which we know as spreading oleaster, and common alder (Alnus glutinosa). These species were chosen for their nitrogen-fixing capabilities. They found that all nurse species increased walnut growth, but only on certain sites.
The Elaeagnus nurse promoted walnut height increases of up to 351% over non-nursed walnuts. Problems with rapid growth from the black locust necessitated severe control of nurse height (coppicing or ring-barking). The alder nurse had a high mortality after 5 years due to an allelopathic reaction with the walnut.
The planting pattern within the multi-site trials used a triangular spacing for the walnut trees. Tree nurses are planted on the opposite pattern to give the walnuts early growing room but also to provide micro-climatic shelter and to encourage vertical growth. The shrub species surround the bases of the walnut trees providing shade and therefore weed control and, with some species, a nitrogen-fixing capability.
Source: http://www.northmoortrust.co.uk/home/estate/forestry-research/species/walnut/walnut-st