Sue Says
Directly underplanting fruit canes is likely to cause disruption to their roots and rob them of water and nutrition. Planting directly beneath the fruit bushes will have similar consequences. The main difficulty with this type of planting is getting the balance right. Forest gardening using trees is effective because they have a higher canopy which allows a shaded understorey to be grown beneath. Given the low canopies of your fruit plants, anything that you grow beneath them is likely to end up in a struggle for light and space and the results may be disappointing.
However, if it is possible to plant a little further from their immediate root zone (at least 30 45cm) then I would suggest that you grow some low growing annuals or some salad crops. Quick growing plants that are removed at the end of the season will allow you to use this space temporarily while the fruit bushes are still small. Just dont let them get overcrowded.