If you want to plant peas to benefit the trees, plant Jack Beans. They are very strong plants and will make a lot of plant mass and get a lot nitrogen. Maybe there is not enougth sun, so Arachis repens is a possibility too. If you want beans to eat them, plant Cajanus cajan in other places, they live for some years and make a good amount of beans. In a small garden, climbers beans don't make too much food in my experience. In the other hands, I dont't know if this plants are good in your climate.
