Your idea of using thorny branches as a protective layer for seeds is interesting! For your area, multiflora rose is a great choice, but you could also consider using blackberry or hawthorn. These plants have dense thorns that can discourage rodents, and their roots won't compete much with tree roots. Raspberry canes, as you mentioned, would also work well because they have fine thorns and grow densely.
It's important to ensure that the thorns don’t obstruct root growth, so be mindful of the thickness of the layers. I hope your experiment goes well!
