Answer: There are several things to consider here. First, the mulch deteriorates as any organic material will do over time and helps feed the soil, this helps to keep the soil healthy and consequently helps keep the trees healthy. So in some ways the mulch is an investment in keeping the trees growing well.
Different mulches will break down faster than others. Using a larger particle size can slow the process, so you might look into using chunks rather than shredded bark for instance. Also, it should be no deeper than about three inches at any given time. Raking it periodically to fluff it can restore the recommended depth and reduce the amount you use. Fluffing also prevents it from compacting and actually shedding moisture, which means rain will penetrate the soil better.
You could plant groundcovers, but both of these trees are difficult to plant beneath. The willow has many fine roots throughout the soil and sucks moisture from the soil very quickly. This makes it very difficult for the groundcover to get enough moisture (and nutrients) to survive. The magnolia casts such deep shade, and drops such thick leathery leaves that it can smother a groundcover.
Some groundcovers you might consider would be liriope, English ivy, and possibly Vinca minor. Keep in mind though that these will not feed the soil the way your mulch would. They will require constant care with watering and fertilizer to help them become established, and thereafter since they are in the limited root space of a planter area with the added drying due to being in a raised bed situation. With the magnolia, you may need to rake off fallen leaves and debris to prevent smothering. I hope this helps you evaluate the situation. Good luck with your project!
Q&A Library Searching Tips