Certified GKH Gardening Expert
It really depends on the types of trees, but in most cases they will provide a breeding ground for mold. It is best to remove them, and substitute with a proper mulch or inoculate the entire area with beneficial fungi. Lepista and Stropharia have many in the genus that will grow in leaf litter, and help provide food and nutrients over the Autumn and Winter.
Here are some articles that will help: