If you are planning on not having any plants in the pond, a little shade from trees is not a bad thing. Yes the do have roots that cold potentally puncture the liner, but a proper underlayment will help prevent that. The shade they provide helps with water clarity and quality. Lillies also shade the surface very well, but it sounds like you are planning a plantless system. The leaf litter can be an issue, but a little maint. goes a long way. Besides, if your looking to build up "muck" just let them sink to the bottom.
