Have your pond liners been poked by tree roots?
I've got a EPDM liner, and I'm not going to use underlayment.
There are an avocado tree, an apple tree, an orange and a willow. They are about 4 feet away from the border of my pond. The pond is just 15 in below the ground.
I wonder if the tree roots can poke the liner? or it will just run around the liner?
Your liner will be fine with those trees. Bamboo and banana trees poke through liners, no others. But it is safer to use an underlayment. Use 15# roofing felt. It's cheap and easy.