These need to be removed for the overall tree health.
Poplar: including Quaking Aspen, Big toothed Aspen, Cottonwood and Bolleana Poplar. These trees should be pruned in fall after the leaves have dropped or in early spring, before the sap starts to flow (March).
If needed, a few small branches can be removed in summer after the leaves have reached full size. The time to make a long lasting effect on the form and structure of the plant is when the tree is. Your pruning cut should expose the least amount of surface area. To begin, make a cut on the bottom of the branch. Your second cut should be on top of the branch. That should remove most of the branch.
The third cut should trim the remaining part of the branch closest to the tree. The trees require little pruning maintenance to keep the crown shapely and appealing, but benefit from trimming in the early years of growth and yearly to remove dead, damaged and diseased.