Trees will cause leaks. I had all the trees, including several trees up to probably 16 inches diameter taken off my first pond's dam in August 2015 when it was renovated. The guy must have failed to get all the roots out, because it leaked prior to renovation, and has leaked ever since.

Cedars might be OK, if the dam is tall enough to support the shallow root system above the water line, but cedars are messy, shedding branches and needles. Plus, they make fishing difficult.