What happens to pine trees?

Weeds don't harm pine trees. Your tree has lost it's needles on the lower branches because that's what pines do. Have you ever seen a large pine tree in the woods that had lower branches? It has to do with the sun or lack of it. The tree shades those branches--the sun can't get to them so the tree loses the needles on those branches in favor of the ones that do get the sun. Trim off those branches for a neater look or leave them but they will never have needles again