There is a row of White Pine that was planted several years ago as a barrier between my back yard and the street. As the trees grow taller, will the bottom branches fall off and result in an open area at the base of the trees? Will pruning the top of the trees help retain the density at the base of the trees?

Greetings Tom:

Depending upon the initial spacing, these trees may or may not "lose" foliage on their lower branches. Regardless, the tree's bottom branches simply do not "fall off" -- many conifers will only drop their photosynthetically active foliage (e.g., needles) due to low light conditions, high physiological maintenance costs, and density (i.e., light-enduced spatial competition from adjacent trees and canopy structure).

I certainly would not encourage you to "top" your white pine trees--topping is a cultural practice that is strongly discouraged!

I'm sorry I can't provide you with a better answer due to the simple fact that I am unaware of the spacing and size of these trees. If you require a more precise or immediate answer to your question, please contact a local urban forester, certified arborist, or ornamental horticulturist.

Best of luck!