Complex sentence: Dwarf mistletoe also causes major trunk swellings on red spruce, significantly reducing the value of the lumber; these swellings are not common on black spruce, perhaps due to the fact that black spruce is a much shorter-lived species.
Split into simpler sentences: Dwarf mistletoe also causes major trunk swellings on red spruce, significantly reducing the value of the lumber. These swellings are not common on black spruce, perhaps due to the fact that black spruce is a much shorter-lived species.