Birches like these are among the first of the larger trees to colonize a disturbed habitat during the process of natural succession. They grow very quickly and act as "nursery" trees for the saplings of later successional trees like spruce and maple. Eventually, within about 60 years, the birches shown in the photo will die and these spruces will take over. Unfortunately in many cases invasive species get a foothold first and form dense monocultures that prevent native successional trees from growing.