This is a tree.  It is a member of the Poplar family, possibly Eastern Cottonwood.  These are the kind of trees that come into an area after the trees that were there are gone... cut down or possibly burned.  The seeds could have been there a long time and did not grow because there was not enough light.  Poplars act as "nurse trees" and give shade to an area so more long-living trees can grow and prosper.  Poplars often have short lives and are soon gone but the other trees are now established and ready to grow larger.