Tinker, if you need to know the area without obstacles needed to land an helicopter, you can read the annex 14 of ICAO:
http://www.bazl.admin.ch/dokumentati..._JjKbNoKSn6A--
In any case you can consider 2D (D is the overall dimension of te helicopter that will land there) for the clearance blade tip-trees.
About the problems with the air moved by h/c (that may damage the trees), I can't help you as dipends on h/c weight.
EDIG: gramm. correction