WordNet (r) 3.0 (2006):

black cottonwood
    n 1: North American poplar with large rounded scalloped leaves
         and brownish bark and wood [syn: swamp cottonwood, black
         cottonwood, downy poplar, swamp poplar, Populus
         heterophylla]
    2: cottonwood of western North America with dark green leaves
       shining above and rusty or silvery beneath [syn: black
       cottonwood, Western balsam poplar, Populus trichocarpa]