Princeton's WordNet

1. swamp cottonwood, black cottonwood, downy poplar, swamp poplar, Populus heterophylla(noun)

   North American poplar with large rounded scalloped leaves and brownish bark and wood

2. black cottonwood, Western balsam poplar, Populus trichocarpa(noun)

   cottonwood of western North America with dark green leaves shining above and rusty or silvery beneath