ID by Ken Owen (Channel Islands Restoration) and Steve Junak (Santa Barbara Botanic Garden)
More tree-like than shrub-like. Petioles not flattened as in P. tremuloides.
The leaves of a lot of young cottonwood trees we've observed lately are shaped like this--they seem to become more triangular as the tree increases in size.
Trinity County, California, US
Cottonwood forest and Bald Eagle perch at confluence Bonanza Ck and Slocan Lake
Old and large tree near the lake in the picnic area of the park
Picture of seeds (western redcedar in background)
Populus trichocarpa (black cottonwood; also known as western balsam poplar or California poplar) is a deciduous broadleaf tree species native to western North America. It is used for timber, and is notable as a model organism in plant biology. Its full genome sequence was published in 2006. It is the first tree species to be sequenced.