Growing from stump.
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.
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.