Among biologists in general, and botanists in particular, says Mark Behan, there are "lumpers," and there are "splitters." The lumpers like to keep the science simple, and they minimize the number of species names; the splitters prefer to be more specific, and name "new" species and subspecies.
Populus tricocarpa ("people's tree with hairy fruits"), which botanists now class as Populus balsamifera (bal-sum-IF-er-a, "aromatic") is but one of three different species of cottonwood Lewis and Clark found in Montana. The other two species are angustifolia ("narrow leaf") and deltoides ("triangular," from the Greek delta), or plains cottonwood. But Mother Nature hasn't read the books, and she doesn't condone generalizations, especially when it comes to cottonwoods. She seems to favor the splitters.
Sometimes cottonwood species are hard to distinguish from one another. The leaves on the left, above, would appear to belong to Populus tricocarpa or balsamifera because they are broad, dark green, and shiny, at least compared with those on the right. Those on the right from a nearby tree, are narrower, more lance-shaped, and lighter green, which might be taken to indicate Populus angustifolia. However, the cottonwood tree's grip on life is so tenuous that Nature allows it to hybridize in order to prevail. These two instances are essentially balsamiferae, with some angustifolia in their blood. This, incidentally, illustrates one reason botanists don't rely on leaves to determine the identities of plants, but rather on fruits.
Donald Culross Peattie, A Natural History of Western Trees (Boston: Houghton Mifflin Company, 1981), pp. 324-330.
Elbert L. Little, National Audubon Society Field Guide to North American Trees: Western Region (New York: Knopf, n.d.), pp. 341-345.