Populus Sect. Aigeiros

Populus Sect. Aigeiros

Populus deltoides L.
Populus fremontii
Populus nigra L.

Populus section Aigeiros is a section of three species in the genus Populus, the poplars. Commonly known as cottonwoods, the species are native to North America, Aigeiros and western Asia. In the past, as many as six species were recognized, but recent trends have been to accept just three species, treating the others as subspecies of P. deltoides.

They are large, deciduous trees 20–45 m tall, distinguished by thick, deeply fissured bark and triangular-based to diamond-shaped leaves that are green on both sides (without the whitish wax on the undersides of balsam poplar leaves) and without any obvious balsam scent in spring. An important feature of the leaves is the petiole, which is flattened sideways so that the leaves have a particular type of movement in the wind.

Male and female flowers are in separate catkins, appearing before the leaves in spring. The seeds are borne on cottony structures that allow them to be blown long distances in the air before settling to ground.

The cottonwoods are exceptionally tolerant of flooding, erosion and flood deposits filling around the trunk.

Although each of the three cottonwood species has a different leaf pattern, they all have the same general diamond leaf shape.

Populus deltoides (eastern cottonwood) is one of the largest North American hardwood trees, although the wood is rather soft. It is a riparian zone tree. It occurs throughout the eastern United States and in extreme southern Canada. The leaves are alternate and simple, with coarsely-toothed (crenate/serrate) edges, and subcordate at the base. The leaf shape is roughly triangular, hence the species name, deltoides.

In the typical subspecies deltoides (Vermont south to northern Florida and west to about Michigan), the leaves are broad and triangular, 7–15 cm across at the base. Further west (Minnesota south to eastern Texas), the subspecies monilifera (plains cottonwood; syn. P. sargentii) has somewhat narrower leaves, 5–10 cm wide at the base. This is also the state tree of Nebraska, Wyoming, and Kansas. In West Texas, New Mexico, and Colorado, the subspecies wislizeni (Rio Grande cottonwood; syn. P. wislizeni) occurs.

Populus fremontii (Fremont cottonwood) occurs in California east to Utah and Arizona and south into northwest Mexico; it is similar to the eastern cottonwood, differing mainly in the leaves having fewer, larger serrations on the edge, and small differences in the flower and seed pod structure. Some consider fremontii to be a subspecies of Populus deltoides.

Populus nigra (black poplar), native to Europe and western Asia, is distinct in its much smaller leaves, 5–11 cm across, with a more rhombic (diamond) shape.