In many parts of the United States, birch trees grow in the same woodland areas as poplar trees. Telling one species from the other can be difficult for someone with little experience in distinguishing tree species. Since birch trees frequently grow to the same heights as many poplars, the task is even more problematic. You can use other features of these trees though to tell one from the other, including the sounds the leaves make in the wind, their bark and their methods of reproduction.
Remember that people frequently refer to poplar trees as cottonwoods. This nickname comes from the fact that their flowers produce a seed capsule that opens up in the late springtime and into the early weeks of summer and releases seeds attached to cottonlike fibers. These fibers will blow all around before finally settling somewhere on the ground. You will readily recognize the cottonwood from these hanging seed capsules and the light feathery seeds coming off them.
Look for what botanists call catkins on male birch trees. These will look like tassels in the spring as they pollinate the female flowers, which will in turn develop into a greenish conelike structure that will scatter the seeds later in the summer and early fall after turning brown. While the poplar trees have both male and female flowers on the same tree, the birches are monoecious, meaning the tree is either male or female.
Listen carefully when even the smallest amount of wind blows through the forest. A slight breeze will make the leaves of a poplar tree rustle, since the long stems combine with the large triangular leaves to catch the air just right. When no other leaves are making a sound up in the trees, you will hear poplar leaves making noise as they shake back and forth. Birch leaves tend to turn golden in the autumn, which can help you recognize the tree. These leaves are typically oval with a tapered end and smaller than the poplar leaves, which are rounded in most species and almost as wide as they are long.
Examine other aspects of these two tree species to tell them apart. Some birch trees such as the black birch and the yellow birch have twigs that give off a wintergreen aroma when you break them. Paper birch is an easy tree to identify, with its white bark that tears in strips from the trunk. The bark of most poplars is dark colored, hard, fissured or furrowed on the mature specimens. The most abundant poplar, the eastern cottonwood, is more massive than any birch, as this cottonwood can grow to 100 feet and has a spreading canopy. Birch trees often grow in stands together in large numbers. Poplars thrive in moist soils, so look for them along rivers and streams.