Ulmus americana

The American elm, known scientifically as *Ulmus americana*, is a large deciduous tree native to eastern North America. It is a remarkably hardy species capable of surviving extreme winter temperatures. These trees can live for several hundred years. They are also highly tolerant of various urban conditions. This resilience made them a favorite choice for lining streets during the 19th and early 20th centuries. Today, the American elm is the state tree of both Massachusetts and North Dakota.

The physical structure of an American elm is quite distinctive. In ideal conditions, these trees can reach significant heights. The trunk may reach a diameter at breast height of more than 1 meter. This thick trunk supports a high, spreading canopy that often resembles an umbrella. The leaves are long with double-serrate margins, meaning they have jagged edges. In the autumn, these leaves turn a bright yellow. The flowers are small, purple-brown, and apetalous, which means they lack petals. Because they are wind-pollinated, they do not need bright colors to attract insects.

Biological processes in the American elm are finely tuned to the seasons. The flowers are protogynous, meaning the female parts mature before the male parts. This timing helps reduce self-fertilization. These flowers emerge in early spring before the leaves appear. After pollination, the tree produces fruit called samaras. A samara is a flat fruit, often around 1 centimeter long, with a circular papery wing surrounding a single seed. These seeds are dispersed by the wind. Because the seeds are wind-dispersed, the species can spread rapidly into new habitats.

Historically, the American elm has a long relationship with human civilization. Carl Linnaeus first described and named the species in 1753. During the 1800s, the tree became a staple of urban landscaping. People loved the way the branches arched over streets to create leafy tunnels. However, this popularity led to a dangerous problem. Many cities planted the same types of trees, creating a monoculture. This meant the trees had very low genetic diversity. Because they were so similar, a single disease could threaten the entire population.

A massive biological crisis began in the 1920s with the arrival of Dutch elm disease, or DED. This is a fungal disease that causes catastrophic die-offs. The disease is spread by elm bark beetles, which act as vectors. There are three species of elm bark beetles in North America: the native species, the smaller European elm bark beetle, and the banded elm bark beetle. The infection is most dangerous during a 30-day period of rapid terminal growth. During this time, the tree's new springwood vessels are fully functional. If spores enter the xylem, the fungus can spread quickly through the tree.

Despite this devastation, some elms have found ways to survive. It is estimated that only about 1 in 100,000 American elms is naturally tolerant to DED. Some trees, like those in Manhattan's Central Park, survived because they were isolated in an urban setting.

Scientists have also worked to develop resistant strains. In the late 1990s, researchers developed DED-resistant elms after 20 years of study. Some triploid trees, such as the 'Jefferson' cultivar, show a high degree of resistance. There is also hope that wild trees will eventually hybridize with these resistant varieties.

The American elm plays a vital role in many different ecosystems. It is a principal member of several major forest cover types. These include forests containing black ash, silver maple, or sugarberry. The tree's leaves provide essential food for many types of Lepidoptera. This group includes various butterflies, such as the Mourning Cloak and the Painted Lady. It also supports many moth species, like the Columbian Silkmoth.

By supporting these insects, the American elm helps maintain the health of the entire food web.

Understanding the American elm requires looking at its complex genetics. For over 80 years, scientists believed the species was a tetraploid. This means it had double the usual number of chromosomes, specifically 2n = 56. However, a 2011 study by the USDA Agricultural Research Service changed this view. Researchers discovered that about 20% of wild American elms are actually diploid, or 2n = 28. These diploid trees have markedly smaller cells than the tetraploid trees. This discovery suggests that these diploid trees might even constitute a separate species. This genetic variety is a key part of why some trees continue to persist in the wild.