Ulmus americana

American Elm is a widespread and common tree species of floodplain forest, but it is far less common than several decades ago due to depletion by an exotic fungus (Dutch elm disease). Some forests impacted by this disease regenerate by individuals not yet infected, but older trees succumb to the disease: this cycle is causing a demographic shift in populations towards younger and smaller trees. Other forests have seen steady declines of the species. The iconic large, arching elms that create a cathedral like effect in nature are quite rare. There are efforts to introduce dutch elm resistant plant material at some sites and there is evidence of some disease resistance among certain populations. The development of disease resistant material was largely driven by the loss of this species as a beloved ornamental tree. The Conservation Status of this species should be reviewed frequently to detect changes in the impact of the pest.

Ulmus americana occurs throughout the eastern U.S.A., and along riparian habitat in the Great Plains, and northward into southern Newfoundland, southern Quebec and Ontario, Manitoba and eastern Saskatchewan (FNA 1997).

The number of sites for this species is not quantified. It was once a codominant or dominant species of floodplain forests, but is still very common (Marks 2017).

The primary threat to this species is Dutch Elm Disease, a fungal pathogen that is spread by elm bark beetles. American Elm trees reach sexual maturity around the age of 15, around the same age that the risk of morality from Dutch Elm Disease also increases. Trees of this age are sometimes able to produce seeds before succumbing to the disease, even if they don't have resistant to the pathogen. Interestingly, the die off of larger trees allows physical space for more smaller trees, not yet killed off by Dutch Elm Disease. It is possible that at some sites, the number of individuals has increased but shifted towards smaller individuals. Isolated trees are less vulnerable than those in dense stands and represent some of the remaining specimens with the mature cathedral like form. Some impacts of the disease are a demographic shift in the population structure, loss of dominance in floodplain forests, and a shift in the forest structure. There are efforts to introduce dutch elm resistant plant material at some sites and there is evidence of some disease resistance among certain populations (Marks 2017).