Tree of Heaven is extremely difficult to eradicate, and has spread throughout Boulder over the last few years.

https://arapahoe.extension.colostate.edu/2021/09/05/tree-fro...

America's favorite (/s) the Bradford Pear. It is invasive and will choke/push out other species.

https://extension.umd.edu/resource/bradford-pear

The North American Black Locust tree, a continental native tree, of the legume family, is considered invasive within North America, in New York and Connecticut and Massachusetts and the rest of New England, mid-west prairie areas, and the west of the continent.

Its original range, before European arrival, is believed to be in Appalachian Mountains, Pennsylvania to Georgia, and the Ozarks.

  It has been used as a pioneer species to restore treeless land in other continents, including Europe, Asia and Africa and Australia. Spain has used them to restart forest in desertified areas, for example.

- Black Locust (Robinia_pseudoacacia) -- Wikipedia https://en.wikipedia.org/wiki/Robinia_pseudoacacia

- Black Locust -- New York PRISM https://www.wnyprism.org/invasive_species/black-locust/

- Black Locust -- Massachusetts Audubon Society https://www.massaudubon.org/learn/nature-wildlife/invasive-p...

- Rethinking Black Locust -- by Maureen Sundberg April 15, 2019 Ecological Lanscape Alliance https://www.ecolandscaping.org/04/landscape-challenges/invas...

Buckthorn in the US is very resilient, crowds out other species of both undergrowth and trees, and is a host to a number of pests such as aphids and fungi which will then attack crops and native species.

You can't simply cut it down, it requires a nasty chemical applied to the stump, or to be completely pulled out by the roots. They grow tightly packed together, so clearing out even a small area is best done with a team of people.

it’s not a tree but kudzu is a pretty famous example. it was imported from japan (i believe —may be mistaken on that point) into the American Deep South for erosion control, it quickly grew in the warm, wet environs and now literally smothers nearly all native vegetation. Its incredibly invasive and difficult to eradicate.

Amur Honeysuckle, though its sort of more of a shrub.