The answer to your question depends on the type of tree. There are several ailments affecting trees in the Northeast. And yes, some non-native tree species, such as Norway maple, crowd out native species.

Globalization of plant and animal species has introduced many non-native pests against which plants in our hemisphere are defenseless. Perhaps the most notorious of these is Dutch elm disease, a fungal blight which originated in Asia, decimated the American and European elm populations in the 20th century, and is still wreaking havoc on American elms. Beetles carry the disease into the trees. Although the disease has killed thousands of elms, it can be treated and some trees survive. In Manhattan, many tall, old elms still stand in Central Park and along Fifth Avenue. In locations where elms have died, more disease-resistant species are often planted as replacements.

A more recently arrived pest is the emerald ash borer, first observed in Michigan in 2002. Invasive outside its native habitat, this green beetle infests all major types of American ash tree, killing them in 1-3 years. In urban areas, all ash trees are often removed once an infestation is found, to reduce borer population and the likelihood of further spread. Insecticides and biological controls are also being used to combat this virulent pest.

Another import, the Asian long-horned beetle, attacks a variety of tree species. Like the the emerald ash borer, it's believed to have arrived in wooden packing material imported from China. Discovered in the New York City in the 1980s, this beetle is a serious ongoing threat.

The hemlock woolly adelgid is a small, aphid-like insect that threatens the health and sustainability of eastern hemlock in the eastern United States. This Asian intruder was first reported in 1951 near Richmond, Virginia. By 2005, it was established in portions of 16 States from Maine to Georgia, where infestations covered about half of the range of hemlock. Impact has been most severe in some areas of Virginia, New Jersey, Pennsylvania, and Connecticut.

For information on these and other tree diseases in New York State, see NY DEC's Tree Pests and Diseases.

There are several non-native tree species, often deliberately planted for their attractive appearance, which push out native trees. Norway maples, which grow easily from seed in many soil conditions, leaf out earlier than other maples, thus depriving later-leafing trees of needed sunshine. The fast-spreading tree-of-heaven produces toxins that stop other plants from getting established.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service