The Clean Air Act is a comprehensive federal law that regulates air emissions from stationary and mobile sources. It authorizes the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) to protect public health and public welfare and to regulate emissions of hazardous air pollutants. If a neighbor's vegetation, such as a large number of trees, were to significantly affect air quality on your property, it might be possible to seek relief under the provisions of this act, although such cases are rare and typically involve industrial rather than residential concerns.
The Endangered Species Act provides for the conservation of species that are endangered or threatened throughout all or a significant portion of their range, and the conservation of the ecosystems on which they depend. If a neighbor's tree or vegetation is considered critical habitat for an endangered species, it may be protected under this act. This could limit your ability to trim or remove the tree, even if it extends onto your property.
The Americans with Disabilities Act prohibits discrimination against individuals with disabilities in all areas of public life, including jobs, schools, transportation, and all public and private places that are open to the general public. If a neighbor's trees or shrubs create a barrier or hazard that interferes with the mobility or access of a person with a disability, the ADA may offer a legal avenue for requiring the neighbor to remedy the situation.
The Migratory Bird Treaty Act makes it unlawful to take, possess, import, export, transport, sell, purchase, barter, or offer for sale, purchase, or barter, any migratory bird, or the parts, nests, or eggs of such a bird except under the terms of a valid permit issued pursuant to Federal regulations. If a neighbor's tree is a nesting site for migratory birds, it may be protected under this act, which could affect your rights to trim or remove the tree.