i - Tree: a peer-reviewed software suite run through the USDA Forest Service that provides urban forestry analysis and benefits assessment tools.
Science.gov: Use this as a gateway to science research provided by the U.S. government.
United States EPA: The Environmental Protection Agency maintains an enormous amount of information on everything from ecosystems to laws and regulations. This site is extremely easy to use.
USDA: U.S. Forest Service: This agency manages the 155 national forests and the 20 national grasslands in the United States. There are three major divisions including National Forest System, State and Private Forestry and the Research and Development branch (which are accessible on the very bottom on the webpage)