Sugar maple (Acer saccharum) is the most dominant and widely distributed tree species of the northern hardwood forests of the northeastern US and southeastern Canada. Recent studies have shown that sugar maple is also a unique and critical species with regard to nitrogen cycling in forest ecosystems, because forest stands dominated by sugar maple tend to have high rates of nitrification and nitrate leaching to surface waters. In some areas, sugar maple populations may be increasing due to reduction of one of their main competitors, American beech (Fagus grandifolia). However, several factors threaten populations of sugar maple in the near future, including acid deposition, climate change, and the introduction of a new insect pest. Changes in the abundance of sugar maple could lead to major alterations in nitrogen retention by forested watersheds in eastern North America.