In recent decades, high-altitude balsam fir (Abies balsamea) forests of northern New England and adjacent areas have been fragmented by canopy openings associated with several stand disturbances. To document the causes of extensive tree dieback and mortality and the source of canopy gaps, we studied the tree growth patterns of live balsam fir from five stands in four sites in the northeastern United States (NUS): Whiteface Mtn. in New York state and Mt. Moosilauke (two stands), Mt. Blue, and Mt. Lafayette in New Hampshire. Additionally, tree-ring patterns of dead fir were documented at two sites, Whiteface Mtn. and Mt. Mégantic (southern Québec). Organic horizons from four of the five NUS stands were analyzed for identification of insect remains and plant macrofossils in 11 and seven soil cores, respectively. Four of the five balsam fir tree-ring chronologies from the NUS stands showed a major growth decline in the 1970s, which was also apparent in the two chronologies from dead trees at Whiteface Mtn. and Mt. Mégantic. At the NUS stands, spruce budworm (Choristoneura fumiferana) head capsules were found in all 11 soil cores from forest and open stands. In spite of large variations in the number of head capsules among levels (range: 1–34; mean: 9.5 ± 8.8 for 71 levels analyzed), their occurrence at all the levels analyzed indicates that the insect had sustained activity through time. Macrofossil remains from the organic horizons included plant assemblages similar to the aboveground forest vegetation. Rubus idaeus, an intolerant species, indicated that gap conditions prevailed sometime in the past. Our tree-ring and macrofossil data along with insect surveys from northern New England provide evidence for the major role played by spruce budworm in tree dieback and mortality during infestations and subsequent gap formation in high-altitude fir forests. The impact of other stand disturbance factors is discussed.