Long-term changes in biomass and demography of a second-growth forest in central New York were quantified based upon re-measurement of 16 permanent plots (0.4 ha) originally established in 1935. We hypothesized that as the average age of the forest overstory reaches 125 years, biomass, structure, and composition would be approaching steady state. Aboveground live biomass (AGB) of the forest continued to increase steadily and significantly from 1985–2008, reaching 233 Mg ha⁻¹. Empirical observations closely matched predictions by the Forest Vegetation Simulator (FVS) from 1935–2008, and the FVS projected peak in AGB of 247 Mg ha⁻¹ at age 174 years. Aboveground net primary productivity averaged 772 g m⁻² yr⁻¹ and wood production was significantly higher on well-drained than more poorly-drained upland sites. Recent changes (1993–2008) in composition and structure continued long-term trends with increasing biomass of sugar maple, red maple, white ash, eastern hemlock and red oak, while beech and basswood declined, owing to effects of exotic pests. Until recently, beech was the only species exhibiting abundant regeneration in the forest, through vegetative sprouting. In the future, mortality from exotic insect pests is likely to alter trends in biomass, composition and structure of this forest and probably others in the region.