Forest health deteriorated in eastern North America as a result of depletion of available soil base cations by elevated inputs of acid deposition. We experimentally restored available calcium (Ca) to soils of a forested watershed at Hubbard Brook, New Hampshire, and measured the response of fine root biomass 14 years after treatment. In this northern hardwood forest, fine root (<1 mm diameter) biomass declined significantly in response to the Ca-addition treatment relative to pretreatment and reference forest conditions. The decline was greatest in the mid- and high-elevation hardwood zones of the watershed, where soils are thinnest and have the lowest base saturation and exchangeable Ca pools. Restoration of soil Ca appears to have reduced the allocation of carbon (C) to root systems, coincident with observed increases in aboveground biomass and productivity. Therefore, we suggest that relatively higher tree C allocation to roots in the past contributed to the depressed aboveground productivity observed in northern hardwood forest ecosystems impacted by acid deposition.