To determine how trees of a temperate to boreal ecotone forest have responded to climate change and nitrogen deposition we investigated tree-ring growth and stable carbon isotope responses of sugar maple and white spruce at Turkey Lakes Watershed (TLW), in Ontario, Canada. Trees at TLW underwent a dramatic surge in age-adjusted tree basal area increment (ΔBAI) between 1900 and 1970 and have declined thereafter. The early ΔBAI pulse was acutely expressed in sugar maples and white spruce whereas recent growth declines were less dramatic for spruce. Such a long growth pulse is inconsistent with disturbance dynamics. Dendroclimatic analyses of ring-width indices (RWI) demonstrated positive responses to warmer temperatures. Additionally, tree-ring carbon isotopes combined with leaf-level gas exchange measurements indicated stomatal conductance has declined only for sugar maples growing on wet sites during recent decades. Therefore, warming and drought stress has not been the primary cause of growth declines. For the years 1985 to 2016 maple RWI was positively related to precipitation, nitrate concentration, or wet deposition whereas spruce showed weak or even negative relationships. Across a longer timescale, 1900 to 2016, ΔBAI was linearly related to nitrogen oxide emissions from the United States for sugar maple growing on either wet or dry sites (r2 = 0.74 and 0.41, respectively) whereas for white spruce the growth response was sigmoidal, or saturating (r2 = 0.79). Overall, evidence points toward the growth pulse at TLW being primarily caused by increasing N deposition during much of the 20th century followed by a reversal in response to the U.S. Clean Air Act and its amendments after 1970.