Excess N deposition has aroused concerns about its negative impacts on forest ecosystems. A two-year study was conducted to assess the responses of stomatal conductance ( G c ) and carbon assimilation ( A net ) of dominant tree species ( Liquidambar formosana , Quercus acutissima and Quercus variabilis ) to increased N deposition at a canopy and understory N additions experimental platform in a temperate deciduous broadleaved forest. Five treatments included N addition of 25 and 50 kg ha −1  y −1 onto either the canopy (C25 and C50) or the understory (U25 and U50), and a control treatment (CK, without N addition). Our results showed that neither canopy nor understory N addition had an impact on carboxylation capacity ( V cmax ), the light saturated rate of electron transport ( J max ) and leaf-level net assimilation ( A nL ) of the studied tree species. Higher concentrations of N addition (U50 and C50 treatments) exerted negative impacts on G c and A net of L. formosana and Quercus acutissima under lower precipitation conditions, while lower concentrations of N addition (U25 and C25 treatments) had minimal impacts on overall ecophysiological function. The U50 treatment increased tree water use efficiency ( WUE ) of L. formosana in the second experimental year. Canopy and understory N addition generated differential effects on forest vegetation. The traditional approach with understory addition could not fully reflect the effects of increased N deposition on the canopy-associated assimilation processes.