Understanding the effffect of increasing atmospheric nitrogen (N) deposition on xylem growth of trees is critical topredict tree growth and carbon sequestration under global change. Canopy N addition (CAN) is generally be lieved to realistically simulate atmospheric N deposition on terrestrial ecosystems given it takes all processes ofN deposition from forest canopy to belowground into account. However, whether CAN is more effffective inreflflecting the effffect of atmospheric N deposition on xylem growth of trees than understory N addition (UAN) hasbeen rarely reported. To address the question, we conducted a CAN vs. UAN experimental study to weeklymonitor xylem growth of two dominant broadleaf species (Quercus acutissima Carruth. and Quercus variabilisBlume) in a warm temperate forest of China during 2014–2015. Weekly xylem increment during the two yearswas measured. Mixed-effffects models were used to quantify the effffects of N addition on xylem growth and detectthe difffferences among treatments. We found that CAN of 50 kg N ha−1 yr−1 plays a more signifificant role inpromoting xylem growth of Q. acutissima than UAN of 50 kg N ha−1 yr−1, and signifificantly enhanced the for mation of difffferentiating xylem (zones of radial enlarging and wall-thickening cells) of Q. acutissima in the earlygrowing season (April-June) and the rate of xylem increment, but no signifificant difffference in xylem increment ofQ. variabilis was detected between CAN50 and UAN50. This is the fifirst study to quantitatively demonstrate thatprevious UAN studies may have underestimated the effffects of atmospheric N deposition on tree growth byignoring the N interception through forest canopy. Furthermore, our study also suggested a species-specifificresponse of xylem growth to N addition. Under a certain amount of atmospheric N deposition in the future, thexylem increment of Q. acutissima may be superior to that of Q. variabilis.