Aims Temperate forest is one of the most important components of the global forests and main carbon pools. Nitrogen (N) is considered as the limiting nutrient for the forest growth. However, the heterogeneities in plant species and stem sizes were largely ignored in previous researches on the effects of N addition on plant growth. Quercus wutaishanica is one of the most common and dominant tree species in the temperate forests in North China. In this study, we investigated the responses of growth of trees and forests to N addition in the Quercus wutaishanica forests in Mt. Dongling in Beijing. Methods We conducted a 7-year N fertilization experiment in Quercus wutaishanica forests in Mt. Dongling, Beijing, since 2011. The N addition was conducted at three treatment levels, i.e., 0 kg hm −2 •a −1 , 50 kg hm −2 •a −1 and 100 kg hm −2 •a −1. Nitrogen was added at the beginning of each month from May to October each year. We used electronic vernier caliper to measure tree growth rate for each year. All trees were divided into three groups based on their diameter at breast height (DBH), namely small trees (DBH = 3-10 cm), median trees (DBH =10-20 cm) and large trees (DBH > 20 cm). Particularly, we considered growth at species level for all Quercus wutaishanica and the growth at community level for all tree species in the stands. Important findings (1) At species level, N addition enhanced the growth rate of Q. wutaishanica. (2) At community level, the growth rate showed no difference among different N addition treatments. (3) Small trees were restrained, while median and large Q. wutaishanica trees were not significantly influenced, by the N addition.
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