Response of Photosynthetic Capacity to Nitrogen Addition in Trees in Different Crown Classes.

We explored the response of photosynthetic capacity to nitrogen (N) deposition among trees in different crown classes (e.g., suppressed, intermediate, and dominant trees) in a 12-year field experiment in a forest in the Greater Khingan Mountains in Northeast China. Four N-addition treatments were established: control (CK), low N (LN), medium N (MN), and high N (HN) (0, 25, 50, and 75 kg N·ha·year, respectively). Photosynthesis and its influencing factors were measured in 2023. Nitrogen addition significantly increased the maximum net photosynthetic rate (), maximum carboxylation rate (), and maximum electron transport rate () of suppressed and intermediate trees. The suppressed trees showed maximum and in MN and HN, and maximum in HN. The intermediate trees showed maximum , , and in MN. For dominant trees, was increased in LN and MN and decreased in HN, and was increased by N addition and peaked in MN. Nitrogen addition significantly increased the leaf N content (N), chlorophyll content (Chl), the ratio of N to phosphorous (N:P), and photosynthetic enzyme activities in all crown classes. N had significant nonlinear relationships with , , and . Enzyme activity and Chl positively affected the photosynthetic capacity of suppressed and intermediate trees, and N:P negatively affected the photosynthetic capacity of dominant trees. The promoting effect of N addition on photosynthetic capacity was stronger in suppressed and intermediate trees than in dominant trees. Therefore, the crown class should be considered when studying the effect of N deposition on the boreal forests.