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

We explored the response of photosynthetic capacity to nitrogen (N) deposition among <i>Larix gmelinii</i> 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^-1·year^-1, respectively). Photosynthesis and its influencing factors were measured in 2023. Nitrogen addition significantly increased the maximum net photosynthetic rate (<i>P</i>_max), maximum carboxylation rate (<i>V</i>_cmax), and maximum electron transport rate (<i>J</i>_max) of suppressed and intermediate trees. The suppressed trees showed maximum <i>P</i>_max and <i>V</i>_cmax in MN and HN, and maximum <i>J</i>_max in HN. The intermediate trees showed maximum <i>P</i>_max, <i>V</i>_cmax, and <i>J</i>_max in MN. For dominant trees, <i>P</i>_max was increased in LN and MN and decreased in HN, and <i>V</i>_cmax was increased by N addition and peaked in MN. Nitrogen addition significantly increased the leaf N content (N_mass), chlorophyll content (Chl_m), the ratio of N to phosphorous (N:P), and photosynthetic enzyme activities in all crown classes. N_mass had significant nonlinear relationships with <i>P</i>_max, <i>V</i>_cmax, and <i>J</i>_max. Enzyme activity and Chl_m 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.