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 Larix gmelinii 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 (P_max), maximum carboxylation rate (V_cmax), and maximum electron transport rate (J_max) of suppressed and intermediate trees. The suppressed trees showed maximum P_max and V_cmax in MN and HN, and maximum J_max in HN. The intermediate trees showed maximum P_max, V_cmax, and J_max in MN. For dominant trees, P_max was increased in LN and MN and decreased in HN, and V_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 P_max, V_cmax, and J_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.