Gas exchange of current-year stems of Fagus sylvatica L. measured as CO₂ efflux in the dark (dark respiration rate, R_d), CO₂ efflux under saturated light (R_l) and calculated rates of gross photosynthesis (P_g), for each N treatment (T1, T2, and T3). Data are means ±1 SE, with n = 3. Letters (a, b, and c) indicate significant differences between N treatments, for each parameter (R_d, R_l and P_g).