Effect of nitrogen levels on photosynthetic parameters, morphological and chemical characters of saplings and trees in a temperate forest

Exploring the response differences of leaf physiology parameters to enhanced nitrogen deposition between saplings and trees is vital for predicting the variations of terrestrial ecosystem structure and function under future global climate change. In this study, the ecophysiological parameters of saplings and trees of Fraxinus mandshurica Rupr. were measured at different levels of nitrogen addition in a temperate forest. The results show that ecophysiological parameters maximum net photosynthetic rate (P-max), apparent quantum efficiency (alpha), dark respiration (R-d), light saturation point (L-sp), photosynthetic nitrogen use efficiency (PNUE), specific leaf area (SLA) and stomatal conductance under saturated light intensity (G(smax)) were higher in saplings than in trees. These physiological parameters and not N-leaf (leaf nitrogen content) led to relatively lower P-max and R-d in trees. For both saplings and trees, low and median nitrogen addition (23 and 46 kg ha(-1)a(-1)) resulted in significant increases in P-max, R-d, L-sp, Chl, PNUE, SLA and G(smax). These parameters tended to decline under high additions of nitrogen (69 kg ha(-1)a(-1)), whereas N-leaf was always enhanced with increasing nitrogen. Variations in P-max and R-d with increasing nitrogen were attributed to variations in the strongly related parameters of, L-sp, Chl, PNUE, SLA and G(smax). Overall, the response sensitivity of physiological parameters to enhanced nitrogen levels was lower in trees compared with saplings.