Divergent Responses of Dominant Forest Tree Species to Manipulated Canopy and Understory N Additions in Terms of Foliage Stoichiometric, Photosynthetic, and Hydraulic Traits

Nitrogen (N) deposition effects on the stoichiometric balance and photosynthetic and hydraulic couplings in subtropical forests has drawn wide attentions. The previously adopted understory application of N fertilization is criticized because it might ignore foliar N retention for different species. This paper reports a fertilizing application from the canopy (CAN) and under the canopy (UAN) in a phosphorus (P) limited ecosystem. Foliage stoichiometric, photosynthetic, and hydraulic traits of six dominant species were measured and analyzed. Both treatments equally enhanced foliage N and N/P, but not foliage P, who was highly species-specific depending on tree height, which implied enhanced P limitation. Decreased isotope abundance of N-15 (delta N-15) that approaching to the level in the urea fertilizer under CAN suggested the existence of canopy retention of N. Besides, N response sensitivity of N, P and delta N-15 that positively related to tree height (H) under CAN indicated different exposure to the added N, which promoted stoichiometric imbalance among species. The photosynthetic traits represented by net photosynthesis (A(n)) increased under both treatments. A divergent foliar photosynthetic and hydraulic traits variation was identified by significant decreased stomatal conductance (g(s)) and A(n)/g(s) for CAN treatments, which induced the elevated isotope abundance of C-13 (delta C-13). Correspondingly, foliage hydraulic traits that shifted to water use efficiency axis were identified only under CAN in principal component analysis. Overall, our results proved that the canopy absorption and species heterogeneity should be considered regarding foliar safety versus efficiency trade-off in response to nitrogen additions in the future.