Increasing atmospheric nitrogen (N) deposition could profoundly impact structure and functioning of forest ecosystems. Therefore, we conducted a two-year (2014-2015) experiment to assess the responses of tree sap flux density (J(s)) and intrinsic water use efficiency (WUEi) of dominant tree species (Liquidambar formosana, Quercus acutissima and Quercus variabilis) to increased N deposition at a manipulative experiment with canopy and understory N addition in a deciduous broadleaved forest. Five treatments were administered including N addition of 25 kg ha(-1) year(-1) and 50 kg ha(-1) year(-1) onto canopy (C25 and C50) and understory (1125 and 1150), and control treatment (CK, without N addition). Our results showed neither canopy nor understory N addition had an impact on leaf N content and C:N ratio (P> 0.05). Due to the distinct influencing ways, canopy and understory N addition generated different impacts on j s and WUEi of the dominant tree species. Canopy N addition increased WUE e of Q. variabilis, whereas understory addition treatment had a minimal impact on WUEi. Both N additions did not exert impacts on WLEi of L. formosana and Q. acutissima. Canopy N addition exerted negative impacts on J(s) and its sensitivity to micrometeorological factors of Q. acutissima and Q. variabilis in 2014, while understory addition showed no effect. Neither canopy nor understory N addition had an influence J(s) of L. formosana in 2014. Probably owing to the increased soil acidification as the experiment proceeded, J(s )of L formosana and Q variabilis was decreased by understory N addition while canopy addition had a minimal effect in 2015. Thus, the traditional understory addition approach could not fully reflect the effects of increased N deposition on the canopy-associated transpiration process indicated by the different responses J(s) and WUEi to canopy and understory N addition, and exaggerated its influences induced by the variation of soil chemical properties.

Canopy nitrogen addition; Understory nitrogen addition; Sap flux; Water use efficiency; Nitrogen content; C:N ratio

South China Botanical Garden, Chinese Academy of Sciences