title: Variation in nitrogen supply changes water-use efficiency of Pseudotsuga menziesii and Populus x euroamericana; a comparison of three approaches to determine water-use efficiency.
abstract: We studied the effects of three nitrogen (N) supply rates (low, intermediate and high) on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings and poplar clone "I-214" (Populus x euroamericana (Dole) Guinier) cuttings growing in mini-stands. Our specific objectives were to: (1) evaluate the effects of N supply on water-use efficiency (WUE) and biomass production; (2) determine if N affects WUE through control of carbon assimilation rates or through stomatal control of water loss; and (3) compare three methods of estimating WUE: one short-term method (WUE(i), based on gas exchange measurements) and two long-term methods (WUE(T), based on the ratio between biomass production and transpired water, and Delta, based on leaf carbon isotope discrimination tested as a proxy of WUE). In both species, biomass production, WUE(i) and WUE(T) increased with increasing N supply, but there was no effect of N supply on either transpiration or stomatal conductance and Delta was negatively related to leaf N concentration. Plots of Delta versus both WUE(i) and WUE(T) revealed negative trends, but the regression between WUE(i) and Delta was significant only for Douglas-fir, and the regression between WUE(T) and Delta was significant only for poplar. Thus, the mechanisms underlying the response of WUE to N supply were mainly related to a positive effect of N supply on photosynthetic rates. The data confirm that carbon isotope discrimination may be a useful proxy of WUE. The finding that N availability enhances both biomass production and WUE may have practical implications in regions where these factors impose constraints on forest productivity.
authors.affiliation: Dipartimento di Produzione Vegetale, Università della Basilicata, 85100 Potenza, Italy. rf464agr@unibas.it
authors.forename: Francesco
authors.initials: F
authors.lastname: Ripullone
authors.forename: Marco
authors.initials: M
authors.lastname: Lauteri
authors.forename: Giacomo
authors.initials: G
authors.lastname: Grassi
authors.forename: Mariana
authors.initials: M
authors.lastname: Amato
authors.forename: Marco
authors.initials: M
authors.lastname: Borghetti
mesh_terms: D018533:Biomass; D009584:Nitrogen / Q000502:physiology; D018515:Plant Leaves / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology; D032107:Populus / Q000502:physiology*; D028224:Pseudotsuga / Q000502:physiology*; D014197:Trees / Q000502:physiology*; D014867:Water / Q000502:physiology
chemical_list: D014867:Water; D009584:Nitrogen