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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 Δ, based on leaf carbon isotope discrimination tested as a proxy of WUE). In both species, biomass production, WUEi and WUE T  increased with increasing N supply, but there was no effect of N supply on either transpiration or stomatal conductance and Δ was negatively related to leaf N concentration. Plots of Δ versus both WUE i  and WUE T  revealed negative trends, but the regression between WUE i  and Δ was significant only for Douglas-fir, and the regression between WUE T  and Δ 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.

tree physiology

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