Mean canopy stomatal conductance responses to water and nutrient availabilities in Picea abies and Pinus taeda

We compared sap-flux-scaled, mean, canopy stomatal conductance (Gs) between Picea abies (L.) Karst. in Sweden and Pinus taeda (L.) in North Carolina, both growing on nutritionally poor soils. Stomatal conductance of Picea abi es was approximately half that of Pinus taeda and the sensitivity of G(s) i n Picea abies to vapor pressure deficit (D) was lower than in Pinus taeda. Optimal fertilization increased leaf area index (L) two- and threefold in P inus taeda and Picea abies, respectively, regardless of whether irrigation was increased. Although it increased L, fertilization did not increase G(s) in Picea abies unless irrigation was also provided. In Pinus taeda growing on coarse, sandy soils, the doubling of L in response to fertilization red uced G(s) sharply unless irrigation was also provided. The reduction in G(s ) with fertilization in the absence of irrigation resulted from the product ion of fine roots with low saturated hydraulic conductivity. When Pinus tae da received both fertilization and irrigation, the increase in L was accomp anied by a large increase in G(s). In Pinus taeda, a reference G(s) (define d as G(s) at D = 1 kPa; G(SR)) decreased in all treatments with decreasing volumetric soil water content (theta). In Picea abies, theta varied little within a treatment, but overall, G(SR) declined with theta, reaching lowest values when drought was imposed by the interception of precipitation. Desp ite the large difference in G(s) both between Picea abies and Pinus taeda a nd among treatments, stem growth was related to absorbed radiation, and ste m growth response to treatment reflected mostly the changes in L.