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 abies was approximately half that of Pinus taeda and the sensitivity of G(s) in Picea abies to vapor pressure deficit (D) was lower than in Pinus taeda. Optimal fertilization increased leaf area index (L) two- and threefold in Pinus 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 reduced G(s) sharply unless irrigation was also provided. The reduction in G(s) with fertilization in the absence of irrigation resulted from the production of fine roots with low saturated hydraulic conductivity. When Pinus taeda received both fertilization and irrigation, the increase in L was accompanied by a large increase in G(s). In Pinus taeda, a reference G(s) (defined 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. Despite the large difference in G(s) both between Picea abies and Pinus taeda and among treatments, stem growth was related to absorbed radiation, and stem growth response to treatment reflected mostly the changes in L.