Effects of nutrition and soil water availability on water use in a Norway spruce stand

We investigated effects of nutrition and soil water availability on sap flux density, transpiration per unit leaf area (E-L), and canopy stomatal conductance (G(s)) of Norway spruce (Picea abies L. (Karst.)) in northern Sweden during the 1996 growing season. Our objectives were to determine (1) if artificially imposed drought (65% rain diversion) reduces soil water sufficiently to cause physiological limitations to whole-tree and plot-scale water transport, and (2) whether increased capacity for water transport resulting from fertilization-induced increases in leaf (> 3-fold) and sapwood areas (> 2.3-fold) deplete soil water sufficiently to cause a negative feedback on G(S) and E-L. We monitored soil water content (theta) and soil water potential (Psi (S)) in control (C), drought (D), fertilized (F) and irrigated + fertilized (IL) treatment plots, along with site meteorological conditions. Ten trees per plot were monitored for sap flow. Although there were significant treatment differences in mean daily E-L(C > D > F; P < 0.01)and G(S) (C > D > F; P < 0.05), variation in absolute magnitudes was small. Therefore, transpiration differences on a unit ground area basis (E-c) were nearly proportional to leaf area differences. Precipitation was well distributed throughout the study period and so Psi (S) remained high, except during short dry periods in Plot F when it declined rapidly. Thus, although soil water was not limiting to G(S), E-L or E-C when precipitation was uniformly distributed throughout the growing season, we cannot conclude that water availability would not limit G(S) in fertilized stands if the seasonal distribution of precipitation were altered.