MINERAL ELEMENT DYNAMICS IN THE XYLEM SAP OF 30-YEAR OLD SPRUCE

Xylem sap fluxes and mineral contents in branches and boles of a 30-yr-old spruce stand were monitored during the 1990 growing period. On average, four trees were felled fortnightly. Three portions of the bottom of each felled trunk and four branches per tree from the third and fourth whorl were sampled before sunrise. Sap extractions were carried out immediatly after felling. Bole xylem sap was displaced by the pressure of a 1-metre high column of water applied to the end of each portion sampled, whereas twig sap was extracted by applying slight air pressure (+1 MPa over potential) in a Schollander bomb. Xylem sap rise was recorded continuously in four trees with a heating probe device (fig 2). Sap water fluxes were multiplied by sap mineral contents in order to calculate the flux of mineral elements rising annually in the xylem tissues. This flux was compared to the mineral content and annual uptake of the stand, previously evaluated by a biomass and mineral content inventory (Le Goaster et al, 1991; Dambrine et al, 1991). Sap element content was found to have a systematically higher concentration in the crown compared to the bottom of the trunk (fig 3). Seasonal variations in the mineral concentrations of bole xylem sap depend on the element Ca, Mg, K and P peak during or after bud break, then decrease and remain low and steady during summer with a final increase at the end of the growing period. Si seems to increase with drought (fig 4). Seasonal variations of branch xylem sap contents show a less pronounced pattern (fig 6). Concentrations of Ca, P and K peak at the beginning of shoot or needle growth, after the peak of bole sap concentrations. Mg content does not change greatly whereas Si increases in parallel with the drought The ratio between branch and bole sap concentrations (fig 6) is highest for Ca and Mg, which are available to the trees in very limited amounts. Annual fluxes of Ca and Mg in bole xylem sap were found to be lower than (Ca) or of the same order of magnitude (Mg) as the gross uptake of the stand, assessed by a biomass and mineral content inventory (table IV). This discrepency between the Ca flux in sap and gross Ca uptake could be partly due to uncertainties in gross uptake evaluation. Conversely, fluxes of K and P in bole xylem sap are far higher than the gross uptake. Fluxes of all elements in branch xylem sap are higher (Ca, Si) or far higher (Mg, P, K) than the gross uptake. For P and K, fluxes are of the same order of magnitude as the total content of the stands crown (table V). These results strongly suggest the existence of a mobile pool of mineral elements, cycling continuously in the tree, and particularly in the crown. This is in agreement with previous investigations showing that root and branch sap concentrations were higher than those from the bole (table VI).