EFFECTS OF DROUGHT ON THE YELLOWING STATUS AND THE DYNAMICS OF MINERAL ELEMENTS IN THE XYLEM SAP OF DECLINING SPRUCE (PICEA-ABIES L)

In order to test the effects of drought on the occurrence of spruce ye llowing, in the Vosges Mountains (northeastern France), the soil of a plot of a 30-year-old spruce stand was protected from throughfall inpu t by a roof for two months during the summer. The degree of yellowing of the trees from the 'dry' and control plots was measured before and after the experiment. Sap flow and sap concentrations were measured in the control plot during the whole of the growing season, and in the ' dry' plot during the drought period. Drought brought about an increase of needle yellowing linked to a reduction in Mg uptake. A decrease in sap concentration of concentration of Ca, Mg and K occurred in the 'd ry' plot as compared with the control plot at the end of the drought p eriod. It is concluded that climatic stress could have been the trigge ring factor of spruce yellowing in the Vosges during the eighties.