Water Regime and Magnesium Deficiency: Manipulative Experiments in Young Norway Spruce Stands

Two main symptoms are typical of silver fir (Abies alba Mill., hereafter shortened to “fir”) and Norway spruce (Picea abies (L.) Karst., hereafter shortened to “spruce”) decline in Europe: (a) crown thinning; and (b) yellowing of old needles, the latter being mostly related to Mg deficiency on acid soils (Roberts et al. 1989; Bonneau 1989). Crown thinning, when not associated with strong mineral deficiencies, may be caused by various abiotic and biotic factors. In the Vosges, it has been mainly related to water stresses, often increased by high stand density (Lévy and Becker 1987). In fact, both needle loss (when not derived from acute deficiencies) and reduced growth appear to be long lasting effects of the sequence of dry years which occurred during the 1960s and 1970s.