Dynamics of “acute yellowing” in spruce connected with Mg deficiency

Abstract

The development of “acute yellowing” and spontaneous regreening in spruce was followed by visual observations and by pigment, mineral and soil analyses. Studies of the population dynamics of “acute yellowing” in mapped natural regenerations of spruce (2000 trees) revealed a small-scale spatial and temporal mosaic of yellowing and spontaneously regreening trees, which was reflected in the Mg content of the needles, but not in changes in pH and in the amount of ammonia-exchangeable Mg in soil samples taken from the rooting areas of diseased and of healthy trees. These findings are not compatible with the assumption of cumulative damage by air pollutants and their depositions, which should lead to an irreversible and much more uniform decline within the stands. The findings rather suggest that “acute yellowing” is a syndrome of complex etiology. In extreme cases, it may be caused solely by too little plant-available Mg in the soil, or by severely restricted Mg uptake resulting from root diseases or antagonists of the rhizosphere flora or mycorrhizae. In most cases, probably both determinants are involved. Magnesium fertilizing will be beneficial in any case since it abolishes the soil-born Mg deficiency and compensates the increasing of the threshold for Mg uptake by a biotic factor.