Magnesium deficiency in spruce. With insufficient Mg available in the soil the young needles obtain their Mg by re-translocation from the old needles via the phloem, and so the old needles become golden yellow at the time of bud break of new growth in spring. This process is exacerbated by an excess of nitrogen, especially when nitrogen is absorbed into the above-ground biomass directly from the atmosphere, bypassing the roots and hence not being regulated. The above-ground N uptake occurs from gases (NOx, NH3) via the stomata, or as ions (NHj, NOi) via the young bark or needles. The exacerbation of Mg deficiency by nitrogen input is because N has a much larger influence on growth than does Mg. However, the amount of Mg required for growth is larger than the Mg availability, and therefore an excess of N results in Mg deficiency. This phenomenon became well known in the 1980s as the "mountain yellowing" syndrome of forest decline. Fichtelgebirge, Upper Warmensteinach, Germany. Photo E.-D. Schulze
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