ALLOCATION OF MASS IN TREES SUBJECT TO NITROGEN AND MAGNESIUM LIMITATION

It has previously been shown that lack of magnesium, potassium or mang anese depresses allocation of new mass to roots in trees, and that the se elements often occur in row amounts in declining forest stands. A m odel for tree growth and nutrient uptake was used to examine the effec t of allocation pattern of new plant mass on growth and development in soils with low nutrient concentrations. Growth parameters and empiric al allocation functions from steady-state experiments with birch (Betu la pendula Roth.) and Norway spruce (Picea abies (L.) Karst.) limited by N or Mg were used in the simulations. The simulation results show t hat plant nutrient concentration is sensitive to decreases in soil Mg concentration and is not counteracted by an increased allocation of pl ant mass to roots. A 50% reduction in the optimal soil Mg concentratio n decreased the plant Mg concentration to the threshold of survival, w hereas a 50% reduction of soil N had much less effect. Birch was more sensitive to low soil Mg concentration than Norway spruce. We conclude that, in the discussion of forest decline, it may be important to con sider the decreased allocation of mass to roots when Mg (or K or Mn) i s in short supply.