Allocation of mass in trees subject to nitrogen and magnesium limitation.

It has previously been shown that lack of magnesium, potassium or manganese depresses allocation of new mass to roots in trees, and that these elements often occur in low amounts in declining forest stands. A model for tree growth and nutrient uptake was used to examine the effect of allocation pattern of new plant mass on growth and development in soils with low nutrient concentrations. Growth parameters and empirical allocation functions from steady-state experiments with birch (Betula pendula Roth.) and Norway spruce (Picea abies (L.) Karst.) limited by N or Mg were used in the simulations. The simulation results show that plant nutrient concentration is sensitive to decreases in soil Mg concentration and is not counteracted by an increased allocation of plant mass to roots. A 50% reduction in the optimal soil Mg concentration decreased the plant Mg concentration to the threshold of survival, whereas 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 consider the decreased allocation of mass to roots when Mg (or K or Mn) is in short supply.