assessment of changes in the phosphorus status of forest ...

The frequency and the amount of mast production has increased in recent decades
probably due to increased N atmospheric inputs and high vitality of trees. This may
increase the amount of P required for the growth in beech forests, which may be 2-3
times the amount of P required during mast years when compared to non-mast years.
Bauhus (1994) reported for a beech stand in the Solling area that the concentration of
P in seeds (4.8 mg g -1 ) was more than 5 times that in leaves (0.9 mg g -1 ).
That phosphorus can become growth limiting during vegetation succession was
shown conclusively by Wardle et al. (2004) who studied six long-term
chronosequences, in Australia, Sweden, Alaska, Hawaii, and New Zealand. During
succession, ecosystem development occurred; but in the long-term absence of
catastrophic disturbance, a decline phase eventually would follow. The decline phase
was associated with a reduction in tree basal area and an increase in the substrate
nitrogen–to-phosphorus ratio (in the litter layer), indicating increasing phosphorus
limitation over time. These changes were often associated with reductions in litter
decomposition rates, phosphorus release from litter, and biomass and activity of
decomposer microbes.
1.4 Possible causal mechanisms of decrease in P status of trees
A number of causal mechanisms have been proposed to explain the association
between the appearance of P deficiencies in forest trees receiving high atmospheric
inputs of acids and nitrogen. Some of these are given below: (i) Soil acidification is
hypothesised to cause an increase in the inorganic P sorption capacity of the soils and
concomitant decreases in P solubility (Van Breemen et al., 1983; Pare´ and Bernier
1989b). (ii) Soil acidification may reduce the rate of organic P mineralisation
(Harrison 1982; Pare´ and Bernier 1989a). (iii) High atmospheric N inputs may
enhance the tree demand for other nutrients (especially P), potentially resulting in
nutritional imbalances (Nihlgard 1985; Binkley et al. 1989). (iv) P uptake may be
reduced due to a decrease in fine roots and mycorrhizal associations (Arnebrant and
Soderstrom 1992, Gundersen 1998) due to high acidity and high levels of N in soils.
However, the establishment of clear linkages between observed P deficiency
symptoms and any of the proposed causal mechanisms has been difficult in the abovecited,
field-based studies describing ‘natural’ variability.
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