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

Below, these hypotheses are being discussed on the basis of a critical literature review
including data from the long-term studies. Moreover, there is very little compiled
information on the use of other plant parts (leaf or needle litter, fine roots) to assess
the P nutritional status of trees, which may be used as a better indicator. There is very
little known information about the consequences on plant growth, decomposer
activity and other ecosystem functions for various P levels in forest ecosystems. The
review will consider some of them by including relevant data from the literature.
1.1 Changes in P supply in forest soils due to acidifying inputs.
Acidifying inputs can decrease the plant phosphorus status (normally observed in the
foliage samples), which is related to acidification and Al toxicity causing a decrease
in P supply in soils or reduced P uptake capacity. A number of soil processes may be
associated to such changes in P availability and uptake through changes in vitality of
the mycorrhizal fungi. Several studies conducted in temperate forests have presented
evidence of phosphorus limitation in forest stands, where the availability of nitrogen
was high (Binkley and Hogberg 1997; Harrison et al. 1999; Corbin et al. 2003; Brown
and Courtin 2003). High N levels may result from natural or man-made atmospheric
inputs or from fertilizer additions for experimental or management reasons.
(a) Natural or man-made inputs of acidifying substances.
Nihlgard (1985) proposed the hypothesis that forests will run into nutrient imbalances
because of increased N deposition. Commonly N/P ratios in the foliage samples are
used to assess the imbalances in P status. An increase in N/P ratios indicates an
imbalance owing to low P uptake under high N input conditions. Linder (1995)
suggested that the ratios between nutrients are much more important for judging the
nutrient status than the concentrations themselves as the concentrations are influenced
by the amount of non-structural carbohydrates in the leaves. Deficiencies of P in
forest stands receiving high levels of acidifying atmospheric inputs have been
reported in North America and Europe. For example, Bernier and Brazeau (1988)
reported P deficiencies in sugar maple stands in Quebec and Mohren et al. (1986) and
Houdijk and Roelofs (1993) in forests in the Netherlands, especially in Douglas fir
stands. Harrison et al. (1999) reported that in the UK the poor canopy condition was
related to P stress in beech, Scots pine and Sitka spruce stands. Polle et al. (1992)
reported P deficiency in needles of three Norway spruce stands growing at three
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