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

mostly during periods of active growth, making it important to chose the sampling
period carefully for collecting needles for nutrient assessment. There are two common
hypotheses pertaining to the fraction and amount of nutrients re-absorbed from leaves.
(a) The first one considers that the amount of nutrient resorption increases as an
inverse function of soil nutrient availability (Grime 1979; Chapin 1980; Berendse
& Aerts 1987). Nutrient resorption from senescing leaves is therefore expected to
be more highly developed on sites of low fertility.
(b) The second hypothesis considers that resorption is influenced by sink strength
within the plant (Nambiar & Fife 1991) so that resorption efficiency is positively
correlated with plant growth rates, because the production of new tissues
constitutes a strong nutrient sink. However sink strength is unlikely to explain the
strong resorption shown by many deciduous trees in autumn where carbohydrate
flux from leaves (i.e., source-sink interactions) may govern the retranslocation
rather than the hydrolysis of nutrient-containing fractions.
Interactions with other nutrients and the use of P/nutrient ratios
Interactions among various nutrients should be considered both in the plant and in the
soil. It has been shown (Sterner and Elser 2002) that stoichiometry of major elements
(C:N:P) may be a major ecological driver from gene to biospheres. Constancy in the
N:S ratios has also been reported in many ecosystem components. Of interest for this
review is the N:P ratio in the leaves, litter layer and soils. A high N/P ratio in leaves is
commonly interpreted as P limitation to plants. For litter layers, the N:P ratio above
which P becomes limiting for biological processes relative to N is considered to be
16:1 (Redfield 1958) which may lead to reductions in litter decomposition rates,
phosphorus release from litter, and activity of decomposer microbes. Wardle et al.
(2004) used this ratio on six long-term chronosequences to report that a decline of
natural forests, which is often observed in the long-term absence of catastrophic
disturbance, may arise through increasing limitation by P and reduced performance of
the decomposer subsystem. Leuschner et al. (2006) studied properties of soils under
50 beech stands and observed that N/P ratios in the litter layer increased with an
increase in soil acidity which they ascribed to the possibility of P deficiency in these
stands.
27