Abstract
Seasonal and spatial variations in foliar nitrogen (N) parameters were investigated in three European
forests with different tree species, viz. beech (Fagus sylvatica L.), Douglas fir (Pseudotsuga menziesii (Mirb.) Franco)
and Scots pine (Pinus sylvestris L.) growing in Denmark, the
Netherlands and Finland, respectively. The objectives were
to investigate the distribution of N pools within the canopies
of the different forests and to relate this distribution to factors
and plant strategies controlling leaf development throughout
the seasonal course of a vegetation period. Leaf N pools
generally showed much higher seasonal and vertical variability
in beech than in the coniferous canopies. However,
also the two coniferous tree species behaved very differently
with respect to peak summer canopy N content and N retranslocation
efficiency, showing that generalisations on tree
internal vs. ecosystem internal N cycling cannot be made on
the basis of the leaf duration alone. During phases of intensive
N turnover in spring and autumn, the NH+
4 concentration
in beech leaves rose considerably, while fully developed
green beech leaves had relatively low tissue NH+
4 , similar
to the steadily low levels in Douglas fir and, particularly, in
Scots pine. The ratio between bulk foliar concentrations of
NH+
4 and H+, which is an indicator of the NH3 emission potential,
reflected differences in foliage N concentration, with
beech having the highest values followed by Douglas fir and
Scots pine. Irrespectively of the leaf habit, i.e. deciduous versus
evergreen, the majority of the canopy foliage N was retained
within the trees. This was accomplished through an
effective N re-translocation (beech), higher foliage longevity
(fir) or both (boreal pine forest). In combination with data
from a literature review, a general relationship of decreasing
N re-translocation efficiency with the time needed for canopy
renewal was deduced, showing that leaves which live longer
re-translocate relatively less N during senescence. The Douglas
fir stand, exposed to relatively high atmospheric N deposition,
had by far the largest peak summer canopy N content
and also returned the largest amount of N in foliage litter,
suggesting that higher N fertility leads to increased turnover in the ecosystem N cycle with higher risks of losses such as leaching and gas emissions.
