Interactions between leaf nitrogen status and longevity in relation to N cycling in three contrasting European forest canopies
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 re-translocation 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 NH 3 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.