Interactions between leaf nitrogen status and longevity in relation to N cycling in three contrasting European forest canopies

[摘要] Seasonal and spatial variations in foliar nitrogen (N) parameters wereinvestigated 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 inDenmark, the Netherlands and Finland, respectively. The objectives were toinvestigate the distribution of N pools within the canopies of the differentforests and to relate this distribution to factors and plant strategiescontrolling leaf development throughout the seasonal course of a vegetationperiod. Leaf N pools generally showed much higher seasonal and verticalvariability in beech than in the coniferous canopies. However, also the twoconiferous tree species behaved very differently with respect to peak summercanopy N content and N re-translocation efficiency, showing thatgeneralisations on tree internal vs. ecosystem internal N cycling cannot bemade on the basis of the leaf duration alone. During phases of intensive Nturnover in spring and autumn, the NH₄⁺ concentration in beechleaves rose considerably, while fully developed green beech leaves hadrelatively low tissue NH₄⁺, similar to the steadily low levels inDouglas fir and, particularly, in Scots pine. The ratio between bulk foliarconcentrations of NH₄⁺ and H⁺, which is an indicator of theNH₃ emission potential, reflected differences in foliage Nconcentration, with beech having the highest values followed by Douglas firand Scots pine. Irrespectively of the leaf habit, i.e. deciduous versusevergreen, the majority of the canopy foliage N was retained within thetrees. This was accomplished through an effective N re-translocation(beech), higher foliage longevity (fir) or both (boreal pine forest). Incombination with data from a literature review, a general relationship ofdecreasing N re-translocation efficiency with the time needed for canopyrenewal was deduced, showing that leaves which live longer re-translocaterelatively less N during senescence. The Douglas fir stand, exposed torelatively high atmospheric N deposition, had by far the largest peak summercanopy N content and also returned the largest amount of N in foliagelitter, suggesting that higher N fertility leads to increased turnover inthe ecosystem N cycle with higher risks of losses such as leaching and gasemissions.

[学科分类] 地球化学与岩石