Abstract
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 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 NH4+ concentration in beech leaves rose considerably, while fully developed green beech leaves had relatively low tissue NH4+, similar to the steadily low levels in Douglas fir and, particularly, in Scots pine. The ratio between bulk foliar concentrations of NH4+ 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.
Highlights
Solid EarthForests cover 44 % of the land area in Europe (FAO, 2006).Nitrogen (N) fluxes and turnover in forests are relevant for biogeochemical cycling of N in Europe (Sutton et al, 2011). adapted to NFolriemstitaetciooTsnyhrsateethmeCsr tarhryaenogtesonpNerhaelxelcyrecesosn(VsiidteoruesdektoanbdeHowarth, 1991; Rennenberg et al, 1998; Aerts, 1996; Xia and Wan, 2008)
Thereafter, the N concentrations remained constant until the onset of senescence, when N became remobilised from the leaves to perennial woody parts, leading to a rapid decrease in both the N concentrations and chlorophyll concentrations (Figs. 3-A1 and 2-A1; see Bauer et al, 1997; Millard and Grelet, 2010)
Our results show that the investigated deciduous forest supplied the ecosystem internal N cycle with only 1.8 g N m−2 yr−1 through leaf litter production compared to 5.8 g N m−2 yr−1 in the temperate evergreen Douglas fir forest (Table 2), pointing to strong differences between trees with different leaf habit
Summary
Solid EarthForests cover 44 % of the land area in Europe (FAO, 2006).Nitrogen (N) fluxes and turnover in forests are relevant for biogeochemical cycling of N in Europe (Sutton et al, 2011). adapted to NFolriemstitaetciooTsnyhrsateethmeCsr tarhryaenogtesonpNerhaelxelcyrecesosn(VsiidteoruesdektoanbdeHowarth, 1991; Rennenberg et al, 1998; Aerts, 1996; Xia and Wan, 2008). Due to deposition of atmospheric NH3, the critical N load for European forest ecosystems is often exceeded (Cape et al, 2009; Pinho et al, 2012) This may in turn lead to elevated concentrations of NH+4 and organic N compounds in the trees and to NH3 emission events (Rennenberg et al, 1998; Fowler et al, 2009). Wang et al.: Interactions between leaf nitrogen status and longevity drivers controlling the direction and magnitude of NH3 exchange between trees and the atmosphere are not well established For this reason, more information about seasonal changes in plant N status and turnover is required to improve modelling of NH3 exchange over forest ecosystems (Massad et al, 2008, 2010)
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