Does nitrogen deposition increase forest production? The role of phosphorus
Introduction
Anthropogenic nitrogen emission from combustion and agricultural activities has led to eutrophication of forest ecosystems in large parts of the industrialized world (Nihlgård, 1985, Aber et al., 1989, Galloway, 1995). As a consequence, previously N limited ecosystems are getting saturated with nitrogen (Aber et al., 1998) and leaching nitrate (Dise and Wright, 1985, Gundersen et al., 1998). The release of N limitation stimulates growth at least initially; an increase in foliar N is expected to increase photosynthesis (Reich et al., 1995). It has therefore been postulated that nitrogen will increase carbon sink strength of forests by a substantial amount (Townsend et al., 1996, IPCC, 2001, Reich et al., 2006). A larger forest growth has indeed been observed in the second half of the last century that was attributed to higher N availability (Spiecker et al., 1996). There are, however, observations that this growth increase after N addition is not sustainable as other limitations will come into effect. The newest IPCC report acknowledges these limitations (Nabuurs et al., 2007). Aber et al. (1989) suggest that phosphorus is getting limiting to plant growth when N limitation is removed. This concept of nutrient limitation would, however, explain the absence of further growth stimulation but not a decrease below pristine situation as observed e.g. by Nellemann and Thomsen (2001) and by Boxman et al. (1998) in Norway spruce, by Magill et al. (2004) in pine and by McNulty et al. (2005) in red spruce.
A network of forest observation plots in Switzerland, with a gradient of modeled N deposition ranging from 12 to 46 kg N ha−1 yr−1, allows to analyze N effects on growth and nutrition by epidemiological methods. The mechanistic background for such statistical analyses is given by complementary N addition experiments with saplings of Fagus sylvatica and Picea abies in different regions. Both datasets will be presented here and the conclusions compared.
Section snippets
Permanent observation plots
135 permanent observation plots are maintained in different regions of Switzerland, with a wide range of altitudes, soil types, and pollution climates (Flückiger and Braun, 1998, Braun et al., 1999). Each plot consists of 60–70 beech (F. sylvatica L.) and/or Norway spruce trees (P. abies Karst.) in pure or mixed stands. Nitrogen deposition for each plot was modeled by Rihm (Meteotest) after the method described in Kurz et al. (1998).
Stem diameter was measured every 4th year at marked points at
Relation between growth and nutrition
Stem increment of Fagus and Picea was significantly related with foliar P concentrations and N deposition (Fig. 1, Table 2). In the case of stem increment of Picea, the N relations were weak, the regression with N deposition being only significant in the multivariate analysis, not in a univariate regression. This result may, however, be affected by an intercorrelation between N deposition and altitude which was more serious in the spruce dataset than in beech. Whereas stem increment was more
N status
Both the observation plots and the experimental plots have to be characterized as saturated with N. This is suggested by N leaching data which exceeds the 4–5 kg N ha−1 yr−1 set as limits by the working group on Critical Loads for Nitrogen (UNECE, 1992) in more than half of the 18 examined forest observation plots and by the high proportion of added N leaching from the experimental plots. Foliar N concentration in the forest was unrelated with N deposition in both beech and Norway spruce (data
Conclusions
The observation plots leach nitrogen in considerable amounts, suggesting saturation with N. The low P concentrations (<1 mg g−1 d.w.) in the foliage of mature forest trees, the high N:P ratios especially in beech, the relation between P and arginine concentration and the observed relationship between foliar P concentration and stem increment suggest P limitation in the forest trees. The decrease of P concentrations after N addition in the experiments indicates that N deposition may have played
Acknowledgements
The forest observation was supported by the forest agencies of the cantons of AG, BE, BL, BS, FR, SO, TG, ZG, and ZH, the N addition experiment by the Federal Office for Environment. We thank Beat Achermann, Jürg Froelicher, Paul Gruber, Heinz Kasper, Ueli Meier, Alain Morier, Walter Schwab, Hansruedi Walther, Martin Winkler, and Richard Volz for their support and their interest in our work. We thank also Christian Schindler for statistical advice, Beat Rihm for modeling N deposition, Harald
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