Whole-tree seasonal nitrogen uptake and partitioning in adult Fagus sylvatica L. and Picea abies L. [Karst.] trees exposed to elevated ground-level ozone
Introduction
Elevated tropospheric ozone (O3) can affect stomatal conductance in leaves and is known to impair carbon (C) acquisition of trees and subsequent allocation with consequences below-ground (e.g. Matyssek and Sandermann, 2003; Karnosky et al., 2003; Andersen, 2003, Matyssek et al., 2010). This can also affect the uptake and allocation of nutrients, such as nitrogen (N) (Haberer et al., 2007; Samuelson et al., 1996, Luedemann et al., 2005, Zak et al., 2007a). Decreased concentrations of newly acquired N in leaves were found in 60-year-old European beech trees in response to elevated O3, following reduced stomatal conductance and transpiration, and possibly increased below-ground N sink strength (Kitao et al., 2009, Weigt et al., 2012a). However, N concentrations as affected by altered N- and C-allocation do not necessarily reflect N uptake and demand of the whole tree, potentially being biased through resource accumulation or dilution related to growth-mediated source-sink balances (e.g. Timmer and Stone, 1978, Timmer and Armstrong, 1987).
Tree-internal N cycling that buffers changes in soil nitrogen availability might be affected if storage pools are changed in response to stress. Effects of altered N retranslocation in response to O3, as indicated in mature trees (Samuelson et al., 1996), may become significant in ageing trees which increasingly depend on internal N cycling (e.g. Miller, 1984, Nambiar and Fife, 1991). Changing leaf C/N ratio as a consequence of O3 stress can influence litter decomposition, which in turn may alter N availability (Andersen, 2003). Most studies considering N allocation in trees in response to elevated O3 have been performed with young trees or seedlings (e.g., Wright et al., 1991, Kytöviita et al., 2001, Bielenberg et al., 2002, Luedemann et al., 2005, Kozovits et al., 2005, Yamaguchi et al., 2010). In young trees, reduced N uptake in beech – at unchanged N allocation – under elevated O3 was coupled with reduced biomass production, while young spruce trees did not show such negative effects (Luedemann et al., 2005, Kozovits et al., 2005). In contrast, to our knowledge, very few comparable studies on adult trees are existing, one focussing on fast-growing pioneer tree species (Zak et al., 2007a), and another on a northern red oak seed orchard (Samuelson et al., 1996), showing partly contrasting effects of O3 on N allocation. Very little is known about long-term effects of O3 exposure on N uptake and allocation of adult trees at established forest stands of late successional species. In view of future forest management under current climate change scenarios, knowledge about such effects on nutrient status of different tree species is, however, important. With this study we therefore aim at contributing to filling this gap.
A previous study in the same experiment, based on N concentrations, indicated reduced uptake and altered allocation of newly acquired N in adult European beech (Fagus sylvatica L.) in response to elevated O3, but no clear response in Norway spruce (Picea abies L. [Karst.]; Weigt et al., 2012a). In the present study, we quantified concentrations of total and newly acquired nitrogen of above- and belowground tree compartments under experimentally enhanced O3 stress and the biomass of the same trees. This up-scaling allowed for a whole-tree perspective on N pools and partitioning of new N, with outcome for weighting O3 stress-induced changes in N and C-allocation in an established forest stand. It was hypothesized that the whole-tree pool sizes and partitioning of new N reflect the pattern of O3-induced changes in concentrations of newly acquired N with stronger effects on beech than on spruce.
Section snippets
Study site
The study was performed on 62-yr-old European beech (Fagus sylvatica L.) and 52-yr-old Norway spruce trees (Picea abies [L.] Karst.) in a mixed forest near Freising, SE-Germany (48°25′12″N, 11°39′42″E; 485 m a.s.l.). The stand was dominated by spruce with scattered groups of beech according to common silvicultural practice in central Europe, and at a stand density of 50.77 m2 ha−1 (J. Dieler, pers. comm.). Forest management had been stopped at the research site for the last 30 years allowing
Recovery of labelled N at the whole-tree level
Sixteen months after application, approximately 72% of the Nlabelled was recovered in trees and soil. On average, 23 ± 4(SE) % was recovered in the target trees and 29 ± 3% in the soil across both species and treatments. Recovery in neighbouring trees within a distance of up to 2 m accounted for ca. 14 ± 2% (sum of up to five neighbouring trees per each target tree), showing the 15N-labelled plots to be dominated by the roots of the target trees. Additionally, about 5% of the labelled N was
Discussion
The recovery of the labelled N within the plant-soil system was comparable to other field studies ranging between 56 and 100% in the total tree-soil system and between 2 and 57% in trees (Feigenbaum et al., 1987, Buchmann et al., 1996, Weinbaum and van Kessel, 1998, Schleppi et al., 1999, Gebauer et al., 2000, Nadelhoffer et al., 2004). About 28% of the label was not recovered in the analysed stand compartments, potentially being translocated across roots and soil to outside the labelled plots
Conclusions
Assessment of N pools and flux sizes in an established forest stand demonstrated the relevance of O3 impact on N uptake and partitioning and improves our understanding on stand-level N cycling. The distinction between ten different N pools revealed changes in N and C sink dynamics, also of rather short-lived tree organs such as fine roots and mycorrhizal root tips, under the influence of O3 stress. Increased allocation of new N into roots and mycorrhizae in beech indicated higher below-ground N
Acknowledgements
This study was part of the 'SFB607 – Growth and parasite defense – Competition of resources in economic plants from forestry and agronomy', funded by the German Research Foundation DFG. We would like to thank Philip Wipfler and Gerhard Schütze (Forest Yield Science, Department Ecology and Ecosystem Management, TU München) for providing data of breast height diameter measurements of the study trees. Michael Goisser, Joseph Heckmair, Franziska Jäger, Peter Kuba, Johanna Lebherz, Lena Penzkofer,
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