How closely does stem growth of adult beech (Fagus sylvatica) relate to net carbon gain under experimentally enhanced ozone stress? Mitsutoshi Kitao a, * , J. Barbro Winkler b , Markus Löw c , Angela J. Nunn d , Daniel Kuptz d , Karl-Heinz Häberle d , Ilja M. Reiter d,1 , Rainer Matyssek d a Department of Plant Ecology, Forestry and Forest Products Research Institute, Tsukuba 305-8687, Japan b Helmholtz Zentrum München, Institute of Biochemical Plant Pathology, Research Unit Environmental Simulation, Ingolstaedter Landstr.1, 85764 Neuherberg, Germany c School of Land and Environment, Department of Forest and Ecosystem Science, University of Melbourne, Water St Creswick, Victoria 3363, Australia d Ecophysiology of Plants, Technische Universität München, Am Hochanger 13, 85354 Freising, Germany article info Article history: Received 19 December 2011 Received in revised form 4 March 2012 Accepted 6 March 2012 Keywords: Adult forest trees Long-term free-air O 3 fumigation Fagus sylvatica Light gradient Seasonal carbon balance abstract The hypothesis was tested that O 3 -induced changes in leaf-level photosynthetic parameters have the capacity of limiting the seasonal photosynthetic carbon gain of adult beech trees. To this end, canopy- level photosynthetic carbon gain and respiratory carbon loss were assessed in European beech (Fagus sylvatica) by using a physiologically based model, integrating environmental and photosynthetic parameters. The latter were derived from leaves at various canopy positions under the ambient O 3 regime, as prevailing at the forest site (control), or under an experimental twice-ambient O 3 regime (elevated O 3 ), as released through a free-air canopy O 3 fumigation system. Gross carbon gain at the canopy-level declined by 1.7%, while respiratory carbon loss increased by 4.6% under elevated O 3 . As this outcome only partly accounts for the decline in stem growth, O 3 -induced changes in allocation are referred to and discussed as crucial in quantitatively linking carbon gain with stem growth. Ó 2012 Elsevier Ltd. All rights reserved. 1 Introduction Tropospheric ozone (O 3 ) levels have increased globally since pre-industrial times (IPCC, 2001; Stockwell et al., 1997), and are predicted to stay high or even continue to rise, depending on the geographical region (Ashmore, 2005; Dentener et al., 2006; Fowler et al., 1999, 2008; Vingarzan, 2004). Such an enhancement has the potential of limiting the C sink strength of forest ecosystems (IPCC, 2007; Pretzsch et al., 2010; Sitch et al., 2007; Wittig et al., 2009), due to reduced net photosynthesis, accelerated leaf senescence and increased dark respiration (for review see Matyssek and Sandermann, 2003; Matyssek et al., 2010a). In a preceding study that made use of a novel canopy-level free- air O 3 fumigation system (Karnosky et al., 2007; Nunn et al., 2002; Werner and Fabian, 2002), experimentally enhanced chronic O 3 exposure incited stomatal closure as the primary cause of photo- synthetic limitation in adult beech trees (Fagus sylvatica) under forest stand conditions (Kitao et al., 2009). In addition, the exper- imental O 3 regime decreased the apparent quantum yield of photosynthetic carbon gain under low light in shade leaves and increased the dark respiration rate. Leaf-level O 3 effects on the photosynthesis of the adult beech trees at the same site were also reported by Nunn et al. (2005a, b), such as adult trees were less sensitive to O 3 than juvenile ones in phytotrons, and leaf responses differed within forest canopies to O 3 between sun and shade crowns. Nevertheless, the maximum rate of Rubisco carboxylation (V c,max ) did not change during the early growing season under 2  O 3 (experimental twice-ambient O 3 regime, <150 nmol mol 1 ) and even displayed delayed senescence relative to 1  O 3 leaves (i.e., the control under the ambient O 3 regime at the site) during the late growing season (Kitao et al., 2009; Löw et al., 2006, 2007). However, at the whole-tree level, the elevated O 3 regime of the 8- year free-air O 3 fumigation experiment did significantly weaken the stem volume increment of the beech trees by about 40% on average each year (Pretzsch et al., 2010). Through a modelling approach, the present study examines the ecological significance of Abbreviations: A, net photosynthetic rate; C i , intercellular CO 2 concentration; ETR, electron transport rate; F 0 m , maximum fluorescence in illuminated leaves; F s , steady-state fluorescence in illuminated leaves; g s , stomatal conductance; J max , maximum photosynthetic electron transport rate; LMA, leaf mass per area; P ml , CO 2 - and light-saturated rate of photosynthesis; PPFD, photosynthetic photon flux density; R d , dark respiration rate during daylight hours; R n , dark respiration rate during night time; V c,max , the carboxylation capacity of Rubisco; a, light-use effi- ciency; F PSII , quantum yield of PSII electron transport. * Corresponding author. E-mail address: kitao@ffpri.affrc.go.jp (M. Kitao). 1 Present address: ECCOREV FR3098, Bât. CEREGE, Europôle Mediterranéen de l’Arbois BP80, 13545 Aix-en-Provence Cedex 4, France. Contents lists available at SciVerse ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2012.03.014 Environmental Pollution 166 (2012) 108e115