Interactions and competition processes among tree species in young 1 experimental mixed forests , assessed with chlorophyll fluorescence and leaf 2 morphology 3 4

26 Chlorophyll a fluorescence (ChlF) and leaf morphology parameters were assessed in two 27 experimental sites in Europe (Kaltenborn, Germany and Satakunta, Finland), within a forest 28 diversity experiment. The trees at Satakunta, planted in 1999, form a stratified canopy; while in 29 Kaltenborn the trees are seven-years-old, with no apparent canopy connection on broadleaf species. 30 The following ChlF parameters obtained from the measured OJIP transient curves were examined: 31 the ratio FV/FM (a proxy of the maximum quantum yield, where Fv is the difference between the 32 minimal (F0) and the maximal (FM) fluorescence in the dark-adapted state); ΨEo (a proxy of the 33 efficiency to move an electron from reduced QA, the secondary PSII electron acceptor into the 34 electron transport chain); the I-P phase (a proxy of the efficiency to reduce the final acceptors 35 beyond PSI), and PItot (the total performance index for potential energy conservation from photons 36 absorbed by PSII to the reduction of PSI end acceptors). At Satakunta FV/FM and ΨEo values in 37 Betula pendula were higher in monocultures and lower in mixed plots, maybe due to the increasing 38 availability of light in mixed plots that can induce photoinhibition. The opposite trend was observed 39 in Picea abies, which was shaded in mixed plots. At Kaltenborn FV/FM decreased in Fagus sylvatica 40 and P. abies in mixed plots. This effect was attributed to competition processes both at aboveground 41 and belowground level. At Satakunta LMA (Leaf Mass per Area) increased in B.pendula leaves with 42 increasing species richness. LA (leaf area of ten leaves) was reduced in F. sylvatica in the mixed 43 plots at Kaltenborn. By upscaling the overall fluorescence response to plot level (PItot_plot), a 44 significant positive correlation with tree diversity was found at Kaltenborn but not at Satakunta. 45 This result may suggest that the competition/facilitation processes in mixed stands play a significant 46 overall role in the first stages of forest establishment, but then tend to be compensated for in more 47 mature stands. 48 49 INTRODUCTION 50 Biodiversity regulates several aspects of ecosystem functioning and the delivery of ecosystem 51 services (e.g. Balvanera et al. 2006; Cardinale et al. 2011). Additionally, the ecological stability of 52 forest ecosystems has been connected to tree diversity (Bengtsson et al. 2000; Thompson et al. 53 2009; Scherer-Lorenzen et al. 2005a). Many forest ecosystem services, such as timber production 54 and carbon sequestration, are directly related to the growth and photosynthesis rates. A recent 55 review (Zhang et al. 2012) emphasized the role of biodiversity in enhancing forest growth and the 56 biological mechanisms and processes leading to an increased biomass production in mixed stands. 57 This is mainly related to a more efficient exploitation of the ecological resources due to niche 58 differentiation and complementary resource use among coexisting species (Tilman 1999; Loreau & 59 Hector 2001), i.e. through species interactions. Such complementarity can occur aboveground 60 within the canopy, or in the soil. For example, different timing of leaf abscission of the various 61 species and increased decomposition rates of litter in mixed stands allow a more homogeneous 62 release of nutrients throughout the year and enhance the biological activity of the soil (Richards et 63 al. 2010). Moreover, the presence of species with symbiotic nitrogen fixation activity increases the 64 soil fertility (Forrester et al. 2012; Nouvellon et al. 2012), representing a classical example for 65 facilitation. Overall, competition or facilitation may be established between different tree species, 66 consequently the performance and growth of trees may be enhanced or depressed in a species67 specific way (Reiter et al. 2005; Lei et al. 2012a; b). 68 The quantification of the role of tree species diversity in producing ecosystem services in 69 naturally grown forests is problematic because of the large variability of the environmental factors 70 (Vilá et al. 2005). For this reason a set of experimental forests with different levels of tree diversity 71 has been established around the world within the framework of several research programs (Scherer72 Lorenzen et al. 2005b; Scherer-Lorenzen et al. 2007). In this context, experimental forests were 73 recently planted in Europe at Kaltenborn (Germany) and Satakunta (Finland). 74 During the growth of a forest stand, trees establish relationships with their neighbors both at 75 root and at canopy level, depending on different growth rates, space occupation strategies and 76 sun/shade tolerance (Kosovits et al. 2005; Lei et al. 2012a;b; Kohyama &Takada 2012). As far as 77 canopy processes are concerned, different height and architecture of tree species result in the 78 formation of micro-environments with a variety of light conditions, thus allowing the appearance of 79 shade tolerant species (Ishii & Asano 2010). A mixed forest creates varying illumination conditions 80 which induce different photosynthetic responses in plants at both stand level and within the crown 81 of individual trees (Ellsworth &Reich 1993; Pearcy 1999; Niinemets et al. 2004; Valladares & 82 Niinemets, 2007; Niinemets 2007; Way & Pearcy, 2012; Mänd et al. 2013). 83 Plant responses to light can be efficiently measured with chlorophyll a fluorescence (ChlF) 84 techniques (Adams & Demming-Adams 2004; Bruce &Vasil’ev 2004). The informative potential of 85 ChlF analysis (Papageorgiou & Govindjee 2004) has been used for forest monitoring surveys, by 86 applying remote sensing techniques (Rossini et al. 2006; Meroni et al. 2009), in applied forestry 87 research (see Ball et al. 1994; de Carvalho et al. 2005; Bussotti et al. 2010 and citations therein) 88 and, more in general, in forest ecology studies (see, for ex. Stylinski et al. 2002; Einhorn et al. 89 2004). Nevertheless, the application of ChlF in extensive terrestrial field surveys on tall trees 90 remains problematic (Mohammed et al. 1995; 2003; Sampson et al. 2000). 91 The survey described here represents the first experience in which ChlF techniques were 92 used in a large scale terrestrial ecological assessment of forests, in relation to biodiversity issues. 93 The specific aim of the present paper was to investigate the dynamics of competition and 94 facilitation between tree species in the experimental mixed forests at Kaltenborn and Satakunta by 95 using their ChlF properties and leaf morphology. The specific hypothesis to be tested was that the 96 interactions between the different tree species and their physiological requirements during forest 97 stand development and stratification -as well as the nature of competition for space and light -are 98 reflected in the ChlF properties. More specifically, the heterogeneity of the canopy layer in mixed 99 forests induces species-specific strategies for the use of light, and photoinhibition conditions, 100 according to the relative growth and crown interaction between the neighboring tree species. In the 101 younger plantation, where the interaction between crowns is lacking, the competition for space, 102 both at aboveground and belowground levels, may have a prominent importance. 103 104 MATERIAL AND METHODS 105 Experimental sites 106 The study was carried out in two experimental plantations, both included in previous projects. 107 Kaltenborn (Thuringia, Germany) is a part of the BIOTREE experiment (Scherer-Lorenzen et al. 108 2005b, 2007), and Satakunta (Finland) belongs to the TreeDivNet platform (Scherer-Lorenzen et al. 109 2005). For details of the experimental sites see the supplementary materials. 110 At Kaltenborn the tree species studied were European beech (Fagus sylvatica L., FS), sessile oak 111 (Quercus petraea Liebl., QP), Norway spruce (Picea abies (L.) Karst. , PA) and Douglas fir 112 (Pseudotsuga menziesii Franco, PM). At Satakunta we analyzed the chlorophyll fluorescence of 113 silver birch (Betula pendula L., BP), European black alder (Alnus glutinosa (L.) Gaert. , AG), 114 Norway spruce (Picea abies (L.) Karst. , PA), Scots pine (Pinus sylvestris L., PS) and Siberian larch 115 (Larix sibirica Ledeb., LS). In both the experimental plantations the tree species were combined in 116 different tree species mixtures (Table 1). 117 118