EV-1 The role of belowground competition and plastic biomass allocation in altering plant mass–density relationships Yue Lin, Franka Huth, Uta Berger and Volker Grimm Y. Lin (yue.lin@ idiv-biodiversity.de) and U. Berger, Inst. of Forest Growth and Computer Science, Dresden Univ. of Technology, PO 1117, DE-01735 harandt, Germany. YL also at: German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Deutscher Platz 5e, DE-04103 Leipzig, Germany. – V. Grimm, Helmholtz Centre for Environmental Research – UFZ, Dept of Ecological Modelling, Permoserstr. 15, DE-04318 Leipzig, Germany, and: Inst. for Biochemistry and Biology, Univ. of Potsdam, Maulbeerallee 2, DE-14469 Potsdam, Germany. – F. Huth, Inst. of Silviculture and Forest Protection, Dresden Univ. of Technology, PO 1117, DE-01735 harandt, Germany. Metabolic scaling theory (MST) predicts a ‘universal scaling law’ for plant mass–density relationships, but empirical obser- vations are more variable. Possible explanations of this variability include plasticity in biomass allocation between the above- and belowground compartment and diferent modes of competition, which can be asymmetric or symmetric. Although complex interactions of these factors are likely to occur, so far the majority of modelling and empirical studies has focussed on mono-factorial explanations. We here present a generic individual-based model, which allows exploring the plant mass–density relationship in realistic settings by representing plasticity of biomass allocation and diferent modes of competition in the above- and belowground compartment. Plants grew according to an ontogenetic growth model derived from MST. To evaluate the behavior of the simulated plants related to the allocation patterns and to validate model predictions, we conducted greenhouse experiments with tree seedlings. he model reproduced empirical patterns both at the individual and population level. Without belowground resource limitation, aboveground processes dominated and the slopes of mass–density relationships followed the predictions of MST. In contrast, resource limitation led to an increased allocation of biomass to belowground parts of the plants. he subsequent dominance of symmetric belowground competi- tion caused signiicantly shallower slopes of the mass–density relationship, even though the growth of individual plants followed MST. We conclude that changes in biomass allocation induced by belowground resource limitation explain the deviations from the mass–density relationship predicted by MST. Taking into account the plasticity of biomass allocation and its linkage to the above- and belowground competition is critical for fully representing plant communities, in particular for correctly predicting their response of carbon storage and sequestration to changing environmental conditions. he relationship between plant performance and density is one of the central topics in plant ecology (Stoll et al. 2002, Deng et al. 2006, Chu et al. 2010, Weiner and Freckle- ton 2010). It is widely accepted that resource competition among plants is the main force in determining plant mass– density relationships induced by density-dependent mortal- ity. In self-thinning stands of plants, the general relationship between density (N) and mean biomass (M y ) of surviving plants can be described by a power function, M y  kN y , where k and g are referred to as the thinning coeicient and exponent, respectively. Based on Euclidean geometry, it was proposed that the value of g is approximately equal to –3/2, which is well-known as the ‘–3/2 power rule’ of self- thinning (Yoda et al. 1963). However, more recently, based on a fractal model and plant energetics, the metabolic scal- ing theory (MST) predicted a ‘universal scaling law’ with g equal to –4/3 (Enquist et al. 1998, West et al. 1999, Enquist and Niklas 2001, Enquist 2002, Brown et al. 2004, Savage et al. 2010). Evidence from theoretical and empirical stud- ies does not consistently support a certain allometric scaling exponent. Several mechanisms are suggested to explain the variable scaling exponents (Coomes 2006, Deng et al. 2006, Weiner and Freckleton 2010, Lin et al. 2013). It is noticeable, however that previous studies on allomet- ric scaling exponent mainly focus on aboveground biomass across diferent types of ecological communities and were usually conducted in environments without major resource limitation or physical stress (Deng et al. 2006). In contrast, indings from arid areas or from plantation experiments with low resource levels often deviate from the theoretical pre- dictions and show signiicantly less negative exponents, i.e. shallower slopes of the self-thinning trajectory or log mass– log density relationship, respectively (Morris 2003, Deng et al. 2006, Liu et al. 2006). Consequently, both the valid- ity and generality of scaling exponents are still unclear and require further analyses of the underlying mechanisms (Coomes 2006, Deng et al. 2006, Coomes et al. 2011). On sites where belowground resources such as nutrients and water are limited, plants tend to allocate more bio- mass to their belowground parts and develop extended root Oikos 000: 001–009, 2013 doi: 10.1111/j.1600-0706.2013.00921.x © 2013 he Authors. Oikos © 2013 Nordic Society Oikos Subject Editor: Richard Michalet. Accepted 6 July 2013