Ecology, 93(1), 2012, pp. 169–179 Ó 2012 by the Ecological Society of America 13 C-labeling shows the effect of hierarchy on the carbon gain of individuals and functional groups in dense field stands FERNANDO ALFREDO LATTANZI, 1,3 GERMAN DARI ´ O BERONE, 1,2 WOLFGANG FENEIS, 1 AND HANS SCHNYDER 1 1 Lehrstuhl fu ¨r Gru ¨nlandlehre, Technische Universita ¨t Mu ¨nchen, Alte Akademie 12, D-85350 Freising-Weihenstephan, Germany 2 Instituto Nacional de Tecnologı´a Agropecuaria (INTA), Estacio ´n Experimental Agropecuaria Rafaela, Rafaela, Argentina Abstract. Measurements of resource capture by individuals, species, or functional groups coexisting in field stands improve our ability to investigate the ecophysiological basis of plant competition. But methodological and technical difficulties have limited the use of such measurements. Carbon capture, in particular, is difficult to asses in heterogeneous, dense field stands. Here we present a new approach to measure in situ daily gross carbon gain of individuals. It is based on measuring the 13 C content of shoots after a few hours of continuous labeling of all assimilated CO 2 . The technique is simple and has few assumptions. A new, fully mobile facility was developed, capable of providing a labeling environment with a CO 2 concentration close to atmospheric air and known, constant 13 C-enrichment, while maintaining temperature and relative humidity within ambient values. This facility was used in seminatural grasslands of Germany and Argentina to explore the relationship between size and carbon gain of individuals of coexisting species growing in contrasting hierarchical positions, and to analyze the carbon gain of functional groups. In general, carbon gain per unit shoot mass increased with increasing size among small individuals, but it became independent of size among the largest ones. In consequence, competition appeared to be size asymmetric between subordinate individuals but size symmetric between dominant individuals. When comparing functional groups, the carbon gain per unit shoot mass of rosette dicots vs. grasses reflected not their relative contribution to stand biomass, but their hierarchical position: irrespectively of mass or growth form, being taller than neighbors was most important in determining carbon gain per unit shoot mass. We believe these results show that in situ measurements of carbon gain can provide valuable insight in field studies of plant competition. Key words: 13 C steady-state labeling; carbon gain; field stands; functional groups; individuals size; photosynthesis; plant height; species coexistence; symmetric and asymmetric competition. INTRODUCTION Mechanistic analyses of plant competition, and thus our understanding of species coexistence, would im- prove if the amount of carbon assimilated over the course of a day by individuals growing in field stands were easy to measure. For instance, the ‘‘which species/ functional group is gaining’’ question (Connolly et al. 2001) could be addressed by assessing the daily carbon gain per unit biomass of all species or functional groups present in a stand. Further, such a measurement could serve to identify which traits affect the most the carbon gain of individuals competing in multispecies or hierarchically structured stands (e.g., Gaudet and Keddy 1988, Ramseier and Weiner 2006). Also, distinguishing size effects from treatments effects on plant growth (Coleman et al. 1994) would become simpler if direct measurements of carbon gain of individuals were feasible. Daily carbon gain of individuals has been modeled by scaling-up leaf photosynthesis (Barnes et al. 1990, Hikosaka et al. 1999, Anten and Hirose 2003). This involves measuring the response of leaf photosynthesis to irradiance and nitrogen, and then integrating it across all leaves of an individual, which in turn requires measurements of both the diurnal course of photosyn- thetic active radiation (PAR) at several canopy depths and the distribution and geometry of leaves within the canopy. In these estimations, photosynthetic and respiratory responses to PAR and nitrogen of each species are assumed equal for all leaves, and the effects of diurnal and spatial (within canopy) variations in other factors (e.g., leaf temperature, leaf age) are generally not considered. Here we present an alternative method to estimate carbon gain based on the continuous 13 C labeling of all assimilated CO 2 at a constant enrichment level (i.e., steady-state labeling), followed by the immediate harvest of labeled plants and determination of their 13 C content. In comparison, this approach does not provide infor- mation on the carbon gain of different parts of an individual: e.g., leaves at different positions within the Manuscript received 28 June 2011; accepted 5 July 2011. Corresponding Editor: J. Weiner. 3 E-mail: lattanzi@wzw.tum.de 169