COMMUNITY ECOLOGY Mari Lepik Æ Jaan Liira Æ Kristjan Zobel High shoot plasticity favours plant coexistence in herbaceous vegetation Received: 18 February 2005 / Accepted: 22 April 2005 / Published online: 28 June 2005 Ó Springer-Verlag 2005 Abstract Several theoretical considerations imply that high shoot morphological plasticity could increase competition symmetry and favour plant coexistence. We tested whether mean plasticity across co-occurring spe- cies is a key trait for explaining ramet density and spe- cies richness in herbaceous vegetation. We used three data sets to test the hypotheses: (a) experimentally achieved estimates of plasticity to light availability for 35 herbaceous species; (b) richness, ramet density and canopy architecture data from 17 herbaceous commu- nities; (c) species richness data from a 5-year permanent- plot study in a calcareous grassland. In herbaceous communities containing species with relatively higher shoot plasticity, ramet density was significantly higher. Consequently, relatively more species were growing per unit area—a greater proportion of the community spe- cies pool was represented on 1 m 2 . In the permanent plot study species-richness was higher in those 40·40 cm quadrats where species with high shoot plasticity pre- vailed—there was a positive regression of richness on the mean plasticity of species. This relationship was highly significant in five consecutive years. Our results suggest that shoot plasticity to light availability is evidently one of the key traits in processes that alter the density of co-existing plants and, therefore, species diversity in herbaceous communities. Keywords Competition asymmetry Æ Morphological plasticity Æ Ramet density Æ Species richness Introduction It is not clear which traits of interacting plants are important in determining the density of locally co- existing ramets of same or different species (Wilson et al. 2000; Rajaniemi 2003). Even in apparently homogenous stands the number of mature ramets per unit area may vary greatly between microsites, not to mention varia- tion between community types—it is easy to observe 100-fold difference in herbaceous plant densities even within a compact geographical region (Zobel and Liira 1997). Often it is not possible to explain small-scale and large-scale differences in the density of co-existing plants through differences in resource availability, environment heterogeneity, plant size, and growth form alone. It may be assumed that part of this variance is attributable to some significant but so far not fully recognised plant trait which varies over different assemblies of locally co- existing species. In mesic herbaceous communities, ramet density at the seasonal biomass maximum is usually a result of intense self-thinning, which means local competitive exclusion of plant modules (ramets, branches, leaves), individuals, and sometimes species. The self-thinning process often oper- ates through asymmetric competition—larger individuals have a disproportionably strong competitive advantage over smaller individuals (Weiner 1990; Schwinning and Weiner 1998; Rajaniemi 2003; Rajaniemi et al. 2003). In plant communities, asymmetric competition usually oc- curs above ground (Weiner et al. 1990; Calhill 2003; but see Rajaniemi et al. 2003; Day et al. 2003). Larger (higher) plants are able to obtain a disproportionate share of the light resource and are likely to outcompete smaller ones by overgrowing and shading (Carson and Pickett 1990; Weiner 1990; Schwinning and Weiner 1998; Berntson and Wayne 2000; Zobel 2001; Day et al. 2003; Falster and Westoby 2003; Rajaniemi 2003). Electronic Supplementary Material Supplementary material is available for this article at http://dx.doi.org/10.1007/s00442-005- 0142-0 Communicated by Christian Koerner M. Lepik (&) Æ J. Liira Æ K. Zobel Institute of Botany and Ecology, Tartu University, 40 Lai St, 51005 Tartu, Estonia E-mail: mari.lepik@ut.ee Tel.: +372-7-376389 Fax: +372-7-376222 Oecologia (2005) 145: 465–474 DOI 10.1007/s00442-005-0142-0