Annals of Botany

Size does not always matter much

Plant functional traits, as discussed by Kahmen and Poschlod (Regensburg, Germany, pp. 541–548), are defined as ‘the biological characteristics of plants that both respond to and determine the dominant processes in an ecosystem’. My interpretation of this is that they represent the interface between the systems’ biology of the plant and the wider systems of which the plants are a component. Germination is an obvious feature that contributes to plant community composition whilst, in turn, germination success can be affected by features of the ecosystem such as grazing. The authors selected two functional traits relevant to germination, namely seed mass (0.5–2.0 mg, ‘large’, or < 0.5 mg, ‘small’) and germination season (autumn or spring). The range of necessary combinations was covered by use of seeds from eight species. The habitat was semi-natural dry grassland subject to three management techniques – mowing, grazing and ‘abandonment’. Seeds were set to germinate in early autumn and seedling establishment was assayed in late autumn and in the following spring and early summer. Several clear trends were seen. Large-seeded species were more successful than small-seeded species. For both groups, most seedlings were established in mown plots and fewest in abandoned plots, although the effect of management system on small-seeded species was very small. This lack of a major effect of management system on small-seeded species confirms some of the authors’ earlier data (Kahmen S, Poschlod P. 2004. Journal of Vegetation Science 15: 21–32) but is contrary to the ‘general idea’ (as the authors put it) that establishment from small seeds is much more successful in more open habitats, such as the grazed or mown plots in these experiments, than in more closed habitats. Mowing very much favoured autumn-germinating species but there was no difference between the two groups in the other management systems. Overall, this study gives a clear indication of how ‘experimental testing of functional trait responses’ can help our understanding of ecosystem dynamics.