Investigating a trade-off in root morphological responses to a heterogeneous nutrient supply and to flooding C. JANSEN,† H. M. VAN DE STEEG and H. DE KROON

1. Flood-sensitive plant species are restricted to highly elevated sites in floodplains, but why flood-tolerant plants do not grow at higher elevations is much less clear. We test the hypothesis that a trade-off exists between flood-tolerance and selective root placement in nutrient-rich patches, a putatively important trait in the more competitive highly elevated sites. 2. Achillea ptarmica , Achillea millefolium , Rumex palustris , Rumex thyrsiflorus , Ranunculus repens , Ranunculus bulbosus , Festuca arundinacea and Festuca rubra were subjected to a drained, waterlogged or partially submerged treatment to test their flood-tolerance. Within each species pair, the former are more tolerant to flooding while the latter are relatively flood-sensitive. In another experiment, plants were grown in soil with either a homogeneous or a heterogeneous nutrient distribution. All species placed their roots selectively in the enriched patch, but overall the flood-tolerant species were less selective than the flood-sensitive ones. The wetland species R. palustris was an exception, with very high root plasticity in response to both flooding and nutrient heterogeneity. 3. The negative correlation between selective root placement and flood-tolerance for seven out of eight species suggests that each of these traits has been selected for in its own environment. The exception of R. palustris indicates that there are no physiological or genetic trade-offs involved in explaining this correlation. Species from more frequently flooded habitats were less able to respond morphologically to nutrient-rich patches in the soil, and are therefore more likely to be outcompeted at the more highly elevated sites. Rumex palustris may use its ability to place roots selectively in nutrient-enriched patches to benefit from the nutrients released during drained periods in its dynamic habitat.