Abiotic constraints at the upper boundaries of two Rumex species on a freshwater flooding gradient

1 Competition is widely thought to be responsible for constraining species limits at the favourable ends of freshwater flooding gradients. However, field studies testing this idea rarely consider the role of competition beyond a species' field distribution limits. 2 To test whether species distributions are, instead, constrained by physiological limits, and only narrowed by biotic interactions, we measured the effect of competition on survival and fecundity of Rumex crispus and Rumex palustris within and above their distribution range along an elevation gradient in a river floodplain. 3 Flooding reduced the fitness of both species at the lowest elevations. A decline in fitness was also noticed at the two highest elevations in control as well as in removal plots, although standing crop measurements indicated that conditions here were the most favourable. 4 Both leaf herbivore damage and likeliness of drought increased with elevation. Because herbivore damage only reinforced prevailing abiotic influences, drought appeared to be the strongest constraint at higher positions. 5 Competition merely narrowed both species' physiological ranges by displacing them downwards from their optimum and maximum elevation. The displacement was strongest for the weak competitor Rumex palustris. 6 Our study provides experimental field evidence for earlier suggestions that both ends of a species niche can be defined in hydrological terms. It thus indicates that spatial variability in water availability may segregate species niches and thereby promote species richness in plant communities.