Flooding regimes and life-history characteristics of short-lived species in river forelands

A large-scale outdoor experiment was carried out to study the effect of flooding on growth, survival and reproduction in relation to developmental stage in three short-lived species. Several cohorts of Chenopodium rubrum, Rumex maritimus and R. palustris were raised in accordance with a flooding regime which was based on the average flooding conditions of their natural habitats in river forelands. Survival of submergence in the pre-reproductive phase was high in both species of Rumex, but relatively low in C. rubrum. Biomass reduction following flooding depended on plant size before flooding and on the mean temperature of the flood-water. Both Rumex spp. were less reduced in size during flooding, and better capable of regeneration afterwards, than C. rubrum. Later-raised cohorts of the short-day species C. rubrum started to flower after a shorter time and at an earlier developmental stage than earlier cohorts. Flooding had little effect on probability of flowering and caused only a slight delay in flowering in this species. In both long-day species of Rumex, flowering stems were not initiated until a minimal leaf number was formed. As a result later cohorts remained vegetative, while flooding delayed flowering until later in the season or even until the following year. All flooded plants of C. rubrum and most flooded plants in early cohorts of R. maritimus attempted to reproduce in the first growing season. Seed number was severely reduced in C. rubrum due to both reduction of plant biomass and destruction of flowers during flooding. In flooded plants of R. maritimus, seed output was correlated with plant size at the time of bolting. Seed number was maximized at the expense of seed size in R. maritimus. These results suggest that there is a trade-off between survival of flooding and early reproduction in these species, and that there are complex interrelationships between highly variable environmental factors and developmental factors which determine the survival and reproductive success of short-lived species in river forelands.