Summary1 Plant performance of species in river floodplains is negatively affected by submergence, due to severely hampered gas exchange under water. Several individual traits have been shown to determine flooding tolerance, but the interrelationships among these traits and their effects on plant performance still remain largely unknown. 2 Here, we aim for a more coherent understanding of submergence tolerance, by investigating whether different traits are alternative strategies to enhance survival under water or whether these traits act in concert.3 Because responses to submergence may be taxon-specific, we chose a multiple-species approach that allowed phylogenetic comparisons. The phenotypic traits investigated in the current study were constitutively different in species originating from frequently flooded habitats than in species from dry habitats: wet species were taller and had a higher specific leaf area (SLA), chlorophyll content, aerenchyma content, and longer longevity of terrestrial and 'aquatic' leaves compared with species from dry habitats. Moreover, the frequently flooded species appeared to be more plastic for these traits in response to submergence. Neither the phylogenetic component nor the light climate under water were important for the expression of the submergence-related traits. 4 Principal component analysis (PCA) showed that these traits were divided in two independent clusters structured either around the petiole (i.e. aerenchyma content and shoot length) or the leaf lamina (i.e. SLA, chlorophyll content and leaf longevity). Shoot length and aerenchyma content of the petiole were also positively correlated, albeit not significantly. A positive correlation between SLA and leaf longevity was observed under water, indicating that thinner leaves may have an increased potential for gas exchange, resulting in increased leaf longevity and plant survival. 5 Path analyses indicated significant positive effects of the two trait clusters on plant performance during full submergence. 6 Our study shows that multiple ecophysiological traits act in concert to fine tune responses to dynamic and unpredictable environments such as river floodplains.

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Summary1 Plant species are known to segregate along small-scale flooding gradients. We tested whether differences in flooding duration can also result in genetic differentiation in the clonal species Ranunculus reptans , which naturally grows in both a lakeside microhabitat and a landside microhabitat with shorter periods of flooding. 2 We compared traits related to fitness, and clonal life-history traits, of 432 plants representing nine genotypes from each microhabitat, grown without flooding or with short or long flooding duration. We also determined aerenchyma contents and carbohydrate use efficiencies during flooding in plants of these 18 genotypes. 3 In the flooding treatments, genotypes from the lakeside microhabitat produced significantly more rosettes and rooted rosettes than genotypes from the landside microhabitat. This indicates small-scale local adaptation to flooding duration in R. reptans . 4 Unexpectedly, genotypes from the landside microhabitat had a higher proportion of aerenchyma in their roots than those from the lakeside microhabitat. Carbohydrate use efficiency was high in all genotypes. These physiological traits cannot therefore explain the observed local adaptation. 5 Genotypes from the lakeside microhabitat produced shorter stolon internodes than genotypes from the landside microhabitat when flooded. Moreover, in the treatment with long flooding duration, there was selection for reduced stolon internode lengths, which might help to reduce respiratory losses. This suggests that local adaptation is a consequence of differences in plasticity of internode length. 6 Our results indicate an important role for flooding in plant microevolution by demonstrating that variation in flooding duration can induce intraspecific specialization even within populations. Physiological traits that determine differences in flooding tolerance between species do not, however, seem to have played a key role in this differentiation.

Summary• Community ecologists often assume a hierarchy of environmental sieves to predict the impact of multiple stresses on species distribution. We tested whether this assumption corresponds to physiological responses using impact of water level and shade in wetland vegetation as a model.• Seedlings of four wetland species were grown under full light and simulated canopy shade, both in drained and waterlogged soils. When subject to both stresses simultaneously, waterlogging and shade independently affected growth of the two waterlogging tolerant species. For the intolerant species, however, waterlogging had the largest impact and the additional effect of shade was smaller than the effect of shade in drained soil. Soil flooding decreased specific leaf area but only if plants were in full light. Waterlogging did also not constrain a higher investment in stems of shaded plants.• These results demonstrate that light limitation in flooded habitats only plays a role if species can tolerate waterlogging and therefore correspond with the notion that water level determines the potential species pool and that standing crop consequently determines which species can actually persist.