Influence of flood timing on the recovery of macrophytes in a former river channel

Over two vegetation cycles we compared the recovery of macrophytes from flood disturbances that occured at different seasons (July vs December) on patches of a former channel of the Rhône River, France. Some patches were disturbed twice; others were disturbed either in summer or in winter; others were never disturbed and were used as controls.The recovery rate of the vegetation was estimated from the duration of recolonization of the disturbed areas and of growth of the recolonizing species. The influence of the summer disturbance appeared to be strong because the disturbance occurred when the development of the vegetation was maximum. The influence of the winter disturbance was apparently much lower since most species had already declined at this time because of their phenology. The repetition of the two disturbances on the same patch had little influence on the vegetation community.In all cases, the recovery of the vegetation occurred rapidly, both for total vegetation cover and species richness. By the following spring, no significant differences appeared between disturbed and reference patches. The effect of the disturbances varied according to the phenology of the plants, and the macrophyte community studied was more sensitive in summer than in winter.     

Seasonal variability in the palatability of freshwater macrophytes: a case study

The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation in palatability of three freshwater macrophytes: Potamogeton lucens, Elodea canadensis and E. nuttallii. For each species, 2–5 populations were investigated in spring and in summer. Preliminary results showed that the feeding rate of similarly-aged snails bred under standard conditions was stable over time. In contrast, snail feeding rate on the three macrophyte species decreased from spring to summer, which was therefore interpreted as a decrease in plant palatability. This decrease was probably due to tissue maturation, as suggested by the concomitant increase in the dry matter content of leaves of the three species. The high palatability of the species studied during the spring may prove detrimental in cases of strong herbivore pressure, and could have consequences for macrophyte distribution among aquatic habitats.     

Recovery of riverine vegetation after experimental disturbance: a field test of the patch dynamics concept

The Patch Dynamics Concept predicts different recovery patterns of communities after disturbance according to the spatial and temporal heterogeneity of the habitat. The aim of this study was to test the predictions arising from the Patch Dynamics Concept on the recovery of macrophyte communities after an experimental disturbance. The test was based on the comparison of the vegetation recovery in three stations located on former channels of the Rhône River, France, differing by their temporal heterogeneity, which was estimated by the frequency of flood scouring. In each station the experimental disturbance was set up by uprooting the aquatic plants in three experimental sets comprising four 1 m²-plots. The aquatic plants were surveyed in these sets as well as in their reference sets from July to November 1991.As predicted, the most frequently disturbed station recovered its species richness (19 species) and its vegetation cover in less than two months. The biological traits of the species occurring in this station are considered as r-strategies. The species colonizing the experimental sets were not necessarily present on the sets before the disturbance, but occurred regularly on other sites station. The rapidity of the community recovery demonstrates the high resilience of this ecosystem. According to the unpredictable character of its recolonization, the macrophyte community of this station could be said to be founder controlled with competitive lottery for establishment.The station with intermediate temporal heterogeneity was overgrown by some r-selected species but the community recolonization was predictable and this station had intermediate resilience. Its species richness was low (6 species).The less frequently disturbed station presented low resilience with a slow recovery of its community (more than 5 months); the species richness (4 species) and the traits of some species were related to K-strategies while others were related to r-strategies. The macrophyte community of this station could be said to be dominance controlled.Different patterns of recovery of the macrophyte communities of former channels of the Rhône River could be depicted according to their temporal heterogeneity; these patterns were consistent with the hypotheses arising from the Patch Dynamics Concept. However, the competitive lottery appeared to be limited at the scale of our experiment.     

Regeneration and colonization abilities of aquatic plant fragments: effect of disturbance seasonality

The regeneration (regrowth into viable plants or production of propagules, such as turions or buds) and colonization (development of roots and establishment in the sediment) of fragments of six aquatic plant species (Elodea canadensis Michaux, Hippuris vulgaris L., Luronium natans (L.) Rafin., Potamogeton pusillus L., Ranunculus trichophyllus Chaix, Sparganium emersumRehmann) occurring in habitats frequently disturbed by floods, were investigated through laboratory experiments conducted in two seasons, and compared to the recolonization patterns depicted after field experiments. Hypothesis was that differences observed between recolonization patterns after spring and autumn flood disturbances should be related to differences in recolonization (via rooting) and regeneration (via propagules) abilities of species fragments. In May and in August, five types of fragments were collected from the plants. Their development and/or rooting abilities were recorded over 10 weeks in the greenhouse. Fragments from E. canadensisand H. vulgarishad higher regeneration and lower colonization abilities in spring and conversely in autumn. Fragments from R. trichophyllusand S. emersumhad high colonization and low regeneration abilities during both seasons. Fragments from L. natans developed new buds in spring, whereas root development occurred only in autumn. Fragments from P. pusillus never rooted into the sediment, but developed turions in autumn. Differences between recolonization patterns observed in the field at the two seasons can most often be related to differences in regeneration and colonization abilities of species fragments. Species that colonize disturbed areas rapidly whenever the disturbance by flooding occurs have at least one type of vegetative fragment with a high colonization potential; this is called the `always-ready strategy' which appears to be an adaptation of aquatic plants to the unpredictability of flood disturbances.     

Recolonization of cleared riverine macrophyte patches: importance of the border effect

Abstract. Recolonization of two experimentally cleared 9 m²-patches by macrophytes in a former channel of the Rhône River, France, was investigated from May to July 1994. Two patterns of recolonization were recognized: (1) Groenlandia densa, Luronium natans and Potamogeton natans invaded the bare areas by propagation from the adjacent vegetation (border effect); (2) Potamogeton pusillus colonized the disturbed areas at random, apparently independently from the position of nearby clumps. An intermediate recolonization pattern was shown by Callitriche platycarpa, Elodea canadensis, Hippuris vulgaris, Ranunculus circinatus and Sparganium emersum. Species recolonization patterns and associated strategies were related to species traits, particularly vegetative and sexual reproduction.     

The ability of aquatic macrophytes to increase root porosity and radial oxygen loss determines their resistance to sediment anoxia

Radial oxygen loss (ROL) has been suggested to be a major process to protect plants exposed to root anaerobic stress. In the present study, we aimed to test the importance of root porosity and radial oxygen loss on the aquatic macrophyte resistance to sediment anoxia. We expected that species living in eutrophic environments characterized by anaerobic conditions in sediments exhibited higher root porosity and radial oxygen loss than species restrained to oligotrophic environments. In this way, we compared the responses to sediment anoxia of two hydrophyte species growing under meso-eutrophic conditions in the field (Myriophyllum spicatum L. and Vallisneria spiralis L.) and three species growing under oligotrophic conditions (Potamogeton coloratus Horne, Elodea canadensis Michx and Sparganium emersum Michx.). Under laboratory conditions, ROL, root porosity, plant metabolism (aerobic respiration, photosynthesis, root fermentative activity) and plant growth were analysed after 3?months of acclimation in anaerobic sediments and compared with control values obtained from aerobic sediments. The results showed that two meso-eutrophic species (M. spicatum and V. spiralis) survived in anaerobic sediments and maintained similar photosynthesis rates than those measured under aerobic conditions. In contrast, the three oligotrophic species (P. coloratus, E. canadensis and S. emersum) suffered net biomass loss and depressed their photosynthesis rates under anaerobic conditions. All variables associated with plant tolerance to anaerobic conditions (maintenance of photosynthesis, aerobic respiration and growth rate, and limitation of root fermentative activity) were positively linked to root porosity and ROL. According to our hypothesis, species that could survive to anaerobic conditions were the species able to increase their root porosity and ROL under these conditions. Thus, in ecological studies, it would be useful to use the root porosity and ROL plasticity as biological traits in order to model the distribution of macrophytes in river floodplains.     

Trade-offs between growth and defence in two phylogenetically close invasive species

The success of an invasive plant species could be explained by trade-off between growth and defence. The aim of this paper was to explore the responses of two non-native aquatic macrophytes Elodea canadensis and Elodea nuttallii to herbivores in their introduced range. We assessed the palatability of the two phylogenetically close aquatic plant species in field and their responses to gammarid consumption in spring, summer and autumn in a microcosm experiment. We measured the variation of functional traits for each season. The traits selected were those judged most closely related to the allocation of resources to growth or to resistance against herbivores. We clearly established that the strategies of the two species were different and that their consumption rate differed in summer. In summer, E. canadensis allocated more of its resources to structural defence (leaf toughness). The increase in leaf thickness reduced the palatability of E. canadensis, whereas E. nuttallii stimulated its growth. Moreover, a decrease in dry matter content in E. nuttallii was found during the growing season in field. In autumn, both plant species accumulated nitrogen and phosphorus in their tissues. We also demonstrated that neither species induced efficient chemical defences against the herbivores. The different strategies of these two Elodea species could be explained by their different resident times in the introduced area and by an adaptation of the naturalised E. canadensis to herbivores.     

Basal rosette or floating leaf canopy – an example of plasticity in a rare aquatic macrophyte

The rare aquatic macrophyte Luronium natans can be encountered with two growth forms: as a bottom-dwelling plant with a rosette of linear leaves, or as a nymphaeid plant with long-petioled oval floating leaves. The change of growth form corresponds to a change in size class (small and submerged vs. tall and canopy-forming). The present study compares through one growing season floating leaf production and patterns of biomass allocation in Luronium ramets from a natural, moderately nutrient-rich habitat and from nutrient-rich transplantation sites. We ask whether differences in gross floating leaf production result from differences in ramet vigour while allocation patterns remain stable, or whether Luronium is able to allocate biomass plastically, and thus to change its growth form with differences in habitat. The study shows that both leaf types were produced in all habitats since the mid-growing season (June), but that biomass allocation to them varied. Floating leaves were dominant at high nutrient levels, whereas at the moderate nutrient level most leaf biomass was found in the rosette. We suggest that Luronium modifies its biomass allocation in a pattern that corresponds to the outlines of Tilman's (1988) mechanistic model of optimal biomass allocation on a light:nutrient gradient. The demonstrated morphological plasticity may enable Luronium to maintain itself in a large range of habitats, but it raises new questions about the reasons for the species' rarity.     

Resistance to herbivory of two populations of <Emphasis Type="Italic">Elodea</Emphasis><Emphasis Type="Italic">canadensis</Emphasis> Michaux and <Emphasis Type="Italic">Elodea nuttallii</Emphasis> Planchon St. John

In this article, we compared the resistance of two introduced populations of Elodea nuttallii and Elodea canadensis to two different herbivores. Samples were collected from the River Rhine and River Rhône in eastern France. The two populations of E. nuttallii differed in their introduction history, whereas E. canadensis was introduced at the same time in the two sites. The Daily Food Consumption (DFC) rates of the two macrophyte populations were evaluated in no-choice experiments using the scraper Lymnaea stagnalis and the shredder Gammarus roeseli. At the same time, we assessed four plant traits: dry matter content (DMC), total nitrogen content, carbon/nitrogen ratio and total phenolic content. The two populations of E. canadensis were consumed at low levels by both the herbivores. L. stagnalis showed a higher DFC on the Rhône population of E. nuttallii than on the Rhine population. No significant difference between the two populations was established with G. roeseli, but the level of DFC was high. This result demonstrates that the assessment of plant palatability should be carried out with several generalist herbivores belonging to various feeding groups (e.g. scrapers or shredders). Although the Rhône population of E. nuttallii had higher levels of phenols than the other populations, it was consistently consumed in greater quantities than E. canadensis. Neither the phenolic contents were not effective against these herbivores, nor the levels of phenolics too low to induce an efficient resistance. The higher DMC and the lower DFC of the two populations of E. canadensis suggest that this introduced plant has co-evolved with indigenous enemies in the introduced range.     

Strategies of reproduction, dispersion, and competition in river plants: A review

The present paper reviews the literature on reproduction, survial, dispersion and competition in aquatic plants of mostly European rivers. The specificity of the strategies of colonization in aquatic plants in comparison with terrestrial plants is noted. The importance of the knowledge of these life history traits in the control of the harmful effects caused by aquatic plants and for ecosystem management is also considered. The difficulty of applying current ecological theories concerning adaptative strategies to aquatic plants is discussed, focusing on the great interest of studying the species traits of aquatic macrophytes for testing such theories and interpreting recolonization patterns of disturbed areas. Knowledge of strategies of reproduction, dispersion and competition among aquatic plants remains very fragmentary, particularly from a quantitative view-point, and further studies are required both for theoretical and practical applications.