Gudrun Bornette

Response of aquatic plants to abiotic factors: a review

This review aims to determine how environmental characteristics of aquatic habitats rule species occurrence, life-history traits and community dynamics among aquatic plants, and if these particular adaptations and responses fit in with general predictions relating to abiotic factors and plant communities. The way key abiotic factors in aquatic habitats affect (1) plant life (recruitment, growth, and reproduction) and dispersal, and (2) the dynamics of plant communities is discussed. Many factors related to plant nutrition are rather similar in both aquatic and terrestrial habitats (e.g. light, temperature, substrate nutrient content, CO2 availability) or differ markedly in intensity (e.g. light), variations (e.g. temperature) or in their effective importance for plant growth (e.g. nutrient content in substrate and water). Water movements (water-table fluctuations or flow velocity) have particularly drastic consequences on plants because of the density of water leading to strong mechanical strains on plant tissues, and because dewatering leads to catastrophic habitat modifications for aquatic plants devoid of cuticle and support tissues. Several abiotic factors that affect aquatic plants, such as substrate anoxia, inorganic carbon availability or temperature, may be modified by global change. This in turn may amplify competitive processes, and lead ultimately to the dominance of phytoplankton and floating species. Conserving the diversity of aquatic plants will rely on their ability to adapt to new ecological conditions or escape through migration.

Comparative abilities of vegetative regeneration among aquatic plants growing in disturbed habitats

Abstract The regeneration (regrowth into viable plants) and colonisation (establishment in the sediment) abilities of vegetative fragments of six aquatic plant species ( Elodea canadensis Michaux, Hippuris vulgaris L., Luronium natans (L.) Rafin., Potamogeton pusillus L., Ranunculus trichophyllus Chaix, Sparganium emersum Rehm.) occurring in habitats frequently disturbed by floods along the Rhone River (France) were compared through a laboratory experiment. Six types of plant fragments were collected from the plants and placed in water over sediment. Their development and/or their rooting into the sediment were recorded over 10 weeks. The species exhibited different survival tactics to survive after fragmentation: (1) fragments from Sparganium and Ranunculus developed roots and rapidly established into the sediment whereas (2) fragments from Luronium , Hippuris and Elodea developed many propagules that could be dispersed but did not establish within the 10 weeks of the experiment. The fragments of P. pusillus never established nor regenerated. The results showed a trade-off between regeneration and colonisation related to the patterns of recolonisation of cleared patches of the species and their ecological requirements. Maintenance of these species in ecosystems frequently disturbed by floods can partly be explained by their high regeneration abilities, and results are discussed in terms of life-history traits and ecological strategies.

Disturbance regimes and vegetation dynamics: role of floods in riverine wetlands

. This study tested whether the frequency of flood disturbances was able to slow down or stabilize vegetation succession in former braided channels over a decade. According to the Patch Dynamics Concept and to succession theory, species richness and diversity should be high but stable in the frequently (40 days/year) flooded channel, and should change over time in the infrequently (1 day/year) flooded one. Within the frequently disturbed channel, composition of vegetation as well as species richness and diversity appeared stable through dynamic equilibrium over the decade. Only one zone, because of particular geomorphological features that decreased disturbance intensity, developed highest diversity and richness as expected from the Intermediate Disturbance Hypothesis. The highest disturbance effect decreased species richness and was related to a higher spatial heterogeneity of the substrate (number of grain-size classes). In the other zones, richness and diversity appeared to be lowest where disturbance frequency was lowest or disturbance intensity was highest. From 1981 to 1987, the infrequently flooded channel underwent succession, and species richness increased in the major part of the channel, whereas diversity increased only in its extreme parts.

Plant resistance to mechanical stress: evidence of an avoidance–tolerance trade-off

External mechanical forces resulting from the pressure exerted by wind or water movement are a major stress factor for plants and may cause regular disturbances in many ecosystems. A plants ability to resist these forces relies either on minimizing the forces encountered by the plant (avoidance strategy), or on maximizing its resistance to breakage (tolerance strategy). We investigated plant resistance strategies using aquatic vegetation as a model, and examined whether avoidance and tolerance are negatively correlated. We tested the avoidance-tolerance correlation across 28 species using a phylogenetically corrected analysis, after construction of a molecular phylogeny for the species considered. Different species demonstrated contrasting avoidance and tolerance and we demonstrated a significant negative relationship between the two strategies, which suggests an avoidance-tolerance trade-off. Negative relationships may result from costs that each strategy incurs or from constraints imposed by physical laws on plant tissues. The existence of such a trade-off has important ecological and evolutionary consequences. It would lead to constraints on the evolution and variation of both strategies, possibly limiting their evolution and may constrain many morphological, anatomical and architectural traits that underlie avoidance and tolerance.

Species traits and recolonization processes after flood disturbances in riverine macrophytes

Macrophyte recolonization after two major flood disturbances (winter 1990 and autumn 1991) was studied for 3 years in 11 contiguous permanent quadrats located along a transect from one bank to the other across a former channel of the Rhône River. Floods induced changes in substrate grain-size from fine to coarser sediment and swept away all the plants present before disturbance. Re-establishment of species started on or near the banks in fine sediment areas that can act as refuges for propagules during disturbance or as regeneration niches for propagules brought in by floods. From the banks, the species expanded towards the center of the channel in coarse sediment. The transect was fully colonized by macrophytes the second year after a major flood. All these species, as well as those observed before the 1990 flood, can be considered as pioneer species adapted to frequent disturbances. Information identified from personal previous observations and collected in literature concerning selected species traits was analysed statistically using a fuzzy-coding technique and confronted with date of re-establishment of each species after disturbance. Species composition before disturbance and selected species traits allow us to predict species composition in disturbed areas. The first species to re-establish were able to produce turions or other non-subterranean vegetative organs. Species that disseminate both by lateral spread and regeneration by stem fragments re-establish later, before helophytes that also disseminate by lateral spread but flower each year.

Ecological complexity of wetlands within a river landscape

The sustainable conservation of diversity in riverine wetlands implies knowledge of the basic geomorphological and ecological processes that inter play at the landscape scale. Eight hypothetical types of wetlands, predicted as differing in their plant communities, were proposed by combination of three factors (geomorphological pattern, river incision vs aggradation, groundwater origin), selected because they determine the effects of flood disturbances (intensity, frequency) and the water supplies. This hypothesis was tested through the comparison of the vegetation and the physico-chemistry of cut-off channels of the Ain River in France. Cut-off meanders were related to meso-eutraphent plant species of standing water, aggraded meanders being more frequently overflowed than incised ones. Braided channels were characterized by flood-tolerant species, some of the incised ones being related to oligotraphent communities, but some remaining connected, through seepage and floods, to the river (mesotraphent species). The ratio of side-hill aquifer to seepage supplying these channels depends on river vs aquifer levels but also on their elevation from the river level, determined by their age. The study demonstrates the effect of geomorphology and of water supplies on floristic assemblages, but some other features must also be considered. The high diversity of functioning implies that conservation of such ecosystems should consider all the parts of the river landscape and all the processes that interfere on several time-scales.

Aquatic plant diversity in four river floodplains: a comparison at two hierarchical levels

The present study focuses on the role ofenvironmental factors in plant diversity and community organization at both water-body and river floodplain levels. The cover of each plant species was measured at 841 sampling plots along 63 cut-off channels located in four river floodplains. Environmental variables were documented either at river level (suspended matter, water physico-chemistry, river slope) or at cut-off channel level (channel capacity, slope, substrate grain-size). At both river and cut-off channel levels, increasing slope (i.e. increasing erosion during floods) and decreasing nutrient-content of the water raise species richness and uniqueness (number of species found only in one river), according to hypotheses on the combined role of disturbances and productivity in biodiversity. Rivers that are not eroded during floods are nutrient-rich and present the lowest richness and uniqueness at the river level, but dewatering combined with high connectivity increase richness at the cut-off channel level.

Propagule banks and regenerative strategies of aquatic plants

Abstract The role of the propagule bank in aquatic plant maintenance was studied in two riverine wetlands. Four sites were selected, characterized respectively by flooding, drying up, both disturbances operating, and neither operating. Our hypothesis was that recolonization after drying up would mostly involve seeds and buds from the propagule bank, whereas recolonization after floods would mostly involve rhizomes. Dry sites were characterized by a high density of seeds, and a high similarity between seed species and established vegetation. Unspecialized fragments remaining in wet parts of the sediment probably also contribute to species maintenance. Species maintenance in sites subjected to flooding was highly dependent on deeply anchored rhizomes, as indicated by the strong floristic similarity between species that occur in the established vegetation and rhizomes in the bank. Regeneration of the community after scouring floods also involved seeds, some species being able to flower under water. When scouring flooding and drying up were superimposed, regenerative strategies exhibited in the bank did not simply result from the ‘addition’ of the two disturbance effects. When the disturbances did not occur too closely together in time, species were able to survive either by: (1) producing many propagules under aquatic conditions or (2) coping with the temporal variability by producing several types of propagules. Nomenclature: Lambinon et al. (1992); Wiegleb & Kaplan (1998) for Potamogeton species and Corillon (1975) for Characeae.

Biodiversity in the floodplain of Saône: a global approach

Biodiversity of European floodplains is seriously threatened mainly due to (1) modifications of river courses such as channelisation or embankments, and (2) changes in traditional agricultural practices (i.e. usually pastures), into intensive production using drainage and fertilisation. A upstream-downstream survey of the Saône floodplain (France) has been done to identify the contribution of habitats to the floodplain biodiversity. Selected taxa were aquatic and terrestrial vegetation, Odonata, Coleoptera, Amphibians, and birds. The taxa were sampled in different habitat types that were: forests, grasslands and aquatic habitats. Tributary confluences with the river and cut-off channels contributed greatly to the floodplain diversity according to their invertebrates and aquatic vegetation communities. The abundance of rare species (benefitting of a national or regional protection status) was the highest in hygrophilous grasslands. Moreover, we demonstrated that diversity of breeding bird communities was correlated with the size of these habitats. We demonstrated also that alluvial forests contributed to maintain some particular species as Middle-spotted Woodpecker (Dendrocopus medius), while new plantations were colonized by openland bird communities sensible to the edge effect. Grassland fragmentation for agriculture appeared to be a major cause in biodiversity loss. Any alteration of the floodplain dynamics must be avoided to preserve the present diversity of riverine wetlands.

Phenotypic plasticity in response to mechanical stress: hydrodynamic performance and fitness of four aquatic plant species

Plastic responses of plants exposed to mechanical stress can lead to modified, performance-enhancing, morphologies, sometimes accompanied by costs to reproduction. The capacity to present short-term plastic responses to current stress, the resulting performance (expected lower mechanical forces), and the costs of such responses to reproduction were tested for four aquatic plant species. Two ramets of the same genet were submitted to running vs standing water treatment. Traits describing the morphology, hydrodynamic performance and reproduction (sexual and vegetative) were measured. For one species, plastic responses led to reduced hydrodynamic forces, without apparent costs to reproduction, indicating that the plastic response could be beneficial for plant maintenance in stressful habitats. For two species, plastic responses were not associated with variations in performance and reproduction, possibly because of the low hydrodynamic forces experienced, even for morphologies produced under standing conditions. For one species, plastic responses were associated with a sharp decrease in sexual reproduction, without variations in performance, revealing the negative impact of currents over a short time scale. Species maintenance is linked to the capacity of individuals to tolerate mechanical forces. The contrasting responses to currents may be a key element for predicting community dynamics.