Sara Puijalon

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.

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.

Aquatic interfaces: a hydrodynamic and ecological perspective

ABSTRACT Ecologically-appropriate management of natural and constructed surface water bodies has become increasingly important given the growing anthropogenic pressures, statutory regulations, and climate-change impacts on environmental quality. The development of management strategies requires that a number of knowledge gaps be addressed through interdisciplinary research efforts particularly focusing on the water-biota and water-sediment interfaces where most critical biophysical processes occur. This paper discusses the current state of affairs in this field and highlights potential paths to resolve critical issues, such as hydrodynamically-driven mass transport processes at interfaces and associated responses of organisms through the development of traits. The roles of experimental methods, theoretical modelling, statistical tools, and conceptual upscaling methods in future research are discussed from both engineering and ecological perspectives. The aim is to attract the attention of experienced and emerging hydraulic and environmental researchers to this research area, which is likely to bring new and exciting discoveries at the discipline borders.

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.

Phenotypic plasticity and mechanical stress: biomass partitioning and clonal growth of an aquatic plant species

Mechanical stresses from wind, current or wave action can strongly affect plant growth and survival. Survival and distribution of species often depend on the plants capacity to adapt to such stresses, particularly when amplified by climatic variations. Few studies have dealt with plastic adjustments in response to mechanical stress compared to resource stress. We hypothesized that mechanical stress should favor plastic adjustments that result in increased biomass production in zones protected from the stress and that altered growth patterns should be reversible after mechanical stress removal. Here we measured plastic adjustments in morphological traits and clonal architecture for an aquatic clonal species (Berula erecta) under two contrasting mechanical stresses in the field-standing vs. running water. Reversion of the morphological changes was then assessed using transplants in standing water. In the case of mechanical stress, size reduction, biomass reallocation within clones (higher allocations to clonal growth and to belowground organs), and a more compact growth form (reduced spacer lengths) contributed to reducing the damage risk. The removal of mechanical stress induced compensatory growth, probably linked to the production of low density tissues. However, most patterns of dry mass partitioning induced by current stress were not reversed after stress removal.

Evidence for biological denitrification inhibition (BDI) by plant secondary metabolites.

Previous studies on the effect of secondary metabolites on the functioning of rhizosphere microbial communities have often focused on aspects of the nitrogen (N) cycle but have overlooked biological denitrification inhibition (BDI), which can affect plant N-nutrition. Here, we investigated the BDI by the compounds of Fallopia spp., an invasive weed shown to be associated with a low potential denitrification of the soil. Fallopia spp. extracts were characterized by chromatographic analysis and were used to test the BDI effects on the metabolic and respiratory activities of denitrifying bacteria, under aerobic and anaerobic (denitrification) conditions. The BDI of Fallopia spp. extracts was tested on a complex soil community by measuring denitrification enzyme activity (DEA), substrate induced respiration (SIR), as well as abundances of denitrifiers and total bacteria. In 15 strains of denitrifying bacteria, extracts led to a greater BDI (92%) than respiration inhibition (50%). Anaerobic metabolic activity reduction was correlated with catechin concentrations and the BDI was dose dependent. In soil, extracts reduced the DEA/SIR ratio without affecting the denitrifiers: total bacteria ratio. We show that secondary metabolite(s) from Fallopia spp. inhibit denitrification. This provides new insight into plant-soil interactions and improves our understanding of a plants ability to shape microbial soil functioning.

Comparison of three life history traits of invasive Elodea canadensis Michx. and Elodea nuttallii (Planch.) H. St. John.

Abstract Three life-history traits, regeneration and colonisation of vegetative plant fragments, resistance to water current and palatability to a generalist herbivore were compared in two invasive macrophyte species, Elodea canadensis Michx. and Elodea nuttallii (Planch.) H. St. John. Few differences in traits were found between these species. Both species showed similar resistance to water current, whilst fragment regeneration and colonisation and palatability were only slightly higher in E. nuttallii than in E. canadensis. These small differences do not explain the displacement of E. canadensis by E. nuttallii as observed in the field.

Effects of Wind Waves versus Ship Waves on Tidal Marsh Plants: A Flume Study on Different Life Stages of Scirpus maritimus

Recent research indicates that many ecosystems, including intertidal marshes, follow the alternative stable states theory. This theory implies that thresholds of environmental factors can mark a limit between two opposing stable ecosystem states, e.g. vegetated marshes and bare mudflats. While elevation relative to mean sea level is considered as the overall threshold condition for colonization of mudflats by vegetation, little is known about the individual driving mechanisms, in particular the impact of waves, and more specifically of wave period. We studied the impact of different wave regimes on plants in a full scale flume experiment. Seedlings and adult shoots of the pioneer Scirpus maritimus were subjected to two wave periods at two water levels. Drag forces acting on, and sediment scouring occurring around the plants were quantified, as these are the two main mechanisms determining plant establishment and survival. Depending on life stage, two distinct survival strategies emerge: seedlings present a stress avoidance strategy by being extremely flexible, thus limiting the drag forces and thereby the risk of breaking. Adult shoots present a stress tolerance strategy by having stiffer stems, which gives them a higher resistance to breaking. These strategies work well under natural, short period wind wave conditions. For long period waves, however, caused e.g. by ships, these survival strategies have a high chance to fail as the flexibility of seedlings and stiffness of adults lead to plant tissue failure and extreme drag forces respectively. This results in both cases in strongly bent plant stems, potentially limiting their survival.

Herbivory mediated by coupling between biomechanical traits of plants and grasshoppers

Summary 1. Despite their potential to provide a mechanistic understanding of ecosystem processes, the functional traits that govern interaction networks remain poorly understood. We investigated the extent to which biomechanical traits are related to consumption in a plant–grasshopper herbivory network. 2. Using a choice experiment, we assessed the feeding patterns of 26 grasshopper species for 24 common plant species from subalpine grasslands. We quantified shear and punch toughness for each plant species, while grasshopper incisive and molar strengths were estimated by a lever mechanics model, following the measurement of mandibular traits. 3. Models incorporating co-phylogenetic effects showed that the ratio between the grasshopper incisive strength and plant toughness, that reflects the cutting effort, is correlated with the mass of plant eaten. Moreover, a strong relationship between the incisive strength of the grasshoppers and the weighed mean toughness of the plants they eat was found. 4. Our results suggest that biomechanical constraints imposed by plants influence the evolution of grasshoppers’ mandibular traits. Such scaling relationships offer promising avenues towards the understanding of trait – function links in interaction networks.

Clonal Plasticity of Aquatic Plant Species Submitted to Mechanical Stress: Escape versus Resistance Strategy

BACKGROUND AND AIMS The plastic alterations of clonal architecture are likely to have functional consequences, as they affect the spatial distribution of ramets over patchy environments. However, little is known about the effect of mechanical stresses on the clonal growth. The aim of the present study was to investigate the clonal plasticity induced by mechanical stress consisting of continuous water current encountered by aquatic plants. More particularly, the aim was to test the capacity of the plants to escape this stress through clonal plastic responses. METHODS The transplantation of ramets of the same clone in two contrasting flow velocity conditions was carried out for two species (Potamogeton coloratus and Mentha aquatica) which have contrasting clonal growth forms. Relative allocation to clonal growth, to creeping stems in the clonal biomass, number and total length of creeping stems, spacer length and main creeping stem direction were measured. KEY RESULTS For P. coloratus, plants exposed to water current displayed increased total length of creeping stems, increased relative allocation to creeping stems within the clonal dry mass and increased spacer length. For M. aquatica, plants exposed to current displayed increased number and total length of creeping stems. Exposure to current induced for both species a significant increase of the proportion of creeping stems in the downstream direction to the detriment of creeping stems perpendicular to flow. CONCLUSIONS This study demonstrates that mechanical stress from current flow induced plastic variation in clonal traits for both species. The responses of P. coloratus could lead to an escape strategy, with low benefits with respect to sheltering and anchorage. The responses of M. aquatica that may result in a denser canopy and enhancement of anchorage efficiency could lead to a resistance strategy.