Littérature scientifique sur le sujet « Vegetation uprooting »

Abstract. The establishment of riparian pioneer vegetation is of crucial importance within river restoration projects. After germination or vegetative reproduction on river bars juvenile plants are often exposed to mortality by uprooting caused by floods. At later stages of root development vegetation uprooting by flow is seen to occur as a consequence of a marked erosion gradually exposing the root system and accordingly reducing the mechanical anchoring. How time scales of flow-induced uprooting do depend on vegetation stages growing in alluvial non-cohesive sediment is currently an open question that we conceptually address in this work. After reviewing vegetation root issues in relation to morphodynamic processes, we then propose two modelling mechanisms (Type I and Type II), respectively concerning the uprooting time scales of early germinated and of mature vegetation. Type I is a purely flow-induced drag mechanism, which causes alone a nearly instantaneous uprooting when exceeding root resistance. Type II arises as a combination of substantial sediment erosion exposing the root system and resulting in a decreased anchoring resistance, eventually degenerating into a Type I mechanism. We support our conceptual models with some preliminary experimental data and discuss the importance of better understanding such mechanisms in order to formulate sounding mathematical models that are suitable to plan and to manage river restoration projects.

Abstract. The establishment of riparian pioneer vegetation is of crucial importance within river restoration projects. After germination or vegetative reproduction on river bars juvenile plants are often exposed to mortality by uprooting caused by floods. At later stages of root development vegetation uprooting by flow is seen to occur as a consequence of a marked erosion gradually exposing the root system and accordingly reducing the mechanical anchoring. How time scales of flow-induced uprooting do depend on vegetation stages growing in alluvial non-cohesive sediment is currently an open question that we conceptually address in this work. After reviewing vegetation root issues in relation to morphodynamic processes, we then propose two modelling mechanisms (Type I and Type II), respectively concerning the uprooting time scales of early germinated and of mature vegetation. Type I is a purely flow-induced drag mechanism, which causes alone a nearly instantaneous uprooting when exceeding root resistance. Type II arises as a combination of substantial sediment erosion exposing the root system and resulting in a decreased anchoring resistance, eventually degenerating into a Type I mechanism. We support our conceptual models with some preliminary experimental data and discuss the importance of better understanding such mechanisms in order to formulate sounding mathematical models that are suitable to plan and to manage river restoration projects.

Riverine ecosystem biodiversity is largely maintained by ecogeomorphic processes including vegetation renewal via uprooting and recovery times to flow disturbances. Plant roots thus heavily contribute to engineering resilience to perturbation of such ecosystems. We show that vegetation uprooting by flow occurs as a fatigue-like mechanism, which statistically requires a given exposure time to imposed riverbed flow erosion rates before the plant collapses. We formulate a physically based stochastic model for the actual plant rooting depth and the time-to-uprooting, which allows us to define plant resilience to uprooting for generic time-dependent flow erosion dynamics. This theory shows that plant resilience to uprooting depends on the time-to-uprooting and that root mechanical anchoring acts as a process memory stored within the plant–soil system. The model is validated against measured data of time-to-uprooting of Avena sativa seedlings with various root lengths under different flow conditions. This allows for assessing the natural variance of the uprooting-by-flow process and to compute the prediction entropy, which quantifies the relative importance of the deterministic and the random components affecting the process.

The aquatic vegetation and physicochemistry of Piccaninnie Ponds are described and recent annual losses of aquatic vegetation investigated. The aquifer-derived waters of the Ponds are characterized by their clarity, low nutrient content, low salinity, and lack of thermal and chemical stratification. In 1985, large areas of aquatic vegetation within the Ponds degraded and were lost. Subsequent faster regeneration of denuded areas by Ruppia polycarpa resulted in the displacement of Lepilaena cylindrocarpa. The annual uprooting of R. polycarpa, which has occurred since, results from Ruppia's comparatively poorer anchorage capacity in the loose sediment floc. Gradual expansion of L. cylindrocarpa into freshly uprooted regions restricts the regrowth of R. polycarpa and hence the area susceptible to denudation in the following year. It is expected that the displacement of R. polycarpa will continue until only small isolated stands remain, which will be prevented from uprooting by the root matrix of surrounding vegetation.

Introduction: Isla del Coco is the only island in the Eastern Tropical Pacific with humid tropical forests; 296 plant species are reported, of them, 22% are endemic. Their ecology is poorly understood. Deforestation and the introduction of rats, feral pigs and white-tailed deer are the primary agents of forest degradation. After more than 120 years, the deforested areas have never recovered the native forest. Objective: To analyse if the deforested area keeps its resilience, we evaluated the natural regeneration and ecological processes associated. Methods: From August 2016 to June 2018, we conducted a restoration experiment consisting of a randomized complete blocks design including vegetation cutting, vegetation uprooting and controls as treatments. Plots were protected with an exclusion fence to avoid herbivores. Results: There were no differences between plant cutting and uprooting in stimulating natural regeneration. We only recorded the seedlings of two tree species, 35 individuals of Cecropia pittieri and three of Sacoglottis holdridgei, both endemic. Their regeneration established during the first 15 months mainly. At the end of the experiment, the structure and composition of the vegetation changed from bushes dominated by Entada gigas (28%) and Clidemia strigillosa (12%) to grasses dominated by Paspalum conjugatum (39%). Entada gigas has a high recolonizing potential with a growing rate of 1.6±0.2m/month. Conclusions: As filters for restoration we determined herbivores, which pose a strong negative impact in the development of the forest; the exhausted seed bank of tree species and scarce or null seed dispersion.

Vegetation controls sediment dynamics and affects the kinematic characteristics of flow in rivers. The uprooting mechanism is strongly affected by mechanical properties, morphology and branching of the roots system. This work presents preliminary results of experimental work conducted in a laboratory meandering flume. The work aims to investigate how the geometrical and mechanical characteristics of the roots of a real, flexible and mature vegetation could vary along the bend. Results show that both the geometrical and the mechanical characteristics of the roots assume higher/lower values in peculiar sections of the bend suggesting that they could be affected by the kinematic characteristic of flow.

In the last decades, the presence of riparian vegetation on riverbanks and floodplains along rivers was acknowledged not only to improve water quality and heal biological diversity but also to contribute to river evolution processes. When water flow runs over vegetated areas, averaged velocity profile is affected by the presence of stem, branches and leaves, sediment transport changes according to modified turbulence and bed shear stresses and soil shear strength is altered by root binding. As a result, bed scour, bank erosion and accretion, bar migration and width adjustment processes lead to different river morphology evolution. Conversely, flow and sedimentary patterns influence vegetation dynamics, by shaping barebed deposits available for colonisation and by affecting mortality rate, through burying and uprooting processes. However, whereas recruitment, establishment and growth represent the transitional dynamics from barebed to vegetated conditions and are mainly related to species properties, plant removal is intrinsically related to species growth stage, flow magnitude and soil properties. Although vegetation uprooting only recently gathered attention from scientific community, there is a rising awareness that vegetation removal is crucial for species selection and location on exposed deposits and floodplains and carbon production and sequestration, at different spatial and temporal scales. This PhD work examines the uprooting process of both pioneer seedlings and established vegetation driven by flow and bed erosion, whose role is to reduce root anchorage, at various spatial scales ranging from a single plant to a river reach. The main purpose of this research is to illustrate the links between temporal scales regarding the hydro-morphological evolution of fluvial systems, such as bed scour development, flood duration and return period, and those proper of biological components with regards to both growth and decay rates of riparian vegetation. For this aim, various methodologies and approaches are followed. Firstly, an intensive analysis of the state of knowledge is presented and discussed. Secondly, the existence of links between growth rate and hydrological return period of flood events and between decay rate and flood duration are proved and investigated by means of an already available eco-morphodynamic equation, and illustrated according to different vegetation cover. Promising results are obtained when relationships are applied to the case studies of the Maggia River (CH) and the Tagliamento River (IT). Thirdly, the uprooting process of juvenile flexible riparian vegetation is investigated by means of flume experiments, field measurements of root resistance and numerical modelling. A new physical relationship able to predict critical conditions of bed shear stress and bed erosion is derived, validated and applied to the case study of the Ombrone Pistoiese River (IT) with good agreement. Lastly, the proposed relationship is combined to a very recent probabilistic model and a stochastic approach to flood events. A new relationship for uprooting process randomness is proposed and the correlation between vegetation removal and flood return period is evaluated, discussed and applied to the case study of the Santa Maria River, Arizona (USA) with very good results. The results of this PhD research show the existence of cross-related temporal scales between riparian vegetation and river morphodynamics and demonstrate their relationships with flood return period and event duration. The adapted comprehensive approach to study the uprooting process of riparian vegetation highlights that multidisciplinary methodology is essential to understand the mechanisms, correctly model the process and formulate the equations. The application to laboratory experiments and to various case studies proves the validity of the relationships as well as the applicability both to small and large spatial scales. As a final result, this research hints the capability for river to select species and cover according to hydrological regime and biological properties. This is crucial in fluvial environments altered by climate change, where alien species may replace native ones. It also underlines the importance of taking into account riparian vegetation dynamics, effects and interactions to guarantee the reliability of long-term river morphodynamics modelling and the success of river maintenance and restoration strategies.