Elena Kornyshova
email: elena.kornyshova@cnam.fr

Eric Gressier-Soudan
email: eric.gressier_soudan@cnam.fr

Introducing Sentient Requirements for Information Systems and Digital Technologies

Keywords: Sentient Requirement, Well-being, Information System, Digital Technology, Museum, Visiting Game

Traditionally requirements for Information Systems are considered as functional and non-functional. However, with current omnipresent Digital Technologies, we believe that new requirements dealing with individuals' wellbeing are emerging. We call them sentient requirements using the term from the animal rights protection field. In this paper, we analyze the existing literature to understand better the deep nature of humans' interactions with digital technologies and we introduce sentient requirements. It is based on a literature review including scientific and science-fiction literature. We apply these requirements to improve user experience in museums through a visiting game as a use case. Our proposal could be used by researchers and practitioners to enforce the design of Information Systems in various application fields to provide a better interaction between humans and digital technologies.

Introduction

We are dealing with visitors' experience improvements in museums through serious visiting games. Our goal is to find an accurate way to enrich user experience (UX). Personalization is usually the way this goal is processed. Information and Communication Technologies (ICT) devices are generally supporting this approach. An important step during design and engineering of IS is requirement engineering at the early phases of the development life cycle [START_REF] Davis | Software Requirements: Objects, Functions and States[END_REF]. Thus, the first step was to identify requirements to provide a personalized experience when using UX featured applications and their associated devices.

From our point of view, the personal use of ICT implies a new kind of need dealing with the well-being of people using them. For example, when a visitor tries to use a portable device in a museum and if the screen is not enough responsive because it is too dirty, the device becomes a source of stress, and its initial goal to provide a better experience to visitors cannot be reached. This kind of concern about ICT devices should deal with self-awareness to provide welfare or well-being to users. The same kind of idea can be found in [START_REF] Kreps | Understanding Digital Events: Process Philosophy and Causal Autonomy[END_REF]. It evokes digital and social media technologies can take control upon human consciousness and, by this way, they imply dissatisfaction about these technologies. For example, social networks stimulate depression when "we feel we have to check, but checking does us no good" [START_REF] Kreps | Understanding Digital Events: Process Philosophy and Causal Autonomy[END_REF]. The current and future Information Systems (IS) and supporting devices should be designed, developed, and updated in a way to enhance and especially not to damage humans and their wellbeing.

The requirements are traditionally seen as functional and non-functional. A functional requirement is a "description of a behavior that a software system will exhibit under specific conditions [START_REF] Wiegers | Software Requirements[END_REF], a non-functional requirement (NFR) is a "description of a property or characteristic that a system must exhibit or a constraint that it must respect" [START_REF] Wiegers | Software Requirements[END_REF]. The former type represents the set of functionalities to be provided by ICT devices, and the latter covers additional aspects like security, availability, scalability.

The main research gap addressed in this paper is based on the following statement: the traditional requirements do not cover the need to provide ICT and ICT devices (called globally ICT in the following) dealing with the well-being of their users. This kind of requirement is not functional by nature. Their purpose does not cover the suitable functionalities addressing the application needs. They don't either address nonfunctional requirements which deal with the operationality of the system architecture. This new kind of feature adds an additional dimension to the usual requirements and also impacts a sub-set of traditional functional and non-functional requirements.

In the case of museum visit experience, the literature review did not identify sources expressing this sort of requirements explicitly. To be able to feed our proposal, we took our main inspiration from science-fiction literature and completed it by related literature reviews. As an example, I. Asimov defined the four laws of robotics (the three first ones [START_REF] Asimov | Runaround[END_REF] and later the zeroth law). The Asimov's laws influenced ethic rules for Artificial Intelligence (AI). We have been inspired by the Powers Trilogy of Anne McCaffrey and Elisabeth Ann Scarborough [5][6][7]. This trilogy describes a living planet, Petaybee, which is also depicted by the authors as a sentient planet. Sentience is defined as an ability "of experiencing an affective state" [START_REF] Feinberg | The Ancient Origins of Consciousness: How the Brain Created Experience[END_REF]. This term was especially developed in the field of animals' rights protection [START_REF] Broom | The evolution of morality[END_REF] [START_REF] Broom | Considering animals' feelings: Précis of Sentience and Animal Welfare[END_REF]. The sentience concept matches our approach of the relationship between humans and technologies; thus, we qualified the new family of requirements as "sentient". This term was first developed in the field of animals' rights protection [START_REF] Broom | The evolution of morality[END_REF][10] and is already used in the field of Artificial Intelligence [START_REF] Husain | The Sentient Machine: The Coming Age of Artificial Intelligence[END_REF]. The Sentient Machine is introduced to define the evolution of machines from Artificial Narrow Intelligence to Artificial Generalized Intelligence by A. Husain [START_REF] Husain | The Sentient Machine: The Coming Age of Artificial Intelligence[END_REF]. The goal of our work is to specify sentient requirements for applications interacting with human through ICT devices. Sentient requirements would add an additional view to the design and development of ICT. In a broader acceptance, our goal is to explore what sentience abilities, or sentientness, imply for ICT.

In Section 2, we present our research approach. Section 3 is dedicated to the presentation of identified sentient requirements that are applied to a use case of ICT in the context of a museum to improve user experience. We discuss results in Section 4. Section 5 concludes the paper and gives future directions of our research work. 1. Define the Research Goal. Our initial research goal was to identify requirements for ICT in Museums in order to provide a better, personalized experience to visitors. After a first literature review, we aimed to provide humans with digital technologies and devices that could contribute to their well-being and could help to avoid negative consequences. To reach this goal, theories and practices of design and engineering should be updated to take into account this new dimension which should be integrated very early in the requirements definition step. For the current research, we refined the research goal as: "Is it possible to identify sentient requirements for the well-being of ICT users?" 2. Review Literature. As sentient requirements represent an emerging field, it is difficult to carry out precise research of the literature. Thus, this first step of the literature review was wide. We proceeded by using the following search string: "ICT" AND "Human" on google scholar in January 2021 and we used the backward/forward strategies to complete the study. We have checked the 20 retrieved pages and stopped after 5 pages without relevant results. We included works directly dealing with requirements and those with the associated terms, like "need", for instance. We excluded several works related to our topic but not specifying any requirements-related information. We classified the gathered literature into the following perspectives: philosophical perspective [

2][12][13][14][15][16][17][18]; IT and business perspective [19][20][21][22][23][24][25]; ethical perspective [26][27][28][29]

; and societal perspective [START_REF] Patel | ICT and Human Development: A Global Perspective[END_REF] [START_REF] Ratan | Welfare, agency and "ICT for Development[END_REF]. Then, we carried out the textual analysis of the selected sources and extracted sentences that could be useful to capture the nature of human interactions with ICT with regards to their well-being. In addition, this step allowed us to identify the list of keywords to be used during a more detailed literature review: sentient requirement, well-being requirement, welfare requirement, smart requirement, affective requirement, and synergy requirement.

Identify Sentient Requirements.

For this step, we used the science-fiction literature to feed the list of sentient requirements. As I. Asimov [START_REF] Asimov | Runaround[END_REF] inspired thoughts on Artificial Intelligence, we have been inspired by the Powers Trilogy of Anne McCaffrey and Elisabeth Ann Scarborough [5][6][7]. This trilogy describes a living planet, Petaybee, which is also depicted by the authors as a sentient planet. We extracted from the first book sentences characterizing the sentient planet abilities and its interactions with humans and other beings. Together with the literature collected from the previous steps, we grouped different ideas depending on their nature.

For instance, to formulate the requirement "Interaction and Involvement", we generalized the following retrieved sentences and ideas: (i) The synergy requirement studied with application to interactions between human beings: ICT should be designed to support synergy between human beings. The synergy could be understood as interaction and involvement as it also covers the relationship between human beings and ICT [START_REF] Whitworth | The Social Requirements of Technical Systems, Virtual Communities: Concepts, Methodologies, Tools and Applications[END_REF]; (ii) Human beings and ICT should co-evolve together through time [START_REF] Kreps | Understanding Digital Events: Process Philosophy and Causal Autonomy[END_REF][17] [START_REF] Kreps | Against Nature: The Metaphysics of Information Systems[END_REF]; (iii) Human-Technology symbiosis "defining how humans will live and work harmoniously together with technology" and Human-Environment Interactions referring to "the interaction of people not only with a single artifact, but with entire technological ecosystems featuring increased interactivity and intelligence" [START_REF] Stephanidis | Seven HCI Grand Challenges[END_REF]; (iv) "It[planet]'s only courteous to communicate", "There's a relationship [between the planet and humans] involved", "Living here, most of us know that and accept the gifts, the protection, and in return, we offer it companionship", "I just know that Petaybee works for us, and for itself, in a unique symbiosis…" [START_REF] Mccaffrey | Powers that Be[END_REF]. We proceeded in the same way for the other requirements. The details cannot be present here due to the lack of space. Based on the analysis we did not include several requirements detailed in different perspectives as we consider them belonging to the two other types of requirements (exclusion criterion). For instance, a requirement "ICT should help to learn on technologies" [START_REF] Buchanan | History of technology[END_REF] is a functional requirement as learning objectives are covered by the ICT functionalities. Another example is "ICT should ensure end-to-end encryption" [START_REF]ICT and human rights: an ecosystem approach[END_REF] is typically a non-functional requirement as is it related to the security of data usage.

Validate Sentient Requirements with a Museum Case Study.

To check the applicability of the sentient requirements, we apply them to a role-playing visiting game in a museum.

Emergence of Sentient Requirements

To qualify the new kind of requirements we used the term of sentience. Broom [START_REF] Broom | The evolution of morality[END_REF] defines: "a sentient being is one that has some ability: to evaluate the actions of others in relation to itself and third parties, to remember some of its own actions and their consequences, to assess risk, to have some feelings and to have some degree of awareness". This author underlines different aspects of sentience [START_REF] Broom | Considering animals' feelings: Précis of Sentience and Animal Welfare[END_REF]: the sentience is related to ethics and moral issues; sentient abilities could be acquired and also be lost; the sentience requires "to act in an acceptable way towards each other person and toward each animal that is used"; and effects should be adapted to a given animal, "some components of sentience being dependent upon cognitive ability". [START_REF] Feinberg | The Ancient Origins of Consciousness: How the Brain Created Experience[END_REF] mentions that sentience covers all aspects of sensory consciousness; to be sentient means to be "capable of experiencing an affective state". By [START_REF] Degrazia | Sentience and Consciousness as Bases for Attributing Interests and Moral Status: Considering the Evidence and Speculating Slightly Beyond[END_REF], the capacity to have pleasant or unpleasant experiences "entails having a quality of life or experiential welfare, from which it follows that sentient beings have interests".

Based on the identified from the literature needs for a better consideration of humans in ICT, we define the sentient requirements as follows:

• Interaction and Involvement: ICT should interact and communicate with human.

• Awareness: ICT should be aware of human beings, know and learn about them, and sense/perceive humans. • Safety: ICT should protect from danger and harm and take care of human; the laws of I. Asimov fall in this requirement. • Belongingness and Felicity: ICT should provide a feeling of being comfortable or happy with ICT. • Adaptability: ICT should be adaptable, thus be able to detect distinctive characteristics of each human and to adapt itself to this given individual. • Sustainability: ICT should be able to stand for a long time and to protect the environment and earth ecosystem. • Respectfulness: ICT should be respectful. Respectfulness is very important as it works in both directions: not only ICT should be respectful with humans, but also humans have the responsibility to make ICT ethic, non-dangerous for other beings. • Inclusiveness: ICT should be inclusive and ensure equality between humans, it should be done with regards to different origins, languages, health situation, and more. • Self-Reflectiveness: ICT should learn about itself and contribute to human development by providing additional capabilities and skills.

The sources of sentient requirements are summarized in Table 1.

X X X Awareness X X X X Safety X Belongingness and Felicity X Adaptability X Sustainability X Respectfulness X Inclusiveness X X X X Self- Reflectiveness X X X
We applied the sentient requirements to the case of a role-playing visiting game in a museum based on our experience with museums and the related literature [START_REF] Vassilakis | Stimulation of reflection and discussion in museum visits through the use of social media[END_REF] The purpose of the game is to learn the process of manufacturing an artifact. The visitors can choose between three different roles. The game is provided through touchable tablets to each player. The visitor has a role and follows different steps of the game to build his knowledge. Across the museum, the visitor solves puzzles related to the content encountered corresponding to different resources and manufacturing tasks. In Table 2, we illustrate the sentient requirements expressed for our use case and deduce related functional requirements to show how the sentient requirements could be implemented in practice.

Awareness

The tablet should be able to identify satisfying conditions to be used by a visitor (clean, enough power, functional).

Alarm if it is dirty, compute the amount of available power, detect any misbehavior.

Safety

The tablet should alarm in case of any kind of danger.

Display/broadcast an alarm and personalized instructions to avoid danger in Belongingness and Felicity Users should feel proud of their accomplishment through the game.

Monitor game achievements, stimulate users, and provide incentives during the game.

Adaptability

The tablet should sense emotions and adapt the game to the mood of the user.

Have sensors, compute emotional states, adapt puzzles.

Sustainability

The tablet should optimize network communications to lower radio waves emission.

Compute and adapt radio communication intensity to the appropriate level.

Respectfulness

The tablet should react to inappropriate usage.

Have sensors to detect unexpected movements, bumps, and drops, display/broadcast warnings.

Inclusiveness

The tablet should detect disabilities and adapt the interaction with the visitor (for instance, switch to speech).

Have sensors, detect how the interaction is going on, and provide accurate help.

Self-Reflectiveness

The tablet should help to improve the user experience.

Gather game data, analyze player behavior, enhance gameplay.

Based on this example, we have obtained the following results: (i) We have succeeded to express the identified sentient requirements applied to the visiting game in a museum, and (ii) We have shown that sentient requirements could be translated as a set of functional requirements.

Therefore, our proposal about sentient requirements could be applied to a real case and it provides useful insights to improve user experience design.

Discussion

To illustrate the role of sentient requirements in the ICT field, we suggest to model requirements hierarchy as a pyramid (Figure 2.). This pyramid is built by analogy with the Maslow pyramid of needs [START_REF] Maslow | A theory of human motivation[END_REF]. It shows the importance of sentient requirements and allows to position them in accordance with the existing requirements typology. As in the Maslow's five levels allow to satisfy human needs, we believe that all three levels of sentient requirements should be achieved in order provide mindful ICT. With regards to the Human-Computer Interaction challenges [START_REF] Stephanidis | Seven HCI Grand Challenges[END_REF], four sentient requirements (interaction and involvement, awareness, inclusiveness, and selfreflectiveness) could contribute to develop the five challenges, namely: Human-Technology symbiosis, Human-Environment Interactions, Well-being, Health and Eudaimonia, Accessibility and Universal Access, and Learning and Creativity.

Sentient Requirements Functional Requirements

Non-Functional Requirements

ICT does satisfy system characteristics and other constraints ICT does provide the expected behavior and functions ICT does contribute to human well-being

Considering current trends in ICT, a set of concepts arose recently: digital humanities, Life 3.0, Health 3.0, Welfare Society 2.0 etc. Digital humanities refer to "the intersection of digital technology and humanities: arts, literature, music, dance, theater, architecture, philosophy, and other expressions of human culture" [START_REF] Drucker | Intro to Digital Humanities: Concepts, Methods, and Tutorials for Students and Instructors[END_REF]. Tegmark [START_REF] Tegmark | Life 3.0: being human in the age of artificial intelligence[END_REF] presents the role of Artificial Intelligence can play for human. He defines three stages of Life: Life 1.0 (biological evolution), Life 2.0 (cultural evolution), and Life 3.0 (technological evolution). The last stage (not yet reached) refers to design not only "software" (meaning knowledge used to process the information issued from senses to decide about action, a kind of living "biological software"), but also "hardware", that is to say, the ability of organisms to change their physical components. To attain this future Life 3.0, Artificial Intelligence (AI) will play a particular role and should be friendly. One of the main requirements to the friendly AI is to align goals, thus, to better accomplish human goals using superintelligence. The goals alignment could be done by resolving three problems: "making AI learn our goals, making AI adopt our goals, making AI retain our goals". Within a visionary editorial, Nash [START_REF] Nash | Health 3.0[END_REF] defines Health 3.0 as using social networking technologies to exchange about symptoms, medication, and providing medical care. Misuraca [START_REF] Misuraca | The Future of Welfare Systems: Exploring the role of ICT-enabled social innovation[END_REF] foresees different sociotechnical trends as, for instance, "Care 3.0 -Robotics for personalized care solutions" and calls it Welfare Society 2.0. These trends underline the necessity to go forward with the definition and application of sentient requirements and make us consider the necessity to take into account the goal-oriented dimension into process-relational philosophy underlying our research.

The ICT device is a kind of blurred border between the real world, and the virtual universe. Sensors from the real-world feed actuators in the virtual world and vice versa [START_REF] Natkin | Adaptive Narration in Multiplayer Ubiquitous Games[END_REF]. The device also supplied by the computing resources behind, is the place to get control from the application and where to instantiate the sentient properties. In some way, this concern dives into the research field of Machine Consciousness [START_REF] Rushby | Technology and Consciousness[END_REF]. But machine consciousness is different. It is more a concern of bringing some intelligence to machines including artificial sentience. Instead, we focus on interaction and sentient properties provided by technology to enhance user experience in the digitalized emerging world of everyday life.

Michel Serres, the French philosopher, states that along his history, Humankind lost abilities, but each time it occurred, it was for a greater benefit [START_REF] Serres | L'Innovation et le Numérique[END_REF]. For example, Human "lost" the head with the invention of the computer. But at the same time, the computer raised new opportunities that we now experience through the digital era and the new tools emerging from it. It extended our minds. Michel Serres also states that Humans have completely changed since the mid 20th century. Humankind lost its connection to earth and nature. We do not feel life the same way anymore. Over the world, most people live in cities and this phenomenon seems far from being stopped [START_REF] Serres | Humain et révolution numérique[END_REF]. Our creed is that this statement shows a missing link in mankind approach to the digital era. As we are losing the connection with nature, we need to reinforce the way we sense our environment.

We believe that the introduction of sentient requirements could contribute to the maintenance of the human sovereignty by aligning properties of technologies to the human needs, by creating this kind of harmony between human beings and technologies. For instance, the application of sentient requirements to social medias inducing the FOMO (Fear Of Missing Out) syndrome (discussed in [START_REF] Kreps | Understanding Digital Events: Process Philosophy and Causal Autonomy[END_REF]) could decrease the risk of becoming depressive. Based on infomateriality of [16][32] and close to the relational synergy of [START_REF] Markus | Is IT Changing the World? Conceptions of Causality for Information Systems Theorizing[END_REF], our point of view is that human beings and ICT should coevolve together through time. In line with these works, we claim that symbiosis between human beings and ICT is often foreseen as two opposite sides. This leads us to sentience that we define as a new kind of harmonious co-existence of human beings and ICT through time.

In our view, the specification of sentient requirements would allow to overcome this gap. According to [START_REF] Leidner | What's in a Contribution[END_REF] detailing a five-point assessment of contributions (with regards to theory, method, framing, phenomenon, and composition), we position our contribution mainly on the phenomenon axis as we deal with a new emerging concept (sentient requirements) applied to ICT. Sentience becomes a topic in scientific Information Technology-related publications. We believe that this term reflects well the concept of requirements contributing to building an emotional link between human beings and ICT. Our proposal will contribute also to the method axis, as the sentient requirements will impact the existing methods of requirements engineering and the whole cycle of IS engineering.

However, the introduction of sentient requirements will produce many additional, often new, questions to be solved, as, for example, can ICT and devices be designed to be sentient, especially when we think about robots and smart Internet of Things? Can we expect a smarter and more sustainable life with the digital surroundings we are providing? The way we embrace the future and the relationship with artificial intelligence-based technology is different from transhumanism [START_REF] Bostrom | A history of transhumanist thought[END_REF] where human beings are extended or augmented with ICT parts.

Conclusion and Future Works

We believe that, using the sentient requirements, we can design a peaceful and sustainable digital technology that will help human beings to reach some kind of bliss in the future. IS and digital technologies are not only a tool that serves the consumer economy; there are tools that should gracefully help humans, and their environment including animals and nature to sustain and to enhance. The applications are numerous, we can cite smart healthcare, smart personal devices, smart home applications, drones, entertainment, cultural heritage, industry 4.0, and all fields covered by augmented, mixed, or, in general, extended reality (XR), and so on. We believe that researchers and practitioners who are interested in the design and engineering of IS should take into account the sentient requirements to design more suitable devices. Sentience is a powerful metaphor that deconstructs the way IS are designed and used nowadays.

In our short-term research, we will study how sentient requirements could be implemented to provide personalized usage of digital technologies based on machine learning techniques. In our further research, we intend to experiment sentient requirements with academics and practitioners working on topics around smart life and robotics.

Fig. 1 .

 1 Fig. 1. Research Process.

  [START_REF] Kuflik | An integrative framework for extending the boundaries of the museum visit experience: linking the pre, during and post visit phases[END_REF][37][38][39][40][START_REF] Paliokas | The use of serious games in museum visits and exhibitions: A systematic mapping study[END_REF][42][START_REF] Doran | Creation of a game-based digital layer for increased museum engagement among digital natives[END_REF], etc.

Fig. 2 .

 2 Fig. 2. Requirements Hierarchy Pyramid.

Table 1 .

 1 Sources of Sentient Requirements.

	Sentient Re-	Philosophical	IT and Business	Ethical	Societal	Science-
	quirement	Perspective	Perspective	Perspective	Perspective	fiction
	Interaction and
	Involvement

Table 2 .

 2 Illustration of Sentient Requirements with a Visiting Game in a Museum.

	Sentient Re-	Sentient Requirement Expression	Corresponding Functional Re-
	quirement		quirement(s)
	Interaction and	The tablet should be able to guess	Include a chatbot service.
	Involvement	visitor's unforeseen needs and
		provide answers/guidelines.


Christophe Proisy
email: christophe.proisy@mpl.ird.fr

Nicolas Gratiot
email: nicolas.gratiot@hmg.inpg.fr

Edward J Anthony
email: edward.anthony@univ-littoral.fr

Antoine Gardel
email: antoine.gardel@univ-littoral.fr

Franc -Ois Fromard
email: francois.fromard@cict.fr

Patrick Heuret
email: patrick.heuret@cirad.fr

Mud bank colonization by opportunistic mangroves: A case study from French Guiana using lidar data

Keywords:

Mud bank colonization by mangroves on the Amazon-influenced coast of French Guiana was studied using light detection and ranging (lidar) data which provide unique information on canopy geometry an sub-canopy topography. The role of topography was assessed through analysis of vegetation characteristics derived from these data. Measurements and analyses of mangrove expansion rates over space and time led to the identification of two distinct colonization processes. The first involves regular step-by-step mangrove expansion to the northwest of the experimental site. The second is qualified as 'opportunistic' since it involves a clear relationship between specific ecological characteristics of pioneer Avicennia and mud cracks affecting the mud bank surface and for which probabilities of occurrence were computed from terrain elevations. It is argued from an original analysis of the latter relationship that mud cracks cannot be solely viewed as water stress features that reflect desiccation potentially harmful to plant growth. Indeed, our results tend to demonstrate that they significantly enhance the propensity for mangroves to anchor and take root, thus leading to the colonization of tens of hectares in a few days. The limits and potential of lidar data are discussed with reference to the study of muddy coasts. Finally, the findings of the study are reconsidered within the context of a better understanding of both topography and vegetation characteristics on mangrove-fringed muddy coasts.

Introduction

Mud in the coastal tropics is often associated with luxuriant mangroves [START_REF] Saenger | Mangrove Ecology, Silviculture and Conservation[END_REF]. This is especially the case of the 1600 km-long muddy coast of the Guianas, from Brazil to Venezuela (Fig. 1), where seafront mangroves often attain a height of 40 m and a trunk diameter of 120 cm (e.g. [START_REF] Fromard | Structure, above-ground biomass and dynamics of mangrove ecosystems: new data from French Guiana[END_REF]. Paradoxically, this coast also provides one of the most dramatically changing examples of mud-vegetation interfaces (e.g. [START_REF] Baltzer | Material exchange between the continental shelf and mangrove-fringed coasts with special reference to the Amazon-Guianas coast[END_REF]. Successive erosion and accretion phases due to the drift of mud banks up to 40-km long from the Amazon River to the Orinoco River continuously reshape the physiognomy of the mangrove-fringed coast (e.g. [START_REF] Allison | Sediment exchange between Amazon mudbanks and shore-fringing mangroves in French Guiana[END_REF]. During erosion phases corresponding to 'inter-bank' phases, mangroves are destroyed (e.g. [START_REF] Fromard | Structure, above-ground biomass and dynamics of mangrove ecosystems: new data from French Guiana[END_REF] whereas during 'bank' phases characterized by mud accretion, the mangrove area increases [START_REF] Fromard | Half a century of dynamic coastal change affecting mangrove shorelines of French Guiana. A case study based on remote sensing data analyses and field surveys[END_REF][START_REF] Gardel | Monitoring of coastal dynamics in French Guiana from 16 years of SPOT satellite images[END_REF][START_REF] Gratiot | Trade-wind waves and mud dynamics on the French Guiana coast, South America: input from ERA-40 wave data and field investigations[END_REF]. Perhaps most significantly, an exceptional feature of this coast is that the highest rates of mud bank migration, which attain up to 3 km per year [START_REF] Gardel | A satellite image-based method for estimating rates of mud bank migration, French Guiana, South America[END_REF], do not seem to be a handicap to mangrove regeneration in areas subjected to erosion: hundreds of hectares of muddy coast will be rapidly colonized after the beginning of a new bank phase. Also intriguing are the observations from repetitive field campaigns at the same sites, of nearly instantaneous mangrove germination over tens of hectares. Whenever this phenomenon occurs, the entire colonized area exhibits mud cracks.

Many of the experiments carried out on sediment transport and biogeochemical processes along this coast (e.g. special issue of Marine Geology, volume 208, issue 2-4) were restricted to small areas and were not repeated at the same locations because of the difficulties involved in accessing such a soft and flooded environment. Elevation profiles of several hectometres are the very most that can be feasibly achieved during ground surveys

ARTICLE IN PRESS

doi:10.1016/j.csr.2008.09.017 [START_REF] Lefebvre | Geomorphic evolution of a coastal mudflat under oceanic influences: an example from the dynamic shoreline of French Guiana[END_REF]. The corresponding spatial scale of measurement is insufficient to fully capture the coastal dynamics since mud mobilization and mangrove colonization processes entail large spatio-temporal scales and thus require topographic measurements over large areas and with a resolution on the order of 10 cm (e.g. [START_REF] Lara | Sediment porewater salinity, inundation frequency and mangrove vegetation height in BraganA ˜ §a, North Brazil: an ecohydrologybased empirical model[END_REF]. Such a degree of accuracy is required because elevation differences of a few centimetres can cause different flooding durations which in turn govern mangrove establishment (e.g. [START_REF] Mckee | Seedling recruitment patterns in a Belizean mangrove forest: effects of establishment ability and physico-chemical factors[END_REF][START_REF] Kitaya | Early growth of seven mangrove species planted at different elevations in a Thai estuary[END_REF][START_REF] Lefebvre | Geomorphic evolution of a coastal mudflat under oceanic influences: an example from the dynamic shoreline of French Guiana[END_REF][START_REF] Fiot | Structural effects of tidal exposures on mudflats along the French Guiana coast[END_REF].

Although the intertidal bathymetry of mud banks can be mapped using a time series of optical satellite images [START_REF] Gratiot | Remote sensing based bathymetry on the highly dynamic Amazonian coast[END_REF], the micro-topography of the mud surface cannot be measured with this kind of data. Indeed, monitoring mangrove expansion from satellite images remains qualitative (e.g. [START_REF] Panapitukkul | Mangrove colonization: mangrove progression over the growing Pak Phanang (SE Thailand) mud flat[END_REF][START_REF] Fromard | Half a century of dynamic coastal change affecting mangrove shorelines of French Guiana. A case study based on remote sensing data analyses and field surveys[END_REF]. For example, fine spatial resolution of typically 1 m and additional spectral bands are required to clearly discriminate algal mat response from mangrove tree signatures (e.g. [START_REF] Kutser | Assessing suitability of multispectral satellites for mapping benthic macroalgal cover in turbid coastal waters by means of model simulations[END_REF]. Moreover, as imaging sensors only provide 2D information, vegetation height remains difficult to measure from image analysis (e.g. [START_REF] Proisy | Predicting and mapping mangrove biomass from canopy grain analysis using Fourier-based textural ordination of IKONOS images[END_REF]. It is nonetheless a fundamental parameter related to vegetation growth. For example, spatial observations of mangrove height may highlight when a mud area was colonized provided that mangrove growth is estimated. As demonstrated by [START_REF] Rosso | Use of Lidar to study changes associated with Spartina invasion in San Francisco Bay marshes[END_REF] and [START_REF] Anthony | Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana[END_REF], emerging remote sensing techniques based on lidar (light detection and ranging) may significantly help in studies of intertidal muddy environments. Lidar data can indeed provide simultaneous and detailed information on topography and 3D vertical as well as spatial distribution of forest canopy structure (e.g. [START_REF] Kimes | Predicting Lidar measured forest vertical structure from multi-angle spectral data[END_REF].

The aim of this paper is to assess the interplay between mud bank micro-topography and mud bank colonization by Avicennia germinans. From lidar data acquired over 230 ha of a major Amazon-derived mud bank located in French Guiana, both a digital elevation model (DEM) of the mud surface and a digital canopy height model (DCHM) have been generated. By assuming a constant annual growth of young Avicennia trees, a spatiotemporal analysis has been carried out both to reconstruct the mangrove expansion history over time and to associate temporal probabilities of mud crack formation. The paper also discusses the technical limitations of lidar measurements over mangrovefringed coasts, and the implications of better characterization of mud-vegetation interface in pristine and imperilled or managed mangrove-fringed coasts.

Study area and data collection

The experimental site is the Macouria mud bank located 514 0 N and 52132 0 W, 20 km northwest of Cayenne in French Guiana (Fig. 1). This mud bank has been described in several studies [START_REF] Gardel | A satellite image-based method for estimating rates of mud bank migration, French Guiana, South America[END_REF][START_REF] Fiot | Structural effects of tidal exposures on mudflats along the French Guiana coast[END_REF][START_REF] Gratiot | Trade-wind waves and mud dynamics on the French Guiana coast, South America: input from ERA-40 wave data and field investigations[END_REF][START_REF] Anthony | Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana[END_REF], and only aspects of interest to the present work are reported here.

Changes on the Macouria bank

The leading edge of the Macouria mud bank was singled out as an experimental site in 1996 after an erosion phase that spared the southern part of the area which always consists of a mature mangrove forest (Fig. 2a). Colonization by A. germinans west of the Macouria River occurred between October 2001 and August 2002 (Fig. 2b). Mud east of this river underwent widespread colonization. One year later, in August 2003, the colonization front had progressed both east of the river and north-westward (Fig. 2c). A high infrared response over this period suggests the presence of pioneer vegetation on linear bar-like features formed seaward of the main colonized zone. In 2005, the area had almost completely been colonized as clearly observed on the aerial photograph (Fig. 2d). A Spot image of April 2006 confirms the spread of colonization to the northwest, but shows that the mangrove front had not progressed in the seaward direction (Fig. 2e). The trailing edge of the bank reached the experimental site position in 2006, heralding the beginning of an erosion (inter-bank) phase.

Tide data

The tide is semidiurnal with a spring range of 2.9 m and a neap range of 0.8 m. Mean water level (MWL) and mean high water level (MHWL) are 2.05 and 2.95 m, respectively, whereas mean neap high water level (MNHWL) is 2.6 m [START_REF] Fiot | Structural effects of tidal exposures on mudflats along the French Guiana coast[END_REF]. The harmonic components were provided by the Service Hydrographique et Oce ´anographique de la Marine (SHOM: www. shom.fr).

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Basic characteristics of A. germinans trees

As observed in the field, A. germinans was undoubtedly the dominant species. In comparison, only few Laguncularia racemosa trees were found. In French Guiana, Rhizophora spp. is absent during the colonization phase of shoreface-attached mud banks [START_REF] Fromard | Structure, above-ground biomass and dynamics of mangrove ecosystems: new data from French Guiana[END_REF]. Avicennia species have an epigeal germination with cotyledons that expand and are exposed. In French Guiana, seed production is prolific from January to April and decreases towards July (personal observations). With a width of 2 cm, about 5 days are needed for anchoring. Propagules can reach a height of 15 cm in 20-30 days and the first leaves appear in 20-30 days (Fig. 2f). The buoyancy capability of A. germinans enables the seeds to float for up to 100 days [START_REF] Delgado | The importance of propagule establishment and physical factors in mangrove distributional patterns in a Costa Rican estuary[END_REF]. The majority of both A. marina (Australia) and A. germinans (Belize, Brazil) often become stranded and establish close to the parent plant population but may be dispersed over greater distances by tidal action, as reported by [START_REF] Clarke | Dispersal of grey mangrovenext term (Avicennia marina) propagules in southeastern Australia[END_REF] and [START_REF] Mckee | Seedling recruitment patterns in a Belizean mangrove forest: effects of establishment ability and physico-chemical factors[END_REF], respectively.

Growth rates of young A. germinans trees are not well known. Within the framework of another study dedicated to the architectural analysis of 2-6 m high Avicennia trees, we recorded large disparities in growth rates of more than 50 individuals monitored between December 2003 and April 2005. Maximum values reached 225 cm per year whereas the lowest did not exceed 100 cm per year. Measurements also showed that Avicennia trees did not grow regularly through time and that parent generations could be outgrown by their offspring growing nearby (unpublished data). This great disparity was probably due to both changes in local environmental conditions and competition among the trees. As described hereafter, we propose an alternative method of evaluating growth rates of young Avicennia trees.

Lidar data

The lidar data were acquired over a 1.2 Â 2 km 2 swath of the Macouria mud bank using the ALTOA system (http://www. altoa.fr/), on October 21, 2004 during a low spring tide. The ALTOA system includes a portable Riegl laser rangefinder (LMS6Q140i-60) embarked on a helicopter flying at a speed of about 30 m s À1 about 150 m above the ground. The system also consists of two dual-frequency GPS receivers coupled to an inertial navigation system, both systems ensuring that a subdecimeter differential position can be calculated for the helicopter in post-processing. The rangefinder system is based on the principle of time-of-flight measurement of 30 kHz laser pulses in the infrared wavelength region (0.9 mm). As laser beam divergence is 3 milliradians, the footprint is about of 0.22 m wide on the ground. The lidar dataset consisted of a cloud of laser echoes originating from terrain and vegetation. For each point, latitude, longitude and elevation are measured in metres and stored into a file. The datum used is WGS84 and the projection North UTM zone 22.

Methods

Probabilities of mud cracking

According to [START_REF] Fiot | Structural effects of tidal exposures on mudflats along the French Guiana coast[END_REF], the probability of mud cracks occurring at a given elevation can be estimated from the number of consecutive days of emersion (CDOE). The number of events (NOE) for different CDOEs is computed over a year by comparing the high tide signal to the terrain elevation derived from a lidar-generated elevation model, as described hereafter. The probability is thus tide-dependent and increases rapidly with terrain elevation, as shown in Fig. 3. For elevations that do not exceed 2.4 m above the hydrographic zero, a single mud crack event has only an estimated 10% chance of developing, whereas this probability reaches 100% when terrain elevation reaches 2.6 m. At elevations of 2.7 m or more, over a period of one year, the durations of emersion and the corresponding number of mud crack events would offer good conditions for Avicennia seeds to be trapped. The optimal conditions for seed trapping and colonization are around or below the MHWL. These probabilities are estimated under favorable conditions of drying, and would be reduced to zero under rainy conditions.

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Lidar data processing

DEM computing

A DEM of terrain topography was generated using a kriging interpolation on lowest points and a cell resolution of 2 m (Fig. 4). DEM accuracy is estimated better than 10 cm for elevations over bare areas, whereas it may decrease to 20 cm or more in dense forested areas due to signal interception by the vegetation layer. Dewatering channel networks were digitised from visual DEM interpretation.

For comparisons with the local tide levels, the DEM was calibrated to the hydrographic zero. This was achieved through a calibration transect using a high-resolution total electronic station in March 2003 (Fig. 4; [START_REF] Anthony | Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana[END_REF]. Although lidar data were acquired one and a half years later, it was considered that both profiles can be overlaid using the least squares method. This means that we assumed that elevation change during this 18-month period was not significant enough to be measurable within the 710 cm of accuracy of the DEM. However, mud deposition probably occurred during this interval but no measurements are available to confirm this. Moreover, mangrove trees are not merely passive colonizers of mud banks. They weaken currents capable of eroding the sediments and can continuously replenish their environments. Rates of vertical accretion are variable but commonly reach 5 mm per year (e.g. [START_REF] Saenger | Mangrove Ecology, Silviculture and Conservation[END_REF].

DCHM computing

Laser echoes from vegetation were separated from those originating from the terrain surface using the DEM as the topographic reference. In other words, points above the DEM value of the corresponding cell were classified as vegetation points, and points below this value taken as echoes from the mud surface. To be confident with this classification, we actually used the DEM cell value plus 25 cm as the real soil-vegetation threshold. Basic statistics on elevation of vegetation-classified points above each DEM cell were then computed and stored in additional raster channels. They included typical statistical data (average, median and maximum, etc). In the following text, we name DCHM the raster with maximum values of laser echoes from vegetation. This lidar-derived product is not shown in a specific figure for reasons of redundancy with the map of germination period described in the following paragraph, as shown later in Fig. 6.

Computing germination periods

During the field experiment in March 2003, it was observed that mangroves had just been established over the mud cape feature indicated in Fig. 4. In October 2004, 19 months later, DCHM-median values over the mud cape yielded a height of about 5.1 m (71 m), corresponding to an annual growth rate of 320 cm. A map of germination periods named 'T0M' was derived by computing the ages of colonized cells from their DCHM values. For example, for a cell with a DCHM value of 3.2 m, germination occurred on 21 October, 2003, one year before the lidar flight time and the T0M value was 12 months. Such germination periods should not be taken as exact time frames of the germination process. As explained in the previous section, it should be recalled that the accuracy of such an age estimation decreases with increasing tree age because of changes in environmental conditions and/or different growth rates throughout the life of any individual tree. The focus is on visualizing how the mud bank surface is spatially colonized time after time, in order to gain a better understanding of the role of topography on the mangrove expansion process.

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Results

Mud bank characteristics

Topographic zones

Three major areas of different elevations and hydrologic patterns were discriminated from visual analysis of the DEM (Fig. 4). They were divided into five zones in the following analysis.

The first was the area of mature mangrove which had a significantly higher topographic elevation than the others with a median value of 3.2 m (710 cm), i.e. 50 cm above the other mud surfaces. In the following sections, this area is discarded from the analysis since it was not concerned by on-going mangrove colonization.

The second area corresponded to the network of dewatering channels between the old mangrove and the Macouria River. It comprises of two zones, A (13.5 ha) and B (20.5 ha) at about (713 cm) and 2.8 m (79 cm), respectively (Fig. 5, top graphs). The presence of these drained channels resulted in a rough topographic surface.

In contrast with these drained areas, the third area was mainly characterized by smooth topography. It was located west of the lidar data site between zone B and the sea, but also included the field measurement site. Although the presence of two 500-m long mud bars was indicated by the bright intensity of the DEM (Fig. 4), the network of dewatering channels had not yet developed. Two zones were delineated in this third area on the basis of their elevation, and named C (25.6 ha) and D (22.6 ha). The frontier between these two zones was taken as the seaward side of the central mud bar, and their median elevations were about 2.7 m (77 cm) and 2.6 m (714 cm), respectively (Fig. 5, top graphs).

Mangroves in the low tidal zones A and D

Apart from the presence of a topographic high (hereafter named mud bar #2) at the seaward margin of zone D (Figs. 4 and5, bottom graph), zones A and D were both characterized by the lowest tidal elevations, ranging from 2.5 to 2.6 m. Although they exhibited a similar elevation, these two zones showed contrasting mangrove and soil roughness patterns. Zone A was completely covered by a well-established young mangrove forest (Fig. 5, top graphs; Fig. 6) while zone D was 95% bare (Figs. 5 and6). Median values of DCHM indicated that vegetation height was about 11 m (72 m) for zone A. DCHM values for zone D indicated a very meagre vegetation signal, and only mud bar #2 showed the presence of vegetation, but with a height that did not exceed 40 cm (Fig. 5).

At the elevations of zones A and D, the probability of occurrence of a mud crack event varies from 50% to 100% (Fig. 3). The presence of mangroves in zone A suggests that mud elevation was not the only factor controlling mangrove establishment. This presence moderates the claim by [START_REF] Fiot | Structural effects of tidal exposures on mudflats along the French Guiana coast[END_REF] of the possible existence of a topographic elevation threshold of about 2.4 m below which mangrove cannot take hold. Such differences in mangrove coverage may be explained by the local geomorphic history (Fig. 1). This suggests that the western river bank (zone A) was sheltered due to both consolidation of the eastern part of the mud bank and proximity to the river channel. In contrast, the sea-exposed and bare zone D was formed more recently, and the action of tidal currents and dampened waves is suggested by the smooth topography of this seaward zone. It is to be noted that this zone has remained uncolonized up to 2006 (Fig. 1).

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Mangroves in intermediate and high tidal zones B and C

Contiguous zones B and C exhibited intermediate terrain elevations ranging from 2.7 to 2.8 m (Fig. 5). Mangrove trees were uniformly distributed across zone B (Fig. 6), i.e., up to a distance of 400 m from the starting point of the transect profile (Fig. 5, bottom graph). This latter area showed a decrease in vegetation height from 7 to 3 m and exhibited a rough topography mainly due to the network of dewatering channels. A strongly contrasting colonization pattern was observed in zone C between 400 to 680 m where transect of analysis indicated a smooth soil surface (Fig. 5, bottom graph). Colonization was sparse and characterized by scrubby mangroves not exceeding 2.5 m high, while at a distance of 650 m on mud bar #1, mangrove density increased significantly with tree heights of up to 5 m. The surface of this latter zone was relatively smooth in comparison with zone B (Fig. 5, bottom graph). Terrain elevation exhibited a slight decrease from mud bar #1 (2.9 m) to the junction with C zone (2.6 m). Given only surface roughness and DCHM, it is difficult to interpret such differences in mangrove expansion over identical elevation ranges, especially since conditions for mud crack formation in both zones were deemed to be excellent, ranging from 100% to optimal (Fig. 3). It is therefore necessary to analyse the temporal aspects of mud bank colonization. This is carried out in the following section.

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Mud bank colonization

The T0M map and temporal evolution of the number of CDOE events from October 2002 to October 2003 were computed (Figs. 6 and7, respectively). For reasons of consistency, germination periods of equal duration (8 months), were used to display the colonization process from June 2001 to October 2004. Visualization of T0M, mainly characterized by the red colour (from October 2002 to June 2003), enabled discrimination of the mud bar #1 region. This elongated feature was disconnected in space from the rest of the area that was colonized during the same period (also in red). This spatio-temporal distance, corresponding to the dark grey and yellow areas (June 2003-October 2004) reached 400 m. The lidar data analysis enabled detail characterization in comparison to what was merely suspected from the interpretation of the Spot image (Fig. 2c).

Regular step-by-step colonization towards the northwest

The colonization process began with about 21 ha of terrain colonized in 8 months from June 2001 to February 2002 (blue colour, zone A typically) in the vicinity of both the old mangrove area and the west bank of the Macouria River. Over the next period, from February 2002 to October 2002 (cyan colour), the mangroves progressed by about 14 ha to the northwest. From October 2002 to June 2003 (red colour), colonization further progressed over 28 ha due to this regular spread from the parent area toward the northwest, zone B being a typical example of this pattern. T0M suggested that this type of colonization process could yield high expansion rates since the computation showed a net advance of about 400 m relative to the adult mangrove frontier in the north direction. This colonization pattern mainly followed that of the dewatering channels. From June 2003 to February 2004, mangrove expansion proceeded exclusively from parent areas and gained 28 ha (orange colour). Notable mangrove progradation was also observed on the Macouria River east bank. After February 2004, T0M displayed an important mangrove expansion of about 32 ha (yellow colour). The surface gained was connected to both sources of mangrove seeds, but expansion was not oriented in any particular direction. In October 2004, 61 ha of bare mud still subsisted.

To summarize, the regular northwest colonization is characterized by step-by-step and continuous pioneer mangrove spread from parent areas in a direction parallel to the Chenier coastline shown in Fig. 1. Close to highly reproductive conspecific adults and in sheltered positions, low intertidal areas also have good chances of being colonized. Network channels probably play an important role in the dispersal of buoyant propagules. The northwest colonization process is at the origin of the regular and typical zonation of coastal mangrove formations (e.g. [START_REF] Panapitukkul | Mangrove colonization: mangrove progression over the growing Pak Phanang (SE Thailand) mud flat[END_REF][START_REF] Fromard | Half a century of dynamic coastal change affecting mangrove shorelines of French Guiana. A case study based on remote sensing data analyses and field surveys[END_REF].

Top-down opportunistic colonization

Pioneer colonization of mud bar #1 occurred from October 2002 to June 2003 at an elevation of 2.9 m. During this period,

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Fig. 6. Lidar-generated map of germination periods named 'T0M' (the mature mangrove area is excluded) using a growth constant of 3.2 m per year. long emersion phases of 5 CDOE at least occurred each month (Fig. 7). As floating Avicennia seeds can reach such elevations and need 5 days for anchoring, the probability of germination should become excellent during the large spring tides of February and March, when seed production and chances for mud cracking are at a maximum, i.e., above 7 CDOE (Fig. 7).

Once established in this forward position, mangroves can spread around as observed from June 2003 to February 2004. Although mangrove progression was mainly confined to the mud bar #1 area, colonization spots were observed between this elongated topographic high and the network of dewatering channels (dark grey colour). This area had a median elevation of 2.7 m, i.e. close to the threshold of significant chances of mud crack formation.

From February to October 2004, as stated above, the colonized area (in yellow) was important and linked both regions of propagule dispersion. With a constant growth of 320 cm, we measured an expansion rate reaching 45 ha per year in zone C, whereas the maximum value measured in zone B (regular colonization) did not exceed 20 ha per year. For information purposes, it is worth noting that with an annual growth limited to 150 cm, expansion rates reach 25 and 10 ha per year, respectively.

Mangrove expansion rate cannot increase twofold without the contribution of a typical and visible event that strongly enhances seed trapping. We postulate that this is not a biological event, but rather a physical event related to the mud surface, i.e., the extensive formation of mud cracks in the area of intermediate elevation represented by zone C. The number of emersion events computed for 2004 indeed shows comfortable conditions for mud cracking and for propagule rooting in February, March, April, May, August and September 2004 (Fig. 7; 45 CDOE).

Consequently, we propose the notion of top-down opportunistic colonization, which involves probabilistic mechanisms of colonization by A. germinans. The first mechanism involves mud crack formation at the highest elevations, i.e., elongated mud bars. The second mechanism is probably due to the great number of seeds readily available because of the precocious maturity of Avicennia trees which can become new parents in less than one year. The third mechanism argues for massive trapping of floating propagules by mud cracks. The fourth mechanism is directly related to the formation of mud bars, which serve as veritable forward positions for mangrove dispersal. The opportunistic colonization is top-down because it is initiated from these topographic highs.

Discussion

Limits and potential of lidar data

Lidar technology supplies information on both micro-topography (DEM) and vegetation height (DCHM) in mud bank areas where in-situ field studies are hampered by substrate instability and rapid change. Although lidar provides suitable data for studying forested surfaces in great detail (e.g. [START_REF] Macmillan | Automated analysis and classification of landforms using high-resolution digital elevation data: applications and issues[END_REF], the accuracy of lidar measurement for mapping ground elevation under vegetation canopy needs to be better determined [START_REF] Silvan-Cardenas | A multi-resolution approach for filtering Lidar altimetry data[END_REF][START_REF] Rosso | Use of Lidar to study changes associated with Spartina invasion in San Francisco Bay marshes[END_REF]. Various filtering techniques enable classification of the point cloud into terrain and vegetation points just on the basis of the spatial relationship of the 3D data [START_REF] Sithole | Experimental comparison of filter algorithms for bare-Earth extraction from airborne laser scanning point clouds[END_REF]. Once the terrain hits are correctly classified, the generation of a DEM is rather straightforward using spatial interpolation methods [START_REF] Fortin | Spatial Analysis: A Guide For Ecologists[END_REF]. New generations of full-wave lidar instruments may improve both terrain-vegetation classifications and the accuracy of DEMs and DCHMs (e.g. [START_REF] Wagner | Gaussian decomposition and calibration of a novel small-footprint full-waveform digitising airborne laser scanner[END_REF].

In this work, vegetation height was given by DCHM values of the colonized cells. However, determining tree height using lidar data is not so trivial since the waveform response of a forest is not yet fully understood [START_REF] Gaveau | Quantifying canopy height underestimation by laser pulse penetration in small-footprint airborne laser scanning data[END_REF][START_REF] Lefsky | Geographic variability in Lidar predictions of forest stand structure in the Pacific Northwest[END_REF] and requires additional theoretical simulations (e.g. [START_REF] Sun | Modeling Lidar returns from forest canopies[END_REF][START_REF] Kimes | Predicting Lidar measured forest vertical structure from multi-angle spectral data[END_REF]. Hence, there is a scope for more significant progress from the coupling of 3D physical modeling with 3D architectural description (e.g. using radar measurements, [START_REF] Castel | Deriving forest canopy parameters for backscatter models using the AMAP architectural plant model[END_REF]. In the case of French Guiana mangroves, assessing Avicennia tree architecture and growth is a goal that may provide many benefits in apprehending the functional dynamics of coastal mud bank ecosystems. Within this context, although lidar data acquisitions are still expensive, their use, coupled with field surveys and knowledge of the local tidal signal, may yield fundamental data on the functional attributes of mangrove-fringed coasts.

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Learning from mud bank topography

The results reported here highlight the close relationships between the spatial dynamics of mud bank topography and the capability of mangrove establishment. This work has shown that, in French Guiana, A. germinans grows between MWL and MHWL. This could be a specific functioning range for the whole Avicennia family since [START_REF] Clarke | The intertidal distribution of te grey mangrove (Avicennia marina) in southeastern Australia: the effects of physical conditions, interspecific competition, and predation on propagule establishment and survival[END_REF] indicated the same intertidal range for Avicennia marina in southeastern Australia. The present study is a significant contribution to the understanding of this elevation criterion. The intertidal range of Amazon mud banks can be decomposed into two topographic domains. The lower part stands between MWL and MNHWL. Mud bank colonization can occur providing that the area is sheltered from currents. In such areas, mangrove expansion is regular (and somewhat inexorable), occurring step by step from the parent area. As stated earlier, this result shows that Avicennia may take hold below an elevation threshold of 2.4 m. Our results confirm the absence of mangrove below 2.4 m only for advanced seaward positions, where the hydrodynamic regime results in a smooth mud surface with little potential for mud crack development, and limits the success of buoyant propagule anchoring.

Between MNHWL and MWL, the rate of regular mangrove expansion increases. But, it would seem that this rate is not enough for opportunistic floating of Avicennia propagules. In comparison propagules of competing Laguncularia species, which may be qualified as 'sinkers' since they loose their buoyancy in a few days, propagules of A. germinans are 'floaters'. This therefore increases the chances of long-distance dispersal and stranding in zones of high elevation [START_REF] Delgado | The importance of propagule establishment and physical factors in mangrove distributional patterns in a Costa Rican estuary[END_REF]. In contrast, A. germinans propagules may have more difficulties becoming established in the lower intertidal zone due to their easy resuspension by tidal currents. As explained by [START_REF] Delgado | The importance of propagule establishment and physical factors in mangrove distributional patterns in a Costa Rican estuary[END_REF], the high number of seeds produced is a way of increasing the chances of assuring at least a minimum number of adult individuals per reproductive period, in an ecosystem where edaphic factors are unfavorable for seedlings during their early stage. In French Guiana, their strategy in colonizing the intermediate and upper tidal zones of Amazon mud banks involves three steps: (1) attainment of an elevated position in the seaward direction, i.e., a macroscale linear bar-like feature, (2) rapid development towards maturity and (3) spread of numerous propagules on the back-bar areas behind the elongated topographic high. Within this context, it would be of significant interest to quantify the proportion of propagules that become established on mud bars and that are transported by the ocean from mangroves shoreward of these bars.

Learning from mud cracks

The present study reinforces the work of [START_REF] Fiot | Structural effects of tidal exposures on mudflats along the French Guiana coast[END_REF] on the probabilities of mud cracking as a function of terrain elevation and confirms the mechanical role of mud cracks in mangrove propagule trapping. A spatio-temporal analysis of mangrove expansion could not have been achieved if probabilities of desiccation for different topographic elevation had not been computed.

Mud cracks have been considered in the past as water stress features that reflect desiccation potentially harmful to plant growth (e.g. [START_REF] Clarke | Dispersal of grey mangrovenext term (Avicennia marina) propagules in southeastern Australia[END_REF][START_REF] Mckee | Acute salt marsh dieback in the Mississippi River deltaic plain: a drought-induced phenomenon?[END_REF]. However, our study has shown that these features play a major role in fostering rapid and extensive mangrove colonization. As observed in the field, they trap opportunistic Avicennia seeds and promote rapid propagule anchorage over mud bars by reducing uprooting due to waves or tidal currents. Their formation strongly varies with the tidal signal from year to year (Fig. 7). In French Guiana, this phenomenon is mainly achieved during the equinox tides of spring and autumn which coincide with the period of proliferation of Avicennia propagules. Further field work should evaluate the proportion of Avicennia seeds that are actually trapped in ephemeral mud cracks and investigate the differences in propagule dispersal between flooding and dewatering periods.

These mud cracks also constitute a typical example of interplay effects between sediments and mangroves. As pointed out by [START_REF] Anthony | Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana[END_REF] in a study dedicated to the topography of the Macouria bank, long linear wave-formed mud bars may have a fundamental influence in this interplay by providing low-energy back-bar areas where fresh mud is deposited, thus increasing the surface area liable to mud crack formation. If this postulate turns out to be correct, it will provide a valuable explanation of field observations such as those presented in Fig. 5.

Lessons from pristine coasts

Rehabilitation of imperilled mangrove coasts (e.g. [START_REF] Lewis | Ecological engineering for successful management and restoration of mangrove forests[END_REF] could also benefit from the hitherto unsuspected role of mud cracks highlighted by our study of pristine mangroves on the Amazon-influenced coast of French Guiana. By providing markers ready to be colonized, mud crack areas can be directly targeted as zones where the chances of successful mangrove replanting aimed at rehabilitation are highest. A simple experimental approach consisting in sowing, from a boat, mangrove seeds over mud crack areas may yield insight into the rehabilitation potential. Although there are numerous types of mangrove forests in a wide variety of topographic settings, such a simple test is a very low-cost one in comparison to tedious manual planting of propagules. Because they are desiccation features, mud cracks are indicative of mud dewatering conditions that can be stressful for vegetation growth. This ambivalent propensity to foster mangrove growth, as in French Guiana, or to reflect environmental conditions that could hinder plant growth, highlights the importance of the relationship between wetting by tidal flooding and mangrove species.

Mangrove establishment on mud banks will depend on biological characteristics such as buoyancy, stomatal conductance, assimilation rate, etc. (e.g. [START_REF] Delgado | The importance of propagule establishment and physical factors in mangrove distributional patterns in a Costa Rican estuary[END_REF][START_REF] Kitaya | Early growth of seven mangrove species planted at different elevations in a Thai estuary[END_REF][START_REF] Ye | Does sea level rise influence propagule establishment, early growth and physiology of Kandelia candel and Bruguiera gymnorrhiza[END_REF]. Consequently, in a mangrove rehabilitation programme, the selection of species must be appropriate in order to be able to follow up the impact of topography and the probabilities of desiccation.

Conclusions

Mangrove sediments have a unique history at any individual site which, when properly apprehended, can provide significant insights into general patterns of mangrove ecosystem functioning [START_REF] Saenger | Mangrove Ecology, Silviculture and Conservation[END_REF]. This is the case of Amazon-influenced coasts which offer intriguing mud-vegetation interfaces with a considerable potential for ground-breaking interdisciplinary research. The work reported here has called for interdisciplinary gap-bridging involving sedimentology, geomorphology, hydrology, mangrove ecology and remote sensing. It is our strong belief that the opportunistic behaviour of Amazon mangroves is hinged on the formation, by waves, of mud bars over mud banks. This may well be the critical factor that explains the strong resilience of French Guiana mangroves in the face of drastic coastal changes induced by mud bank migration from the Amazon to the Caribbean.

Fig. 1 .

 1 Fig. 1. Location of the study site.

Fig. 2 .

 2 Fig. 2. Temporal evolution of the Macouria experimental site from 2001 to 2006 based on IGN aerial photographs (a, d) and Spot images (b, c, e). The lidar coverage is also indicated. Development of an A. germinans seedling (f) (courtesy of M. Herteman).

Fig. 3 .

 3 Fig. 3. Statistical distribution of emersion events of a given duration in relation to mud bank elevation (after[START_REF] Fiot | Structural effects of tidal exposures on mudflats along the French Guiana coast[END_REF]. Emersion periods consist of 3, 5 and 7 consecutive days of emersion (CDOE).

Fig. 4 .

 4 Fig. 4. Lidar-generated DEM. Calibration transect, dewatering channels and observation point are also shown.

Fig. 5 .

 5 Fig. 5. Percentages of bare mud surface versus colonized mud surface for the four zones A, B, C and D (top graph). Laser echoes from vegetation and DEM values along the transect of analysis across zones B, C and D (middle graph). Illustration of mud cracking over tens of hectares colonized by even-aged Avicennia trees, 7 October, 2004, 3 km north of the study site, Macouria bank. The primary phase of colonization, i.e. the tallest trees, is observed on the elongated mud bar at about 600 m in the seaward direction.

Fig. 7 .

 7 Fig. 7. High tide level and the number of consecutive days of emersion (CDOE) from October 2002 to October 2004. The range 2.7-2.8 m of mud elevation observed in zone C is also shown.

Acknowledgements

This research was funded by Grants provided by the Programme National Environnement Co ˆtier (PNEC)-French Guiana Experimental Site and the French Guiana Contract 'Plan Etat-Re ´gion', under the auspices of the IRD, Centre de Cayenne. We thank M. Gue ´roult, F. Lokonadinpoulle ´, M. Laurans, L. Polidori, J. L. Smock and M. Tarcy for aid in the field. We are also grateful to Lisa-Maria Rebelo for her careful reading of the submitted manuscript. We also thank the ALTOA team for their patience and help. The SPOT images were provided by the station SEAS-Guyane & CNES/SPOT image 2006 and aerial photographs by IGN.

AMAP (Botany and Computational Plant Architecture) is a joint research unit in which are associated CIRAD(UMR51), CNRS(UMR5120), INRA (UMR931), IRD(R123), and Montpellier University (UM27); http://www.amap.cirad.fr.