nvestigating the influences of tidal inundation and surface elevation on the establishment and early development of mangroves for application in understanding mangrove rehabilitation techniques 1 3

2.2.1 The evolution of coastal wetlands restoration Early coastal wetland restoration efforts focused on mitigating problems in isolation, without consideration for the connectivity of

Chapter 2 – Literature Review

2.1 The roots of restoration science

The concept of restoration was considered as early as the 19th Century, and the first well-known attempt to restore land began when Aldo Leopold launched the Curtis Prairie project in Wisconsin in 1934 (Palmer et al., 2014) Despite those early efforts,

it was only in the 1980s that ecological restoration entered mainstream scientific thought, evident by the founding of the Society for Ecological Restoration in 1987 (SER, 2015) Restoration ecology is a rapidly growing science, with an exponential growth in research publications appearing across a broad array of scientific journals (Palmer et al., 2014) While restoration ecology is largely an applied science carried out in the field, the theoretical basis of restoration ecology is firmly rooted in classical ecology

To understand the potential for the restoration of an ecosystem, there exists strong emphasis on understanding (i) what factors enhance the restoration of biodiversity (Rodrigues et al., 2011), (ii) the roles of physical habitat heterogeneity in the rate and degree of recovery (Holl et al., 2013), (iii) the use of resilience theory (state changes and feedbacks) (Suding et al., 2004), (iv) spatial ecology (i.e ecosystems connectivity

in relation to dispersal dynamics and metapopulation theory) and (v) the landscape context encompassing the restoration site (Reynolds et al., 2013)

As the concept of ecosystem services gains acceptance by policy makers, restoration managers and funders have shifted the focus of restoration to a pre-disturbance condition, to a state which can provide ecosystem services (Benayas et al., 2009) This translates into a change in the type of science demanded from restoration ecologists

underlying restoration, aiming to understand the functional roles of species in supporting ecosystem processes or products of value to humans (Montoya et al., 2012)

2.2 The restoration of coastal wetlands

Anthropogenic changes in rivers, estuaries and coastal landscapes, accumulated over time, mean that few coastal wetlands remain untouched or have retained complete historical ecosystem functions (Palmer et al., 2014) One of the best examples is the management of river flows where dams have replaced natural hydrological pulsing of water with large and, for most part, uncontrollable flooding events Similar smaller scale yet cumulative effects that affect wetlands have been caused by mining waste, land use change and urbanisation (Simenstad et al., 2006) Restoration initiatives have been on the rise as society attempts to reverse ecosystem degradation for the improvement of future conditions over current state (Reed et al., 1997; Purcell et al., 2002; Milano et al., 2007)

2.2.1 The evolution of coastal wetlands restoration

Early coastal wetland restoration efforts focused on mitigating problems in isolation, without consideration for the connectivity of the ecosystem to the larger landscape (Simenstad et al., 2006)

Recent restoration projects have evolved to include a scale of planning, design and implementation to achieve functional, self-sustaining restoration through the understanding and reinstating of fundamental ecosystem processes on a landscape scale The approach advocates setting clear and realistic restoration goals, monitoring restoration responses and adopting a precautionary, adaptive approach (Simenstad et al., 2006) A core guiding principle is that removal of barriers will direct the

restoration of physical, geochemical, ecological and other ecosystem processes towards a more natural state (Simenstad et al., 2006) The progression of ecosystem recovery over time, referred to as “restoration trajectories” (Hobbs & Norton, 1996), has been characterised as a pathway of ecosystem redevelopment toward a less compromised, or even the attainment of a fully functioning system that proves comparable to “target” reference sites (Figure 2.1; Aronson et al., 1993) Yet, there exists wide variation in response patterns and rates of such trajectories due to variability in restoration approaches, the types and levels of stressors, antecedent conditions, and changes in the landscape setting (Hobbs & Harris, 2001)

Figure 2.1: A traditional view of restoration options for a degraded system, illustrating the idea that the system may proceed along different trajectories and that the goal of restoration is to

guide the trajectory towards some desired state Source: Hobbs & Norton, 1996

2.2.2 Approaches in coastal wetland restoration

There are three basic approaches to ecosystem recovery: passive, active, and creation

(Simenstad et al., 2006) In passive approaches, barriers to natural recovery are removed to allow the reinstatement of pre-degraded ecosystem processes No additional actions are required in facilitating restoration For example, studies evaluating ecosystem responses to removal or breaching of levees surrounding a former wetland often report how flooding events reinstate ecosystem processes such as sedimentation rate (Simenstad & Warren, 2002) Active approaches are achieved through planned actions that specifically re-create wetland structure and processes It usually involves the removal of barriers hampering natural recovery and active management of ecosystem processes (Simenstad et al., 2006) For example, re-establishing tidal hydrology in a drained and levelled estuarine wetland might involve both digging channels to encourage tidal channel development and plantings to promote re-colonisation of native marsh vegetation Creation is the establishment of wetlands where none previously existed (Simenstad et al., 2006) The restoration concept and design will be referenced from wetlands elsewhere but such efforts usually achieve the creation of structure rather than the natural process of ecosystem function

2.3 Adopting the Ecological Mangrove Rehabilitation (EMR) approach

The EMR paradigm largely mirror the active approach used in the restoration of coastal ecosystems (Section 2.2.2) EMR was first published as a presentation abstract

at the 1998 World Aquaculture Society meeting in Las Vegas, Nevada, USA by Lewis and Marshall (1998) It comprised of five steps, but has since been tested and refined

through application to multiple rehabilitation projects (Lewis, 2005; Brown & Lewis,

2006) before a publication comprising of six steps was achieved (Lewis, 2009)

The approach seeks to facilitate natural regeneration to construct and rehabilitate self-sustaining mangrove ecosystems that are valuable to both humans and nature (Mitsch

& Jørgensen, 2004; Lewis & Brown, 2014) EMR is designed to provide a logical sequence of tasks that are likely to succeed in re-establishing a biodiverse ecologically functioning mangrove ecosystem with vegetation structure similar to that of a natural reference mangrove forest, has tidal creeks connected to upland freshwater systems and is able to support a diverse faunal community (Lewis & Brown, 2014) The site is also designed to persist over time without a significant amount of human intervention EMR is advantageous in three ways – (i) rehabilitate ecosystems substantially degraded by humans, (ii) develop new sustainable ecosystems with both human and ecological value and (iii) achieve (i) and (ii) in a cost-effective manner (Lewis, 2005)

2.3.1 The 6-step EMR approach

As summarised in Figure 2.2, the approach comprises of six critical steps, with focus

on (i) pre-rehabilitation planning (Steps 1 – 3) and (ii) the subsequent design and implementation of actual rehabilitation works (Steps 4 – 6) Chosen sites must encompass high potential for successful rehabilitation into a self-sustainable mangrove ecosystem Some essential characteristics for consideration include soil texture, sediment budget, hydrodynamic influences (wave and wind action), hydrology, land topography and landscape connectivity to other mangrove patches to facilitate propagule exchange

Figure 2.2: The 6-step EMR approach (Lewis & Brown, 2014)

EMR prioritises hydrologic restoration without human-mediated planting as the distributions of mangroves and subsequent development are influenced mostly by appropriate surface elevations and thus, inundation durations When the pre-requisites

of appropriate surface elevations, inundation durations and propagule availability are met, rehabilitation sites can undergo secondary succession quickly and with minimal human intervention (Cintrón-Molero, 1992; Stevenson, 1997; Zedler 2000; Bosire et al., 2003) Establishment of naturally recruited mangrove propagules will thus be the dominant event driving reforestation of the rehabilitation site

Surface elevations in abandoned aquaculture ponds are generally modified to be unnaturally low to ensure perpetual flooding As mangroves can only establish and develop at suitable elevations, the introduction of fill material may be necessary to raise the elevation to one suitable for mangrove recruitment and establishment

Additionally, diking and its implication of prolonged flooding are stresses to mangroves (Brockmeyer et al., 1996) Hence, tidal hydrology should be restored through the removal or modification of obstructions to tidal connection (i.e strategic breaching of dike walls; Brown & Lewis, 2006; Dale et al., 2014; Lewis & Brown, 2014) To catalyse the establishment of mangroves, mature propagules are collected from surrounding forests, and hand-broadcast on the rising tide (Lewis & Brown, 2014) The method is most advantageous because composition of the rehabilitated mangroves will be similar to that of surrounding mangroves as the resultant mix of established species is regulated by the composition of locally occurring species Also, incompatibility between site environmental conditions and biological tolerances of mangrove species will be minimised as propagules will naturally establish at appropriate surface elevations This further prevents the conversion of inappropriate habitats (e.g mudflats, sand flats and seagrass beds), where elevation is commonly too low and where the coast is overly exposed to mechanical wind stress and higher wave action, into mono-specific plantations (Erftemeijer & Lewis, 1999; Samson & Rollan, 2008) An alternative or supplementary approach would be to plant locally-sourced propagules and/or wildings (naturally occurring wild seedlings) and nursery-grown seedlings/saplings The caveat however, is to determine the appropriate area for such plantings as species differ in their tidal and inundation ranges and soil type Therefore, any planting remains controversial and should only be attempted as a last resort (Lewis, 2005)

2.4 The importance of surface elevation and inundation hydroperiod for mangrove rehabilitation success

Numerous papers have discussed the science behind mangrove hydrodynamics (Wolanski et al., 1992; Wolanski et al., 1993; Furukawa et al., 1997; Mazda et al., 2007), with focus on tidal and freshwater flows and relationships between mangroves and wave attenuation and sedimentation processes Also, Kjerfve (1990) argued for the importance of topography as micro-topography governs the distribution of mangroves, with physical processes playing a dominant role in the formation and functional maintenance of mangrove ecosystems Surface elevations in mangroves, and its inherent control on periods of inundation and drainage, are therefore critical determinants of forest health For example, hyper- and hypo-salinity resulting from changes to rainfall and normal freshwater flows can kill mangroves (Cintrón et al., 1978; Medina et al., 2001; Biber, 2006) and changes inundation regimes (frequency, duration and depth) through global sea-level rise can stress (and kill) mangroves when they are unable to adapt

The knowledge of appropriate surface elevations and its influence on inundation can

be said to be one of the more important factors that determine the success of mangrove rehabilitation (Lewis, 2005; Lewis, 2009; Gilman & Ellison, 2007; Friess et al., 2012) Surface elevation, tidal frequency and amplitude determine inundation hydroperiod (inundation frequency, duration and depth) whereby lower elevations are inundated more frequently, for longer durations and to a greater depth This relationship between inundation hydroperiod and surface elevation, and the establishment and subsequent survival of mangrove vegetation has been acknowledged by rehabilitation practitioners, and is evident in their use of a comparable reference system to inform rehabilitation planning and design, across both intertidal ecosystems such as tidal

marshes and mangroves (Sullivan, 2001; Vivian-Smith, 2001; Lewis & Brown, 2014) Similarly, this relationship has been studied as evident in the large volume of journal publications focused on elucidating the relationship between surface elevation/inundation regimes and mangroves establishment and physiological development, and how this potentially translates into a control on the distribution of mangrove species (McKee, 1995; Kitaya et al., 2002; Chen et al., 2005; He et al., 2007; Chen et al., 2013)

Surface elevation and inundation durations work to impose differing impacts on propagule establishment and early development in established seedlings For example, propagules subjected to consistent inundation will not be able to overcome their inherent buoyancy to establish Or, if rooting has occurred, mangroves may (i) be exposed to prolonged inundation that thus comprises their development or (ii) not be exposed to sufficient inundation and eventually die Across the studies, there is agreement that mangrove species differ in optimal thresholds to inundation periods, and hence surface elevations The general relationship between inundation durations and physiological functioning of mangroves is that prolonged inundation (from low surface elevations) impedes psychological processes such as aerobic respiration and photosynthesis

2.4.1 Field studies relating surface elevation and mangrove distributions

Studies have recognised distinct zones in mangrove forests, where different species are observed to occupy different areas which are generally delimited from each other Watson (1928) first ascribed control of mangrove distributions to inundation frequency Through this field study, a Watson Classification was developed The classification comprises of five inundation classes, with details on the frequency of

inundation per month and dominant mangrove species for each respective class For example, mangroves in Inundation Class 1 will be flooded by all high tides, with an inundation frequency of 56 – 62 times per month, and has no dominant vegetation species (Watson, 1928; Table 2.1) Subsequently, de Haan (1931) expanded the Watson Classification to include the effect of fresh water, hence increasing the number

of classes from five to eight However, these are site-specific classifications and may not prove relevant across all mangrove ecosystems Thresholds of inundation period and frequency will be influenced by location-specific factors For example, studies

describing the inundation threshold of saltmarsh vegetation (Spartina spp.) suggested a

general tolerance of approximately 5800 – 7800 inundated hours per year (Friess et al., 2012) Yet, site-specific conditions such as unusual tidal dynamics, frost damage stress and water turbidity can act to change such thresholds (Hubbard & Partridge, 1981; Christiansen & Møller, 1983)

Table 2.1 Watson inundation classification and the related Southeast Asian mangrove species

Source: Watson, 1928

Inundation

Class

admirality datum)

Flood frequency (times per month)

Mangrove species

tides

Sonneratia

tides

Ceriops, Bruguiera

tides

Bruguiera, Acrostichum aureum

tides

Phoenix paludosa