Chapter – 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 and is reflected in the scientific focus of studies on testing ecological theories 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 10 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, reestablishing 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 11 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 selfsustaining 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 – 3) and (ii) the subsequent design and implementation of actual rehabilitation works (Steps – 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 12 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 13 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) 14 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 15 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 16 inundation per month and dominant mangrove species for each respective class For example, mangroves in Inundation Class 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 Flooded by Flood frequency admirality datum) Class Elevation (m above Mangrove species (times per month) All high tides Below 2.44 56 – 62 None Medium high 2.44 – 3.35 45 – 59 Avicennia spp., tides Normal high Sonneratia 3.35 – 3.96 20 – 45 tides Spring high Rhizophora spp., Ceriops, Bruguiera 3.96 – 4.57 – 20 tides Lumnitzera, Bruguiera, Acrostichum aureum Equinoctial 4.57 and above tides Up to Ceriops spp., Phoenix paludosa 17 It was only in the late 1960s that this control of mangrove distribution was expanded to include the interaction of several environmental factors such as inundation frequency, soil type and soil salinity (Macnae, 1966, 1968; Santistuk, 1983) Macnae (1968) conducted a review on the zonation schemes that have been proposed for mangroves of the Indo-West-Pacific region, and based it on inundation frequency, soil salinity and generic name of the dominant trees Mangroves were proposed to be considered under three broad zonations (Santistuk, 1983) The fringing forests consist of trees or shrubs which form a thin band facing open sea The next zone is generally a well-developed forest dominated by tall straight trees, principally members of the Rhizophoraceae Further landward is a transition forest which tends to be more floristically diverse than the previous two, with its species composition depending on the type of forest mangroves transition into (fresh water swamp, peat swamp, salt flat or dry land) More recently, Crase et al (2013) tested this theory statistically The study quantified the relationship between species’ dominance and hydroperiod (defined in the study as the duration of inundation in a year), soil salinity and water salinity for three species – Sonneratia alba, Rhizophora stylosa and Ceriops tagal All models indicated that hydroperiod was the key variable influencing mangrove distribution, followed by soil salinity 2.4.2 Experimental studies relating seedling responses to inundation Multiple experimental studies under controlled systems and in the field have established that mangroves exhibit species-specific responses on growth and survival rates in response to varying inundation levels and durations Growth and survival of Avicennia germinans seedlings were compromised at intertidal positions where inundation regimes were either greater or lesser than mean sea level (Ellison & 18 Farnsworth, 1993) Rhizophora mangle responded better to deeper inundation depths and durations, characteristic of lower intertidal positions (Ellison & Farnsworth, 1993) The relative growth rates of Bruguiera gymnorrhiza decreased as inundation duration increased, while Kandelia candel exhibited no such reductions in response (Ye et al., 2003) But with prolonged inundation, the capacity for photosynthetic light saturation levels and photosynthesis was reduced for K candel seedlings (Chen et al., 2005) B gymnorrhiza seedlings subjected to tidal inundation treatments had reduced seedling height, diameter, leaf area, leaf biomass, stem biomass and root biomass relative to controls (Krauss & Allen, 2003) However, tidal inundation unexpectedly enhanced biomass attributes for Xylocarpus granatum seedlings, despite this species’ typical natural preference for low inundation frequencies and durations (Allen et al., 2003) A field survey on Avicennia marina established that elevation had a positive effect on seedling growth – seedlings at lower elevations had less annual biomass accumulation and population productivity (Lu et al., 2013) In the same study where a mesocosm experiment was used to vary inundation durations and depth, both inundation duration and depth exerted significant and negative effects on biomass accumulation, photosynthetic rate, leaf electron transportation and water-use efficiency The negative effects of prolonged inundation were exacerbated by increasing inundation depth as that resulted in complete canopy immersion of A marina seedlings (Lu et al., 2013) To simulate the effects of sea level rise, a 2.5-year experiment subjected R mangle seedlings to three relative tidal inundation regimes of increased sea level, no change and decreased sea levels (Ellison & Farnsworth, 1997) While the experimental simulation of “sea level rise” is questionable as it is a long-term process (decades) and is a continuous, cumulative process that acts on mangroves development, the experiment can be re-interpreted as seedlings were subjected to longer inundation 19 duration and depth in “increased sea level” treatments, with the opposite for “decreased sea level” treatments A comparison of the responses of seedlings highlighted that seedlings in treatment with no change in sea level maintained – 7% fewer stomata per unit area, – 12 % greater photosynthetic rates, and – 23 % greater absolute relative growth rates compared to seedlings in other treatments Under prolonged inundation durations, seedling growth eventually reduced (Ellison & Farnsworth, 1997) This trend was registered in other mangrove species – K candel and B gymnorrhiza demonstrated rapid growth over the first two months after sealevel rise simulations of 30 cm but were unable to maintain this rate beyond the initial period (Ye et al., 2004) In general, species-specific seedling physiological efficiency and growth potential are reduced with increased inundation durations and depths beyond some optimum Under prolonged inundation, substratum oxygen concentrations will generally decrease, and elicit an effect on mangroves primarily through their roots Roots will exhibit lower oxygen partial pressures in the root aerenchyma (McKee, 1996) and enter anaerobic respiration for short durations, allowing some energy production to continue (Chen et al., 2005) Such depressed development efficiency would translate towards low rehabilitation success wherein recovery via secondary succession would be hampered as seedlings would not survive for long 2.4.3 Experimental studies relating propagule establishment to inundation Unlike mangrove seedlings, inundation does not directly impact the growth and survival of propagules through the reduction of photosynthetic capacity and growth potential Instead, optimum inundation durations achieved through optimum elevation ranges have implications on the stranding and subsequent rooting of propagules, before 20 propagule viability is lost Propagule buoyancy, period of obligate dispersal, rooting time, and the action of tides and currents are primary factors determining dispersal and establishment of mangroves and other water-dispersed species (Rabinowitz, 1978; Foote & Kadlec, 1988; Schneider & Sharitz, 1988; Clarke & Myerscough, 1993; Clarke, 1993; McGuinness, 1996; Delgado et al., 2001) Upon reaching a new habitat, inundation frequency and duration become important factors in determining propagule establishment success (Jiménez & Sauter 1991; Clarke & Myerscough, 1993, Balke et al., 2011) Two experimental settings, a field and mesocosm experiment, were employed in studying propagule establishment in Laguncularia racemosa, Avicennia germinans and Avicennia bicolour Successful establishment in the field showed that both L racemosa and Avicennia spp were optimised for establishment in the lower intertidal zone compared to mud flats and upper intertidal zones (Delgado et al., 2001) Under controlled mesocosm conditions, L racemosa had successful establishment in treatments with continuous-inundation and no-inundation whereas A germinans performed significantly better in treatments with no inundation and normal tidal regime Under the continuous-inundation treatment, L racemosa showed significantly higher establishment compared to A germinans L racemosa propagules sank shortly after their radicles protruded, favouring rooting under water, whereas highly buoyant A germinans propagules floated throughout the experiment and never established (Delgado et al., 2001) Similarly, R mangle propagules had difficulty establishing in lower intertidal zones as tidal action regularly buoyed them away soil surface (McKee, 1995b) This again, emphasis species-specific variation in propagule physiology, and its impacts on propagule establishment across different inundation regimes 21 Flume studies and field observations by Balke et al (2011) showed that establishment of A alba propagules require an inundation-free period that allows rapid development of roots long enough to anchor the seedling and to withstand displacement by inundation and wave action Under a fixed tidal regime, probability of an inundationfree period occurring will increase with higher surface elevations As most mangrove propagules remain buoyant for long periods of time without sinking or losing viability, this implies that mangrove establishment will require suitable elevations that optimise the occurrence of such suitable inundation-free periods Two additional thresholds that seedlings must pass before successful establishment are related to hydrodynamics First, roots of seedling must grow to surpass a minimum length to withstand hydrodynamic disturbances from waves and currents Particularly when establishment is occurring on exposed coasts (e.g mudflats) with low elevations and higher exposure to wave and wind action Second, even longer roots are necessary for persistence through high energy events that cause surface sediment mixing and/or erosion, potentially inducing seedling dislodgement (Balke et al., 2011, Figure 2.1) 22 Figure 2.3: Schematic representation using an Avicennia alba propagule to illustrate the three thresholds that have to be surpassed before establishment (1) The propagule first has to acquire a minimum root length during an inundation-free period to resistant against floating up during tidal inundation (2) Then, roots have to be long enough to resist hydrodynamics by waves and currents (3) Rooted seedling may still be dislodged via mixing or erosion of surface sediments Source: Balke et al., 2011 23 ... 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... 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... comprises of five inundation classes, with details on the frequency of 16 inundation per month and dominant mangrove species for each respective class For example, mangroves in Inundation Class will