The zonation system which appears to use different elements of above- and below-ground vegetation densities for optimum soil protection and wave attenuation 

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Context 1

... like other forest stands, are complex living systems that are dynamic, ever-growing, and constantly reestablishing and renewing themselves (Duke 2001). Mangroves differ from terrestrial forests chie fl y because of their special adaptations for survival in tidal marine locations. Situated at the seawater margin, these stands are subject to both land and river runoff, as well as the direct action of the sea itself. By using their uniquely evolved features, mangrove plants have been able to readily occupy, dominate, and stabilize exposed tidal foreshore and estuarine environments ( Fig. 9). In such dynamic conditions, in fl uenced by severe hydrological and physicochemical conditions and faced with pervasive and pro- gressive changes, like human development and sea level rise, it has been essential for these plants to have successful regenerative strategies. Each occurrence of mangrove habitats today demonstrates the success of the regenerative strategies of these species. Mangrove stands are recognized as dynamic ecosystems that are relatively robust and stable while dominated by a small number of specialist species. In addition, certain species (e.g., Avicennia , Sonneratia ) are pioneers which are capable of fast regeneration after periodic disturbance. Disturbance is a natural feature of coasts – dynamic erosion/accretion and typhoons. A forest turnover model (Fig. 10) maps out the natural processes involved in mangrove forest dynamics by quantifying turnover in terms of gap creation and recovery (Duke 2001). The accumulative in fl uence of gap replacement, as numerous small-scale disturbances on mangrove forests, appears to explain the peculiar characteristics and presence of mangrove forests. Small gaps are common in terrestrial forests also, but those in mangroves differ because they rarely involve tree falls. In mangroves, the trees usually die standing in small clusters of 10 – 20 trees. The particular survival advantage of this strategy for mangrove forests is most evident in exposed stands. In these locations, forest structure must be maintained because exposed sediments would rapidly erode and destabilize the forest. Mangrove forests are therefore believed to be at great risk of ecosystem collapse and total loss of habitat. However, their common presence in exposed locations today is testimony to the success of current renewal strategies. But, what about the future? There is growing concern with predictions of future increases in large-scale pollution incidents, more rapid rises in sea level, and increased severity of storms. Each of these factors would seriously challenge the capacity of mangroves to regenerate. The cumulative in fl uence of such factors is therefore expected to seriously threaten the survival of exposed mangrove habitats. Many mangrove-dependent biota form interlinked community assemblages, with a number of unique and in fl uential plant-animal relationships (Saenger 1994). While mangroves have a relatively low number of species compared with adjacent communities like tropical rain forests and coral reefs, the diversity of organisms that reside in, or utilize, mangroves is surprisingly high. For one thing, they uniquely include both terrestrial and marine biota caught up in the regular ebbs and fl ows associated with fl uctuations of tides as well as river fl ows. While canopies often remain emergent, lower parts of mangrove forests are regularly inundated and drained in diurnal rhythms. Dependent marine life must either come and go with the tides or be adapted to exposure during low tides and inundation at high tide. During low tides, mangrove marine residents might either seek shelter in burrows of wood or mud or within a shell or other durable cover. As one example, small mangrove crabs (sesarmids) actively remove and consume leaves and fruits fallen from mangrove forest canopies (Smith et al. 1991). These crabs are considered ecosystem engineers (Lee 1998) because they in fl uence forest growth, diversity, structure, productivity, and function. Instead of leaf biomass being lost from the mangrove system with tidal fl ushing, it is transferred below ground where it is recycled into surrounding trees and forest structure. And, species of mangrove propagules eaten by crabs are likely to be excluded from future mature forests. It is suggested that this type of faunal activity may help explain the bimodal distribution of Avicennia marina across some tidal pro fi les, along with other unexplained local distributional characteristics (Duke et al. 1998). Mangrove forests often display a typical band-like zonation which contributes to potentially effective coastal protection where different assemblages have different aboveground root structures (Fig. 11). The aboveground structure and density of mangroves are important for wave attenuation and differ among mangrove species. Horstman et al. (2014) showed that wave attenuation increases signi fi cantly with volumetric vegetation densities. The latter is about 4.4 ‰ in the Avicennia zones and 5.8 – 32 ‰ in the Rhizophora zones along the Andaman coast of Trang Province in southern Thailand. This results in reduced wave-in fl uenced erosion, starting with Avicennia pioneer species at the seaward edge, backed up by the Rhizophora zone. The differences in belowground root structures also contribute to potentially effective soil stabilization. Mangrove forests worldwide are not only being depleted and lost, but they are also being degraded at an alarming rate (Duke et al. 2007). With continued habitat fragmentation, many believe these forests are losing functionality. And, with this, their long-term survival is at risk along with their notable ecosystem services. For example, many fi sh and shell fi sh spend all or part of their life cycles in mangroves. Pressures on these biota have important and wide-reaching implications for commercial, recreational, and artisanal fi sheries. Depending on location, the importance of mangroves to dependent fi sheries fauna ranges from 50 % to 90 % of commercial species having fundamental links with healthy, mangrove-lined estuaries. Other values of mangrove forests include stabilization of coastal and estuarine shorelines, mitigation of fl ooding and storm impacts, sequestered carbon accumulation in deep peaty sediments, and a supply of low-scale energy needs with timber harvesting. More generally, mangrove habitats are commonly sandwiched between two of the world s iconic tropical ecosystems, coral reefs and tropical rain forests, among which mangroves notably interact and support. In relatively arid places, mangroves cover tidal margins with closed shrubby canopies of greenery that distinctly contrast with parched upland landscapes. Where corals occur in shallow warm seas, mangroves buffer them from the in fl uences of runoff from coastal lands, protecting corals of wet and arid regions from unwanted nutrients and turbid waters. This may be achieved by stabilizing otherwise smothering waterborne sediments and shifting shorelines (Wolanski and Duke 2002). Tidal wetland plant assemblages accordingly provide important ecosystem services that further include structured habitat and nursery sites. Biota-structured habitats of coral reefs, rain forests, and mangroves play a vital role in coastal ecosystem processes via a combination of well-developed links and interconnectivity, coupled with transient biota uniquely adapted to the unusual and often dramatic physicochemical gradients. Such dependent relationships developed over millennia have become vital to the survival of each habitat. For instance, while sediment-loving mangroves fl ourish in waters sheltered by coral reef structures, they in turn protect wave-hardy corals from excessive sediments and nutrients in runoff from surrounding catchments. The consequences in upsetting any one of these links will have unexpected but likely far-reaching impacts on such interrelated habitats. Increased coastal erosion is a serious anticipated consequence of global climate change resulting from rising sea levels coupled with more severe storm waves and winds. Shorelines are protected where mangrove vegetation present acts to reduce and attenuate the eroding forces of coastal waters. The types of mangrove root structures present are thought to help protect vulnerable coastal soils and sediments. Furthermore, Tamooh et al. (2008) showed in a case study in Kenya that the root biomass of species changes according to their position in the zonation. Sonneratia alba has a higher root biomass than Avicennia marina which again had more root biomass than Rhizophora mucronata. Sonneratia which grows at the seaward side is exposed to intensive wave action and thus needs strong root anchorage for support in the unstable substrate. Sonneratia and Avicennia have more belowground root biomass because of their extensive underground cable root systems, whereas Rhizophora species have an extensive aboveground prop root system and a relatively small proportion of belowground biomass (Fig. 11). It is therefore proposed that wave attenuation by root structures along with soil stabilization contributes to coastal protection. Mangroves and tidal wetlands are fundamental to the persistence and survival of highly productive natural coastal environments. Mangroves have many well-acknowledged roles in coastal connectivity supporting enhancements in biodiversity and biomass (Mumby et al. 2004). At another level, commercial advantages note the importance of mangroves, where up to 75 % of the total seafood landed in Queensland comes from mangrove estuarine-related species. These observations indicate that healthy estuarine and near- shore marine ecosystems ...

Context 2

... lost from the mangrove system with tidal fl ushing, it is transferred below ground where it is recycled into surrounding trees and forest structure. And, species of mangrove propagules eaten by crabs are likely to be excluded from future mature forests. It is suggested that this type of faunal activity may help explain the bimodal distribution of Avicennia marina across some tidal pro fi les, along with other unexplained local distributional characteristics (Duke et al. 1998). Mangrove forests often display a typical band-like zonation which contributes to potentially effective coastal protection where different assemblages have different aboveground root structures (Fig. 11). The aboveground structure and density of mangroves are important for wave attenuation and differ among mangrove species. Horstman et al. (2014) showed that wave attenuation increases signi fi cantly with volumetric vegetation densities. The latter is about 4.4 ‰ in the Avicennia zones and 5.8 – 32 ‰ in the Rhizophora zones along the Andaman coast of Trang Province in southern Thailand. This results in reduced wave-in fl uenced erosion, starting with Avicennia pioneer species at the seaward edge, backed up by the Rhizophora zone. The differences in belowground root structures also contribute to potentially effective soil stabilization. Mangrove forests worldwide are not only being depleted and lost, but they are also being degraded at an alarming rate (Duke et al. 2007). With continued habitat fragmentation, many believe these forests are losing functionality. And, with this, their long-term survival is at risk along with their notable ecosystem services. For example, many fi sh and shell fi sh spend all or part of their life cycles in mangroves. Pressures on these biota have important and wide-reaching implications for commercial, recreational, and artisanal fi sheries. Depending on location, the importance of mangroves to dependent fi sheries fauna ranges from 50 % to 90 % of commercial species having fundamental links with healthy, mangrove-lined estuaries. Other values of mangrove forests include stabilization of coastal and estuarine shorelines, mitigation of fl ooding and storm impacts, sequestered carbon accumulation in deep peaty sediments, and a supply of low-scale energy needs with timber harvesting. More generally, mangrove habitats are commonly sandwiched between two of the world s iconic tropical ecosystems, coral reefs and tropical rain forests, among which mangroves notably interact and support. In relatively arid places, mangroves cover tidal margins with closed shrubby canopies of greenery that distinctly contrast with parched upland landscapes. Where corals occur in shallow warm seas, mangroves buffer them from the in fl uences of runoff from coastal lands, protecting corals of wet and arid regions from unwanted nutrients and turbid waters. This may be achieved by stabilizing otherwise smothering waterborne sediments and shifting shorelines (Wolanski and Duke 2002). Tidal wetland plant assemblages accordingly provide important ecosystem services that further include structured habitat and nursery sites. Biota-structured habitats of coral reefs, rain forests, and mangroves play a vital role in coastal ecosystem processes via a combination of well-developed links and interconnectivity, coupled with transient biota uniquely adapted to the unusual and often dramatic physicochemical gradients. Such dependent relationships developed over millennia have become vital to the survival of each habitat. For instance, while sediment-loving mangroves fl ourish in waters sheltered by coral reef structures, they in turn protect wave-hardy corals from excessive sediments and nutrients in runoff from surrounding catchments. The consequences in upsetting any one of these links will have unexpected but likely far-reaching impacts on such interrelated habitats. Increased coastal erosion is a serious anticipated consequence of global climate change resulting from rising sea levels coupled with more severe storm waves and winds. Shorelines are protected where mangrove vegetation present acts to reduce and attenuate the eroding forces of coastal waters. The types of mangrove root structures present are thought to help protect vulnerable coastal soils and sediments. Furthermore, Tamooh et al. (2008) showed in a case study in Kenya that the root biomass of species changes according to their position in the zonation. Sonneratia alba has a higher root biomass than Avicennia marina which again had more root biomass than Rhizophora mucronata. Sonneratia which grows at the seaward side is exposed to intensive wave action and thus needs strong root anchorage for support in the unstable substrate. Sonneratia and Avicennia have more belowground root biomass because of their extensive underground cable root systems, whereas Rhizophora species have an extensive aboveground prop root system and a relatively small proportion of belowground biomass (Fig. 11). It is therefore proposed that wave attenuation by root structures along with soil stabilization contributes to coastal protection. Mangroves and tidal wetlands are fundamental to the persistence and survival of highly productive natural coastal environments. Mangroves have many well-acknowledged roles in coastal connectivity supporting enhancements in biodiversity and biomass (Mumby et al. 2004). At another level, commercial advantages note the importance of mangroves, where up to 75 % of the total seafood landed in Queensland comes from mangrove estuarine-related species. These observations indicate that healthy estuarine and near- shore marine ecosystems are biologically and commercially linked. And, these natural systems are intimately related, connected, and dependent. So, where one is impacted, the effect will be felt more widely than might otherwise be expected. This is the case whether these ecosystems are viewed as sources of primary production with complex trophic linkages, as nursery and breeding sites, or as physical shelter and buffers from episodic severe fl ows and large waves. Tidal wetlands and mangroves are ancient ecosystems evolved over the last 100 million years. During this time, the earth, sea level, and climate have changed dramatically. Mangrove habitats of today are composed of plants that are survivors of previous ages. These surviving ecosystems consequently have strategies for dealing with change. As tidal wetland ecosystems respond to current changes, they rely on inherent adaptive capacity (Duke et al. 1998). Where changes can be identi fi ed, described, measured, and monitored, they form the basis for a more enlightened monitoring and assessment strategy. For example, if a tidal wetland habitat had shifted upland, this might demonstrate and quantify the effects of sea level rise. Two deductions to be made from such observations are that mangroves had responded to sea level rise and that we might evaluate the rate of net change and their success. The value in this approach in combination with direct instrument measures, like sea/tide level elevation stations, is that mangrove plants integrate daily and seasonal fl uctuations. These changes, when viewed in aerial images, are ampli fi ed depending on lower pro fi le slopes in respective locations. Furthermore, these enhanced shift incidents can be measured retrospectively using interpretations of vegetative condition for speci fi c locations from historical aerial/ satellite imagery. These incidents form the basis for questions about the causes of change. In this way, the drivers of change can be both identi fi ed and quanti fi ed at local and global scales. Some might be delivered directly, others indirectly, while others may be considered natural. In all situations, tidal wetlands respond to changing in fl uences in characteristic ways that are useful indicators of change. With the systematic identi fi cation of each type of change in tidal wetlands, then it will be possible to identify each responsible driver and to quantify the importance of each, along with likely, anticipated consequences. Mangrove ecosystems worldwide, and their associated habitats, are seriously threatened (Duke et al. 2007). For instance, coastal intertidal habitats are seriously threatened by smothering plumes of mud that greatly exceed prior natural levels. This has been exacerbated by large-scale land clearing and conversion of coastal forested wetlands (Fig. 12) into agricultural, port, urban, and industrial developments (Wolanski and Duke 2002). Key coastal rivers in populated areas have become little more than drains transporting eroded mud to settle in downstream estuarine reaches, along nearby shorelines, in shallow embayments, and on inshore reefs. Mangrove-lined estuaries had offered some respite and dampening of this effect, but in recent years these fi nal bastions of natural coastal processes and sediment fi lterers are succumbing to the increasing and unrelenting pressures of human population expansion into coastal and estuarine regions of tropical shorelines (Duke et al. 2007). The land-sea interface is a dynamic environment, where subtle natural changes in climate, sea level, sediment, and nutrient inputs have dramatic consequences for the distribution and health of mangroves. Local human disturbance of mangroves includes eutrophication, dredging/ fi lling, over fi shing, and sedimentation. The combined pressures of human disturbances (e.g., Duke et al. 2007) and global climate change have led to mangroves becoming “ endangered communities ” in many places. Small-scale rehabilitation projects have demonstrated the extreme dif fi culty in scaling up to effective, large-scale restoration. Urgent protective measures need to be implemented to avoid loss of mangroves and the resulting environmental degradation of coastal ecosystems, especially in the face of anticipated climate change. In conclusion, we identify some key gaps in ...