Mangrove restoration, a case for an evidence based approach
Machin, James (2015) Mangrove restoration, a case for an evidence based approach. PhD thesis, James Cook University.
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Abstract
Mangrove forest restoration projects undertaken globally to date have had significant issues and limitations and "most attempts to restore mangroves often fail completely or fail to achieve the stated goals" (Lewis, 2005). The root cause of many of the issues associated with mangrove restoration is that knowledge of the underlying factors controlling spatial patterns in mangroves is often lacking or not integrated into design of restoration projects, impeding the successful restoration of these critical ecosystems.
To improve the success of restoration and address many of the issues associated with the current paradigm for mangrove restoration, there is a need to develop an evidence based approach to the planning and implementation of mangrove restoration projects. The current thesis demonstrates the need for an evidence based approach to mangrove restoration through implementation of case studies in lower intertidal mangrove communities of the Ngao river, located in the Kraburi river estuary, Ranong province, southern Thailand and development of a series of recommendations on how to incorporate this improved evidence base into different phases of mangrove restoration projects.
The case studies target identified gaps in knowledge of the different stages of the mangrove tree life cycle through: observational studies on mature tree forest composition and spatial patterns in two forest types; observational studies and experiments examining the dispersal and development of mangrove propagules and seeds and establishment of released propagules in experimental enclosures; and experiments examining the survival and growth of transplanted seedlings in the two lower intertidal forest communities, with distance from the Ngao river and inside and outside canopy gaps.
Chapter 2 of the thesis documents the findings of the first case study and showed that the two lower intertidal forest communities included in the study, the "Aegiceras forest" and the "Sonneratia-Avicennia forest", were low in mangrove tree species diversity and differed based on species similarity in assemblages of mature trees and differences in forest structure parameters of dominant species. Within each of the two forest types, assemblages of mature trees also differed with distance from the Ngao river with the study confirming the presence of distinct spatial patterns or zones within each of the two forest types. Duration of tidal inundation was found to be an important determinant of forest structure in the two forest types, significantly correlated with assemblages of mature trees as well as key forest structure parameters.
These correlations and the differences in natural assemblages of mature mangrove trees between the two forest types and further significant differences with distance from the Ngao river in the "Aegiceras forest", suggest that assemblages of mature trees are related to hydroperiod and duration of inundation but that forest communities in both the "Aegiceras forest" and "Sonneratia-Avicennia forest" could have a broad tolerance to hydroperiod characteristics of the two forest types. Within the two forest types, the physical effects associated with more exposed locations close to the Ngao river appear to determine within forest spatial patterns, with A.corniculatum seemingly less tolerant of these more exposed locations than A.alba and S.alba but a better competitor than A.alba and S.alba at more sheltered sites inland.
Chapter 2 also provided information on the demographics of mature trees in the two forest types, which suggest that secondary succession has possibly taken place within the "Aegiceras forest" in the past with A.corniculatum trees taking over from S.alba and A.alba trees, a pattern which is possibly currently being repeated in the "Sonneratia-Avicennia forest". These patterns of secondary succession have not previously been documented before in these lower intertidal communities.
The second case study, described in Chapter 3 focused on propagules and developing seedlings and had two components. The first component was a propagule dispersal study which showed that assemblages of propagules and developing seedlings differed between the two lower intertidal forest types, as did the propagule and immature seedling forest structure parameters of the three dominant species A.corniculatum, A. alba and S.alba. Assemblages of propagules and developing seedlings also differed with distance from the Ngao river with the greatest density of A.corniculatum propagules and seedlings in the "Aegiceras forest" found in the middle zone of the forest and the lowest density found in the zone immediately adjacent to the Ngao river and inland. A.alba propagules and seedlings had contrasting spatial patterns and were confined to quadrats closest to the Ngao river. In the "Sonneratia-Avicennia forest", A.corniculatum seedling density was also related to the distance from the Ngao river, highest in the zone adjacent to the river and lowest inland. The propagule dispersal study also found clear seasonal variation in propagule and seedling abundance of the dominant species and seasonal differences between forest types, with seedlings of the dominant species tending to persist across the year in the "Sonneratia-Avicennia forest" but not in the "Aegiceras forest".
The second component of Chapter 3 was a propagule release experiment which showed that a greater proportion of A. corniculatum propagules released into enclosures developed into mature seedlings in the "Aegiceras forest" than the "Sonneratia-Avicennia forest". In contrast, a greater proportion of A.alba propagules developed into mature seedlings in the "Sonneratia-Avicennia forest". The rate of propagule development was also related to distance from the Ngao river. For A. corniculatum the highest proportion was found in the middle of the forest dominated by A.corniculatum trees, while in contrast, a greater proportion of A.alba propagules developed into mature seedlings in plots further away from the Ngao river outside the area dominated by mature A.alba trees.
The results suggest that differences in assemblages of propagules and seedlings between the two forest types are closely related to the mature forest composition and the Importance value of conspecific trees in the canopy. Within the two forest types, the results also support the secondary role of duration of inundation, with physical effects associated with more exposed locations close to the Ngao river appearing to determine within forest distribution patterns of A.corniculatum propagules and seedlings which were seemingly less tolerant to exposed locations than propagules and seedlings of A.alba and S.alba.
The results described in Chapter 3 also suggest differences in reproductive strategy amongst species and provides additional evidence of secondary succession through observations of: (1). A wide distribution of A.corniculatum propagules and seedlings across the study area; and (2). persistence of A.corniculatum propagules and seedlings over an annual period in the "Sonneratia- Avicennia forest". These results add weight to the theory espoused in Chapter 2 of the thesis that A.corniculatum is expanding its range in the "Sonneratia-Avicennia" forest, replacing the dominant S.alba and A.alba, perhaps in the same manner as this species has done in past in the "Aegiceras forest".
The third case study documented in Chapter 4, was a transplant study focusing on A. corniculatum and S.alba seedlings. Results showed that survival and growth of transplanted seedlings of both species was greater in the "Sonneratia-Avicennia forest" than the "Aegiceras forest". S.alba seedlings showed a particularly strong preference for environmental conditions in the "Sonneratia-Avicennia forest" type. Survival and growth of seedlings of both species also increased with distance from the Ngao river in both forest types. This trend was especially pronounced for f A.corniculatum seedlings. The proportion of transplanted seedlings of both species surviving also differed with canopy type, with a greater proportion of seedlings surviving in light gaps than under the forest canopy. Canopy type had less of an effect on survival of A.corniculatum seedlings than on S.alba seedlings, which showed a strong preference for light gaps as opposed to locations under the forest canopy. Canopy type had less of an effect on survival of transplanted seedlings in the "Sonneratia-Avicennia forest" than it did in the "Aegiceras forest" and also had less of an effect on survival of transplanted seedlings in the quadrats closest to the Ngao river than it did further inland.
In both forest types the survival and growth of transplanted seedlings of both species was correlated with the percentage of time that they were inundated by tides, and the survival and growth of both species increased as the proportion of the day in which the site was inundated decreased.
Clear differences in survival and growth of transplanted seedlings of the two species appears to be related to significantly different levels of light availability in the two forest types resulting from the differing forest structure of these two lower intertidal communities, the denser, more closed canopy of the "Aegiceras forest" and the more open canopy "Sonneratia-Avicennia forest" as reported in Chapter 2 of this thesis. Within the two forest types, a secondary role of duration of tidal inundation appears to exist, with physical effects associated with more exposed locations close to the Ngao river appearing to influence seedling survival and growth patterns, both of which were significantly greater in plots away from the Ngao river than plots close to it. A.corniculatum seedlings were seemingly less tolerant to these more exposed locations than those of S.alba. The effects of light level and physical disturbance on seedling establishment, although not unequivocal, are consistent with results from the literature and also provide further insights into tolerances of the two lower intertidal species to environmental conditions typically observed at restoration sites.
Chapter 5 of the thesis summarises the results of the case studies and makes recommendations for how to incorporate each aspect of the improved evidence base arising from the case studies, into different phases of mangrove restoration projects. A table making explicit links between the case studies and restoration is included as was a concept for a decision support system for restoration of lower intertidal forests in the form of a flow chart based on results of studies of mature forest, propagules and seeds and mature seedlings. The flow chart demonstrates how each piece of evidence could potentially be applied in practice to enable the best decisions on which mangrove restoration approach to adopt to be made in an objective manner. For example information on hydroperiod, combined with information on site exposure and level of natural recruitment at the site would result in a specific recommendation for restoration of a site with either A.corniculatum, S.alba or A.alba seedlings where natural recruitment at the selected site was insufficient. Incorporated together with the improved evidence base on lower intertidal forests developed through the three case studies, the decision support system can potentially serve as a practical tool for the integration of scientific knowledge about mangrove ecosystems into restoration planning and ultimately improve the results of restoration and assist in the recovery of these essential ecosystems.
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