TDA-IME Project Final Report June, 2013 mangrove stands may provide little or no storm or wave protection (Paphavisit, et al., 2008). The Role of Mangrove Roots It is generally agreed that the dense root systems of mangroves help to promote sedimentation; bind settled coastal sediments; and reduce the rate of soil erosion (Hutchins and Saenger, 1987; Augustinus, 1995; Wolanski 1995; Young and Harvey 1996; Furukawa et al., 1997; Maxwell, 2000). Since different species of mangroves produce roots systems with different architectures, it is reasonable to suggest that a mangrove forest with a diversity of root structures would be more effective in minimizing erosion than one with a single type of root architecture. However, the relative efficiency of different root architectures in reducing erosion has still not been investigated in the field. This seems to be an important oversight given the likelihood that rates of erosion will increase in many locations due to climate change. Another gap in our knowledge of mangrove root systems concerns their resilience to sea level rise, since the effect of rising sea level is likely to depend on the ability of aerial roots to supply oxygen to the trees’ below-ground roots. Species with root systems that can adapt quickly to changes in sea level, or sediment level, are likely to fare better than those with less such adaptive capacity. A superficial analysis of the structure and development of the four main kinds of mangrove root systems described by Tomlinson (1986) suggests that species with knee roots (e.g. Bruguiera) and plank roots (e.g. Heritiera littoralis and Xylocarpus granatum) are not well adapted to dealing with fast rates of sea level rise or high rates of sedimentation. The basis for this conclusion is that they need a significant investment in woody aerial roots, which take both time to construct and a large proportion of the tree’s photosynthetic production. In the case of those species with stilt roots (e.g. Rhizophora) gas exchange is restricted largely to the lower, less woody sections of the stilt root. Adaptation to rising sea level, or rapid sedimentation, requires the production of new lateral extensions to existing stilt roots or the production of new stilt roots from higher up on the trunk, as well as the development of new underground root systems. In larger Rhizophora trees, the development of new root systems above and below ground not only takes time, but also involves the allocation of a considerable part of photosynthetic production to the formation of new roots. Thus in a climate change scenario that involves a wide range of potential stresses that could affect plant metabolism and growth, the capacity of Rhizophora to adapt to rising sea levels might be limited. This apparent, but still unconfirmed, vulnerability of species with plank, knee and stilt roots to rapid sea level rise might help to explain why neither Bruguiera (knee roots), nor Rhizophora (stilt roots), could apparently keep pace with rates of sea level rise above 12 cm per 100 years during the Holocene 10,000 year BP (Ellison and Stoddard, 1991), a rate well below current projections for sea level rise to 2100. In contrast, mangrove genera with upwardly growing pneumatophores as aerial roots, such as Avicennia and Sonneratia, seem to be better equipped to handle moderate rates of sea level rise and sedimentation than Bruguiera and Rhizophora species. Pneumatophores have a much lower dry weight per unit volume or surface area than woody roots, since they contain mostly aerenchyma (spongy root tissue with large air spaces). Pneumatophores, apparently, can be produced rapidly in response to changes in sedimentation patterns (Young and Harvey, 1996). Moreover, since the growth meristem is at the top of the pneumatophore it could, in theory, continue to grow upwards in response to sea level rise, or sedimentation, without the need for the construction of more carbon-expensive woody roots. The capacity for vertical extension appears to be more limited in Avicennia than in Sonneratia species. The pneumatophores of old Sonneratia trees can extend to over 2 m above their point of attachment to the main cable roots, presumably reflecting a long history of adaptation to sedimentation. This assumption is rather speculative, but it does identify a 34
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
To see the actual publication please follow the link above