Friday, May 24, 2013

Mangrove Restoration Should Involve More Biodiversity Much Less Monoculture:


Interest in mangroves has grown considerably since the Asian tsunami, but for reasons other than biodiversity (e.g. green-belt protection, fisheries productivity, sustainable development of local people, ('blue') carbon storage, as a casualty of large-scale aquaculture development and so on.)

A key problem for mangrove biodiveristy stems from the methods and beliefs around restoration after clearance for charcoal, aquaculture or over-exploitation. Mangrove forests vary in their macro-flora biodiversity, from three or four tree and shrub species in the US to 60-80 species around Indonesia. After restoration efforts what replaces the forests, if successful, is often from only the Rhizophora genus. This is due to the fact thatRhizophora propagules are easy to handle, Rhizophora is sometimes believed to be 'the best' mangrove species and that it has a fairly wide range of tolerances to inundation regimes, and bio-chemo-physical gradients. Therefore many of the restoration efforts resemble mono-specific plantations, with Rhizophora planted in straight lines. Plantation-style restoration is unsurprising as many states’ mangroves are the responsibility of the forestry department. And as a result of imitation and perhaps government training, local people’s restoration is similar.

For a review of the issues, see Lewis (2013). For a description of a more appropriate method of ecological mangrove restoration, whereby amended and corrected hydrology encourages natural regeneration and therefore greater biodiversity, see Field (1996) and Lewis (2005).
Evidence about whether other mangrove species colonise mono-specific plantations is mixed. Walters’s (2000) detailed survey of various Filipino plantations revealed that there was very little natural recruitment of other species in mangrove plantation areas, even after taking into account the ‘weeding’ done by local people (supported by Sudtongkong and Webb 2008, Enright, 2012 per. comm.). The process of establishing a plantation sometimes involves the removal of other species nearby to allow space for the plantation, simplifying local biodiversity further (Enright 2012, per. comm., per. ob.). The lack of propagule recruitment might be due to several reasons: the very dense prop roots characteristic of Rhizophora planted at high density, blocking the movement of other propagules into the site; prop root-induced soil accretion making the substrate high relative to sea-level, unsuitable for pioneer species to colonise; exuded phenolic compounds from mangrove tree roots might suppress other species (Alongi 2002); propagule predation (Bosire et al. 2006)  and the gaps in the plantation formed by limited harvesting being too small and open for too short a time (gap-mediated colonisation, see Clarke and Kerrigan, 2000).  Walters (2000) found no correlation between stand age and species richness and thus concluded that if biodiversity is an objective, it has to be designed in at planting.
In contrast, Bosire et al. (2003) and Kairo et al. (2008) found that even mono-specific reforestation facilitated natural recruitment of propagules and seeds (supported by Ellison 2000), by reducing wave energy and trapping propagules. Saenger (2002) also mentions the biochemical improvements live mangrove roots make to the soil by exuding oxygen and carbohydrate, making colonisation more likely.
Whether low macroflora biodiversity leads to lower net primary productivity, nutrient cycling, litter processing and as a result, lower fish productivity appears not to have been comprehensively studied.

References cited
Alongi, D. M. (2002). Present state and future of the world's mangrove forests. Environmental Conservation, 29(3) doi: 10.1017/S0376892902000231
Bosire, J. O., Dahdouh-Guebas, F., Kairo, J. G., Wartel, S., Kazungu, J. M., & Koedam, N. (2006). Success rates of recruited tree species and their contribution to the structural development of reforested mangrove stands. Marine Ecology Progress Series, 325, 85-91.
Bosire, J. O., Dahdouh-Guebas, F., Kairo, J. G., & Koedam, N. (2003). Colonization of non-planted mangrove species into restored mangrove stands in gazi bay, kenya. Aquatic Botany, 76(4), 267-279. doi: 10.1016/S0304-3770(03)00054-8
Clarke, P.J. & Kerrigan, R.A. 2000, "Do Forest Gaps Influence the Population Structure and Species Composition of Mangrove Stands in Northern Australia? 1", Biotropica, vol. 32, no. 4, pp. 642-652.
Ellison, A. M. (2000). Mangrove restoration: Do we know enough? Restoration Ecology, 8(3), 219-229. doi: 10.1046/j.1526-100x.2000.80033.x
Enright, J. (2005). Mangrove and mangrove community expert. Thailand.
Field, C.D., ed. (1996). Restoration of mangrove ecosystems. International Society for Mangrove Ecosystems, Okinawa, Japan.
Kairo, J. G., Lang’at, J. K. S., Dahdouh-Guebas, F., Bosire, J., & Karachi, M. (2008). Structural development and productivity of replanted mangrove plantations in kenya. Forest Ecology and Management, 255(7), 2670-2677. doi: 10.1016/j.foreco.2008.01.031
Lewis, R.R. (2005). Ecological engineering for successful management and restoration of mangrove forests. Ecol. Eng., 24(4 SI): 403–418.
Lewis III, R.R. 2013. Mangrove forest restoration and the preservation of mangrove biodiversity. In: Bozzano, M., Jalonen R., Evert, T., Boshier, D., Gallo, L., Cavers, S., Bordacs, S., Smith, P., and Loo, J. (eds). Genetic considerations in ecosystem restoration using native tree species. A thematic study for the State of the World’s Forest Genetic Resources. United Nations Food and Agriculture Organization, Rome, Italy. (Forthcoming).
Saenger, P. (2002). Mangrove ecology, silviculture, and conservation. Dordrecht; Boston: Kluwer Academic Publishers.
Sudtongkong, C., & Webb, E. L. (2008). Outcomes of state-vs. community-based mangrove management in southern thailand. Ecology and Society, 13(2), 27.
Walters, B. B., Rönnbäck, P., Kovacs, J. M., Crona, B., Hussain, S. A., Badola, R., . . . Dahdouh-Guebas, F. (2008). Ethnobiology, socio-economics and management of mangrove forests: A review. Aquatic Botany, 89(2), 220-236. doi: 10.1016/j.aquabot.2008.02.009
Walters, B. B. (2000). Local mangrove planting in the philippines: Are fisherfolk and fishpond owners effective restorationists? Restoration Ecology, 8(3), 237-246. doi: 10.1046/j.1526-100x.2000.80035.x

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