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.)
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.
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).
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.
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.
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.
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
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Bosire,
J. O., Dahdouh-Guebas, F., Kairo, J. G., Wartel, S., Kazungu, J. M.,
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Lewis
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