Restoration versus natural regeneration in a neotropical mangrove: Effects on plant biomass and crab communities

Highlights

• Mangroves are resilient ecosystems, but forest recovery may need planting sometimes.

• We compare tree development of an area planted with mangrove and other self-recovered.

• Planted trees showed higher development, but lower diversity than self-recovered ones.

• Brachyurans resettle suggest return of some ecological processes at restored area.

• Planting Rhizophora mangle is useful in restoring Neotropical mangals despite monospecificity.

Abstract

Mangrove restoration is a powerful tool for reconstructing degraded tropical estuaries worldwide. The level of intervention necessary for rebuilding a mangrove area is still under study because this system is quite dynamic and some degraded areas are able to recover by themselves. We compared a Restored Area planted with Rhizophora mangle and a Self-recovered Area, to investigate how they would differ in respect to their plant and crab assemblages. In each area, 10 plots were randomly sampled for measuring tree richness, density, diameter at breast height and biomass, as well as crab richness and density after five years from recover. Plant height and biomass, as well as crab density, were significantly higher in the Restored in relation to the Self-recovered Area. However, tree richness was higher in the Self-recovered Area and crab assemblage reached in five years a similar composition in assisted, naturally recovered and natural areas, showing the high resilience of this important functional group. Active planting of R. mangle propagules can significantly improve mangrove recovery in sites with high propagule predation, increasing tree biomass and accelerating the return of animal functional groups such as crabs. However, if resources for active restoration are scarce, passive recovery can be a successful low cost alternative for mangrove restoration that preserves tree diversity and crab assemblages.

Introduction

Mangroves are highly productive biological communities that occupy around 15 × 10⁶ km² of tropical and subtropical coasts worldwide (FAO, 2007, Lacerda, 2002). At least 35% of mangrove forests have been destroyed in the past decades by human settlements, wood extraction and shrimp culture (Valiela et al., 2001) leaving many deforested and abandoned areas in need of restoration (Alongi, 2002). However, to restore a degraded ecosystem to a pristine condition is not an easy task, because most ecosystems are in permanent change, and estuarine mangroves are a good example (Alongi, 2009, Wyant et al., 1995). On the other hand, the effectiveness of mangrove restoration programs to significantly improve ecosystem function has been doubted, due to the use of only one or few tree species, a low richness when compared to natural areas (Ellison, 2000, Lewis, 2005, Rovai et al., 2012, Salmo and Duke, 2010, Walters, 2000).

Tree development is an important and commonly measured feature of a restored ecosystem, because key ecosystem functions and faunal colonization are dependent on vegetation structure, biomass and diversity. Planted mangroves can reach high tree development in height, diameter or biomass (Hong, 1996, Ferreira et al., 2007, Macintosh et al., 2002), but Ross et al. (2001) found that naturally recovered Neotropical mangroves have achieved similar tree development than planted ones. Choosing a better cost/benefit management procedure in such forests is impaired by scarcity on studies comparing equal lifetime planted and naturally recovered areas. Studied natural stands used to compare with planted were of unknown age or older (Bernini, 2013, Martinuzzi et al., 2009, Shafer and Roberts, 2008). Alternatively, the return of faunal functional groups can be an indicator of forest restoration success (Babin-Fenske and Anand, 2010, Gibb and Cunningham, 2013, Gollan et al., 2011, Jansen, 1997). In mangrove forests some studies show evidences that important invertebrate groups like Brachyuran crabs may be more diverse in planted stands despite restoration being implemented with only one or few tree species (Al-Khayat and Jones, 1999, Ashton et al., 2003, Bosire et al., 2004, Bosire et al., 2008, Macintosh et al., 2002).

Crabs of the families Ocypodoidea and Grapsoidea (Brachyura; Decapoda) (Ng et al., 2008) are among the most abundant and ecologically significant mangrove dweller animals. They are ‘ecosystem engineers’ that play important roles on sediment topography and biogeochemistry, vegetation structure and primary production (Jones et al., 1994, Kristensen, 2008, Lee, 2008, Robertson and Daniel, 1989, Warren and Underwood, 1986). Despite their role in affecting overall community structure and ability to serve as important indicators of natural and managed forests conservation status (Ashton et al., 2003, Bosire et al., 2004, Macintosh et al., 2002, Ruwa, 1997), their effects have been rarely assessed in neotropical mangroves (but see Ferreira et al., 2007, Ferreira et al., 2013). Therefore, the relationship between mangrove development and crab assemblage is a relevant aspect that needs evaluation in mangrove restoration programs. Several studies designed allometric equations for calculate mangroves biomass (see Review of Komiyama et al., 2008), while others compared carcinofauna between natural and reforested stands (see Review of Bosire et al., 2008), but there are not studies comparing both carcinofauna and tree biomass differences between these forests types.

The aim of this study was to compare tree and crab diversities and community structure of two mangrove deforested areas of the same age, one planted with propagules of red mangrove, Rhizophora mangle, and the other naturally recovered. Here, we tested the hypothesis that the two areas neither differ significantly in plant richness and biomass nor differ in crab richness or density. A rejection or acceptance of this hypothesis would provide evidence for or against the effectiveness of R. mangle monospecific planting for neotropical mangrove restorations.