Functionality of restored mangroves: A review

Study Number:

22

Author:

J.O. Bosire, F. Dahdouh-Guebas, M. Walton, B.I. Crona, R.R. Lewis III, C. Field, J.G. Kairo and N. Koedam

Abstract:

Widespread mangrove degradation coupled with the increasing awareness of the importance of these coastal forests have spurred many attempts to restore mangroves, but without concomitant assessment of recovery (or otherwise) at the ecosystem level in many areas. This paper reviews literature on the recovery of restored mangrove ecosystems using relevant functional indicators. While stand structure in mangrove stands is dependent on age, site conditions and silvicultural management, published data indicates that stem densities are higher in restored mangroves than comparable natural stands; the converse is true for basal area. Biomass increment rates have been found to be higher in younger stands than older stands (e.g. 12 t ha-1 year-1 for a 12 years plantation compared to 5.1 t ha-1 year-1 for a 80- year-old plantation). Disparities in patterns of tree species recruitment into the restored stands have been observed with some stands having linear recruitment rates with time (hence enhancing stand complexity), while some older stands completely lacked the understory. Biodiversity assessments suggest that some fauna species are more responsive to mangrove degradation (e.g. herbivorous crabs and mollusks in general), and thus mangrove restoration encourages the return of such species, in some cases to levels equivalent to those in comparable natural stands. The paper finally recommends various mangrove restoration pathways in a functional framework dependent on site conditions and emphasizes community involvement and ecosystem level monitoring as integral components of restoration projects.

Main Results and Conclusions:

• Forest structure, biomass and regeneration analysis shows that the success of replanted mangroves is extremely site-specific, and therefore classification of replanted mangrove success is hard to homogenize.
  • Comparing a Kenyan study by Kairo et al. (2008) to Bosire et al.’s (2006) study indicates that replanted mangrove stands will have a higher stem density than natural stands of the same species: “…data from a 12 years old (Table 1) Rhizophora mucronata Lamk plantation in Kenya indicate that reforested plots (after deforestation) have the potential of yielding 4864 stems ha-1 (much higher than the stem density in a natural stand of the same species at the same site of 1796 stems ha-1; Bosire et al., 2006), with a standing biomass and merchantable volume of 106.7 t ha-1 and 60.7 m3 ha-1, respectively (Kairo et al., 2008)” (252).
  • Data show that in some cases, biomass is higher in replanted mangrove stands: “The biomass accumulation rate for the 12-year-old Rhizophora plantation in Kenya was estimated at 12 t ha-1 year-1 (Kairo et al., 2008). This value is higher than the 5.1 t ha-1 year-1 reported for an 80-year-old natural plantation of Rhizophora apiculata Bl. In Malaysia (Putz and Chan, 1986)” (252).
  • “The recruitment rate of saplings has been found to be increasing with age in one R. mucronata Lamk. plantation in Kenya (Fig. 1). The densities observed in this plantation are however, much lower than those observed in a comparable conspecific natural stand at the same location (see, e.g. Kairo et al., 2002; Bosire et al., 2006), suggesting age may be a critical factor in determining the level of natural regeneration”(253).

• Biodiversity in restored mangroves is discussed in detail. This section indicates that some species populations, such as crabs, mollusks, and algae, vary among replanted mangrove sites (235).
• Socio-economics of mangrove restoration is discussed, indicating that certain cases of replanted mangroves offer significant economic returns. The economic returns were directly linked to local communities via tourism, timber, and fishing yields. The practicality of using carbon credits to raise income is also discussed (Walton, 2006a) (254-255).
• A ten-step mangrove restoration plan is offered in the “Opportunities and constraints to mangrove forest restoration” section. The summarized steps are as follows: (1) Acquire an in depth understanding of mangrove ecosystem, (2) Understand hydrology of mangrove ecosystem, (3) Asses factors hampering succession, (4) Survey sustainable utilization practices from local communities, (5) Select appropriate restoration sites based on steps 1-4, (6) Restore hydrology and remove any barrier to natural regeneration, (7) Select appropriate species for planting, (8) Plant selected mangrove species, (9) Based on assessment of planted mangroves (i.e. potential for sustainable utilization, vegetation development, faunistic recruitment, and environmental factors and processes) assess the success and functionality of planted mangroves, (10) Give recommendations for improved site management (256).

Works Cited:

Bosire, J.O., Dahdouh-Guebas, F., Kairo, J.G., Wartel, S., Kazungu, J., and N. Koedam. 2006. Success rates of recruited tree species and their contribution to the structural development of reforested mangrove stands. Mar. Ecol. Prog. Ser. 325: 85–91.

Kairo, J.G., Dahdouh-Guebas, F., Gwada, P.O., Ochieng, C., Koedam, N., 2002. Regeneration status of mangrove forests in Mida Creek Kenya: a compromised or secured future? Ambio 31: 562–568.

Kairo, J.G., Lang’at, J.K.S., Dahdouh-Guebas, F., Hansen, L. Bosire, J.O., and M. Karachi. 2008. Structural development and productivity of replanted mangrove plantations in Kenya. Forest Ecol. Manag. doi:10.1016/j.foreco.2008.01.031.

Putz, F., Chan, H.T., 1986. Tree growth, dynamics, and productivity in a mature mangrove forest in Malaysia. Forest Ecol. Manag. 17: 211–230.

Walton, M.E.M., Samonte-Tan, G., Primavera, J.H., Edwards-Jones, G., Le Vay, L., 2006a. Are mangroves worth replanting? The direct economic benefits of acommunity-based reforestation project. Environ. Conserv. 33: 335–343.