Natural recovery of mangroves in abandoned rice farming areas of the Rufiji Delta, Tanzania

Mwita M. Mangora,Judith A. Okello,Emmanuel Japhet,Carl C. Trettin

doi:10.4314/wiojms.v18i2.3

Abstract

Natural recovery of mangroves in abandoned rice farming areas is important for reviving its multiple ecosystem services including climate change mitigation. This study aims at understanding the natural recovery rate and pattern of mangroves in the abandoned rice farming areas of the Rufiji Delta. Mangrove areas were stratified into early and intermediate succession as well as old growth forest. Forty-five nested plots, fifteen at each site, were randomly selected, where plant and soil data were collected. Based on the Importance Value Index, the most abundant tree species in the early succession was Barringtonia racemosa, a mangrove associated tree species having a value of 67.9. The mangrove Avicennia marina was the most abundant in both intermediate succession and old growth forest having a value of 170.7 and 163.1, respectively. Pairwise comparison of means indicated a significant change (p< 0.05) of structural parameters with fallow age. No significant change (p > 0.05) was detected in the measured soil properties among the three succession categories. The findings demonstrate that even a period of up to 15 years would not allow full recovery of structural attributes for a mangrove forest converted to agricultural land, with grass cover being among factors limiting the recovery due to obstruction of propagule dispersal.

Highlights

The mangrove ecosystem in the Rufiji Delta, Tanzania, has over the years been severely affected by a complex set of factors, including climate change and anthropogenic pressures, triggering their loss (Wagner and Sallema-Mtui, 2016)
The highest plant species richness was found in the old growth forest, followed by the intermediate succession and the early succession sites
The recovery trends in this study shows that structural attributes, stem density, basal area and height increased significantly with fallow age (p = 0.001), but they are much lower compared to the old growth forest (Table 2)

Summary

Introduction

The mangrove ecosystem in the Rufiji Delta, Tanzania, has over the years been severely affected by a complex set of factors, including climate change and anthropogenic pressures, triggering their loss (Wagner and Sallema-Mtui, 2016). The anthropogenic threats include over harvesting of mangrove trees (for fuel wood, building materials), conversion of mangrove areas for other uses, such as salt works, human settlements and agriculture, and in particular rice farming (Semesi, 1992; Mangora et al, 2016). Rice farming is reported to be a major cause of mangrove cover loss in the delta. Most recently (Monga et al, 2018) reported the loss of mangroves due to rice farming at about 7,298 ha between 1991 and 2015, implying an annual loss of about 304 ha per year. South-East Asia has been in the spot light where conversion for aquaculture has led to unprecedented losses of mangroves, with about 22,000 ha lost between 2000 and 2012 (Richards and Friessa, 2016). Mangrove deforestation has implications for the provision of ecosystem services (Richards and Friessa, 2016), as it results in a loss of carbon (Lagomasino, 2019) and other biodiversity supported by the mangroves

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Discussion

Conclusion