Abstract
Mangrove forests are important coastal wetlands because of the ecosystem services they provide especially their carbon potential. Mangrove forests productivity in the Niger Delta are poorly quantified and at risk of loss from oil pollution, deforestation, and invasive species. Here, we report the most extensive stem girth survey yet of mangrove plots for stand and canopy structure in the Niger Delta, across tidal and disturbance gradients. We established twenty-five geo-referenced 0.25-ha plots across two estuarine basins. We estimated aboveground biomass (AGB) from established allometric equations based on stem surveys. Leaf area index (LAI) was recorded using hemispherical photos. We estimated a mean AGB of 83.7 Mg ha–1 with an order of magnitude range, from 11 to 241 Mg ha–1. We found significantly higher plot biomass in close proximity to a protected site and tidal channels, and the lowest in the sites where urbanization and wood exploitation was actively taking place. The mean LAI was 1.45 and ranged fivefold from 0.46 to 2.41 and there was a significant positive correlation between AGB and LAI (R2 = 0.31). We divided the plots into two disturbance regimes and three nipa palm (Nypa fruticans) invasion levels. Lower stem diameter (5–15 cm) accounted for 70% of the total biomass in disturbed plots, while undisturbed regimes had a more even (∼25%) contribution of different diameter at breast height (DBH) size classes to AGB. Nipa palm invasion also showed a significant link to larger variations in LAI and the proportion of basal area removed from plots. We conclude that mangrove forest degradation and exploitation is removing larger stems (>15 cm DBH), preferentially from these mangroves forests and creates an avenue for nipa palm colonization. This research identifies opportunities to manage the utilization of mangrove resources and reduce any negative impact. Our data can be used with remote sensing to estimate biomass in the Niger Delta and the inclusion of soil, leaf properties and demographic rates can analyze mangrove-nipa competition in the region.
Highlights
Mangroves are very productive ecosystems due to their tropical coastal location (Alongi, 2009)
Unchecked logging of mangrove trees leads to a reduction in mangrove extension and has been linked to the expansion of the invasive Nypa fruticans within the Niger Delta (Okugbo et al, 2012; Global Invasive Species Database (GISD), 2015)
We address the following questions: (a) what is the natural variation of aboveground biomass (AGB) and Leaf area index (LAI) across the Niger Delta study site? (b) What are the effects of anthropogenic factors on the structure of mangrove forests? (c) Is mangrove wood exploitation a precursor to nipa palm invasion in the Niger Delta? We hypothesize that higher biomass plots will be closer to tidal channels and farther from human settlements
Summary
Mangroves are very productive ecosystems due to their tropical coastal location (Alongi, 2009). Pristine mangrove forests in Nigeria serve as important sources of seafood including shellfish, finfish and nursery grounds for these organisms (Feka and Ajonina, 2011). These aquatic organisms are vital indicators of undisturbed mangrove ecosystems in coastal Nigeria (Amadi et al, 2014). Mangrove research in Nigeria is restricted to community structure relationship with soil properties, carbon dynamics and productivity, remote sensing of forest area and remediation/ remediation (Ukpong, 1994, 2000b; James et al, 2007, 2013; Jackson, 2011; Fatoyinbo and Simard, 2013; Edu E.A. et al, 2014; Edu E.A.B. et al, 2014). Remote sensing analysis relies on sound ground data for calibration, and fieldwork is still an important requirement
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