Massive mangrove dieback due to extreme weather impact - case of Maputo River Estuary, Mozambique

Abstract

This study documents one of the first massive mangrove dieback in Africa caused by hailstorm, occurring in Maputo Bay, Mozambique. Field observation and satellite imagery prompted this observation of extensive mangrove dieback out of Maputo River Estuary, a regionally significant large mangrove stand, the southernmost large mangrove area in the Western Indian Ocean.This study aimed to determine the causes and extent of mangrove dieback in Maputo River Estuary that occurred in September 2019, identify, and quantify the mangrove loss, describe the structure of the mangrove forest, describe the soil composition in healthy and impacted areas. To address this Sentinel 2 imagery were assessed and calculated the Normalized Difference Vegetation Index (NDVI) to estimate the mangrove dieback area and cover change. This was supported by an extensive field survey, impacted and pristine or natural areas were assessed by sampling 233 plots of 10×10 m, established along transects separated by 50 m and set perpendicularly to the coastal line. Structural parameters assessed included species composition, height, diameter at the breast height (DBH), number of live and dead trees, density of stumps and seedling per species.The mangrove cover reduced nearly half (48 %) of mangrove area in just a year, from 1377.4 Ha in 2019 to barely 716.2 Ha in 2020, representing a loss of 661.2 Ha, and 38.7 % of trees were completely dead. Five mangrove species were identified, Avicenna marina being largely dominant. The regeneration ratio was 142:10:1, way beyond the minimum ecological ratio of standard mangrove forest, 6:3:1. According to local communities and meteorological data, it seems plausible that this event was caused by a hailstorm.These results contribute to understanding climate-related impacts on mangrove forests and its response which is crucial for adopting adequate management measures, recovery and climate change adaptation actions and monitoring of mangrove forests in Mozambique.