Massive mangrove dieback in Maputo River Estuary, Southern Mozambique
Oral presentation, WIOMSA 12th symposium
Tuesday, October 11th 2022, room 5, 11:00 am
Mangrove in Mozambique @IRD- Stéphanie Duvail
Authors
V. Machava-António, Department of Biological Sciences, Eduardo Mondlane University,
H. Mabilana, Department of Biological Sciences, Eduardo Mondlane University,
C. Macamo, Department of Biological Sciences, Eduardo Mondlane University,
A. Fernando, Department of Biological Sciences, Eduardo Mondlane University,
M. Massingue, Department of Biological Sciences, Eduardo Mondlane University,
S. Bandeira, Department of Biological Sciences, Eduardo Mondlane University,
J. Paula, Marine and Environmental Research Centre, Faculty of Sciences, University of Lisbon,
Abstract
Mangrove forests are resilient to multiple environmental changes and human impacts, and despite knowing the natural condition for mangrove colonization and development, it has been difficult to identify causes of rapid massive dieback of mangrove trees. These events have been associated with long-term stress on vegetation by sea level and drought, but the capacity to detect and measure those impacts are still limited, with effects largely unstudied. The overall aim of this study is to determine the extension and the causes for the mangrove dieback in Maputo River Estuary and the impact of this loss to the ecosystem, using the following approach
(1) identify and quantify the mangrove loss using a Normalized Difference Vegetation Index (NDVI),
(2) describe the structure of the mangrove forest in healthy and impacted areas,
(3) identify the causes of the mangrove dieback.
To estimate the change in the mangrove cover area after the observed massive dieback, cover was assessed at a landscape scale through remote sensing images from the Landsat 8 Operational Land Imager, with ground spatial resolution of 30 m per pixel, satellite images were transformed into a NDVI, and a field survey was conducted at both impacted and natural sites in the Maputo River to validate the mangrove imagery data.
The georeferenced survey sites were overlaid on the map, then the field notes, photos and videos compared with mapped damage level categories. To access the mangrove forest structure, in both on natural and impacted areas were established 233 plots distributed in 50 m distant transects perpendicular to the coastal line. Each plot had an area of 100 m2 (10m x 10m), following standard field protocols. In each plot, was determined the mangrove trees height, species identity, diameter at the breast height (DBH), number of live and dead trees and height and density of stumps. Dead trees were defined by the absence of leaves, being classified in three classes, and in each plot were also identified the seedling density per species.
Paletuviers in Europa @ IRD - Pascale Chabanet
Hailstorm impacts on mangrove forests can cause bark damage, loss of floral buds and fruits and major canopy. In the Maputo River Estuary, the mangrove cover was of 1377.39 Ha in 2019 and it decreased to 716.17 Ha in 2020, representing a loss of 48.01% (661.22 Ha) of the mangrove area in a year. In the sampled areas, 38.71% of trees/ha were completely dead and the proportion of live and dead trees were of 1 to 1, in some of the sampled areas. The mean DBH of all species is of 16.63 cm and 2.32 m of Height. Five mangrove species were identified in Maputo River estuary, Avicennia marina, B. gymnorhiza, C. tagal, R. mucronata and X. granatum. The Importance Value Index of the species was higher for A. marina in all sampling sites, followed by Bruguiera gymnorhiza. None of the sampling areas have a regeneration ratio that approaches the minimum ecological ratio of 6:3:1. Local communities conducted some mangrove replantation, but most of the seedlings die before reaching 40 cm of height.
Extreme whether events are becoming more frequent in different parts of the world, and this study documents the first evidence of extreme weather impacts on mangrove forests in Mozambique, that caused a massive mangrove dieback and continuing degraded conditions 2 years after the impact. This was similar to what observed in other coastal wetlands under potential thermal and drought effects, where mangrove recovery is also dependent of other physiological stress of adults and seedlings. The results of this study contribute to understand natural mangrove die back and provide insights to the adoption of adequate mangrove recovery actions, responding to several commitments Mozambique has assumed, including recommendations of the recently approved mangrove management strategy (Estratégia do Mangal, 2020 - 2024) and Millennium SDG 14 target, addressing the unique importance of climate change adaptation actions and coastal monitoring throughout the extensive mangrove forests in Mozambique.