Death and Regeneration of an Amazonian Mangrove Forest by Anthropic and Natural Forces

Abstract

The Amazon Macrotidal Mangrove Coast contains the most extensive and continuous mangrove belt globally, occupying an area of ~6500 km2 and accounting for 4.2% of global mangroves. The tallest and densest mangrove forests in the Amazon occur on the Bragança Peninsula. However, road construction that occurred in 1973 caused significant mangrove degradation in the area. A spatial-temporal analysis (1986–2019) based on optical, Synthetic Aperture Radar (SAR), drone images, and altimetric data obtained by photogrammetry and validated by a topographic survey were carried out to understand how the construction of a road led to the death of mangroves. The topographic data suggested that this road altered the hydrodynamical flow, damming tidal waters. This process killed at least 4.3 km2 of mangrove trees. Nevertheless, due to natural mangrove recolonization, the area exhibiting degraded mangrove health decreased to ~2.8 km2 in 2003 and ~0.73 km2 in 2019. Climatic extreme events such as “El Niño” and “La Niña” had ephemeral control over the mangrove degradation/regeneration. In contrast, the relative sea-level rise during the last several decades caused long-term mangrove recolonization, expanding mangrove areas from lower to higher tidal flats. Permanently flooded depressions in the study area, created by the altered hydrodynamical flow due to the road, are unlikely to be recolonized by mangroves unless connections are re-established between these depressions with drainage on the Caeté estuary through pipes or bridges to prevent water accumulation between the road and depressions. To minimize impacts on mangroves, this road should have initially been designed to cross mangrove areas on the highest tidal flats and to skirt the channel headwaters to avoid interruption of regular tidal flow.

Response Mechanism of Leaf Area Index and Main Nutrient Content in Mangrove Supported by Hyperspectral Data

“…The result of mean percentage shows that there is no significant difference in the retrieval index of mangroves in the three regions (Figure 5a), which shows that although the environment of the three regions is different, the overall health level of mangroves is the same [34,53]. The maximum value calculation results indicate that the maximum values of the other six indicators appear in region A and B, except for the maximum value of Chlb in region C. The index with a large difference is TK, and the maximum values of other indicators are also approximately the same (Figure 5b) [21].…”

“…Natural factors such as temperature, sea level rise, extreme weather events, and coastline length have a macro impact on the distribution of mangroves [32,33]. The whole process of mangrove death caused by road construction was studied based on the analysis of optics, synthetic aperture radar, UAV image, and topographic survey data [34,35]. The spatial and spectral information obtained through remote sensing technology can reflect the growth status and species composition of mangroves, as well as the impact of habitat changes and other factors on the health of mangroves.…”