The growth trend shows that there are two reverse trends of the number of trees and natural regenerations between different vegetation types. A higher number of trees can be found in the least disturbed areas and vice versa. The number of trees and natural regenerations may be influenced by the canopy opening factor. Amir & Duke (2019) found a similar result in Moreton Bay, Australia. They found that there is a higher seedling density in an opened-canopy area, compared to a closed-canopy area. They proposed that the canopy opening will stimulate the growth of the seedlings, which previously was constrained by the competition between bigger trees and seedlings. Subsequently, Irwanto et al. (2020) also found a similar result of high mangrove seedlings and saplings density in the forest gaps in Maluku Island, Indonesia. The gaps allow more sunlight to reach the forest floor, which stimulates the seedling’s growth. Azad et al. (2021) in their study from Bangladesh also concluded that canopy gaps are the main influence on seedling’s abundance. The more canopy gaps are, the higher the seedling density is.
However, the canopy gaps do not necessarily contribute to the seedling’s growth to its full capacity. The change in forest cover due to anthropogenic activities was found to pose significant impacts on the soil structure, nutrients, enzymes, and sulfide accumulation (Chowdhury et al. 2019). These factors will inhibit the seedling’s growth, thus influencing the regeneration from contributing to the forest biomass and carbon storage.
Another study by Machava-António et al. (2022) also found that there is a greater mangrove regeneration potential in the heavily anthropogenically impacted areas in Maputo Bay than in the less impacted areas. They also found that the good mangrove regeneration rates were contributed by dominant species such as Avicennia marina, Ceriops tagal, and Rhizophora mucronata. These may be explained by the viability and availability of seeds respectively.
The study then found that the least disturbed vegetations (MBBF and BNF) have higher AGB and carbon storage than the most disturbed vegetations (RF-D and RF-M). This suggests that the more intact forests have higher biomass and carbon storage than the disturbed forests. A similar result was found by Adotey et al. (2022) in which the protected mangrove forests of Amanzule, Ghana has 28 times higher aboveground carbon densities than the disturbed Kakum forest. The higher carbon densities in the Amanzule forest are contributed by the big tree diameter and height as compared to the low average diameter and height of trees in the Kakum forest. The tree size is often reflected by the forest canopy. Smaller size tree (commonly seedling and sapling) is more often found in broader canopy openings as the seedlings receive more sunlight and are able to grow more. However, dense forest canopy often prohibits seedling and saplings growth, hence the difference in tree size.
The influence of tree size on carbon storage was also found by Rozainah et al. (2021) in their study at Klang Straits, Malaysia. They found that although Telok Gong has the lowest number of trees, the carbon storage is higher than the pristine Pulau Klang forest and disturbed Pulau Ketam forest. Telok Gong consists of larger trees and there are patches of old-aged forests in the area. Swangjang & Panishkan (2021) in their study on the western coast of the Gulf of Thailand, Thailand also found that more carbon can be found in the old-aged mangrove forest (natural forest) as compared to the young-aged forest (disturbed land). The carbon storage increases with the increase of tree age. They estimated that the loss of 1 hectare of mangrove vegetation will result in the loss of 77.71–189.97 tonne of carbon per hectare and 32.54–81.73 tonne of carbon per hectare in disturbed land and natural forest, respectively.
Although natural regeneration has the potential to recover the carbon stocks, the time taken for the recovery process is long. It was found that the rates of carbon sequestration increased over the first 15 years with a mean of 4.0 ± 2.5 Mg C ha− 1 year− 1 and continued at a comparatively continuous rate of 7.0 ± 2.1 Mg C ha− 1 year− 1 up until approximately 40 years (Sasmito et al. 2019). There is a need to restore the carbon stocks to their original levels and promoting rehabilitation is important for effective climate change mitigation.