Effects of dieback on the vegetative, chemical, and physiological status of mangrove forests, Iran

Mangrove forests are valuable resources in tropical and subtropical regions, which have been faced dieback due to various human activities including rapid expansion of shrimp farming, urban development, and pollution, as well as natural factors such as rising sea level, increasing air temperature, drought, and sharp decrease in rainfall. However, the mechanisms of dieback of mangrove forests are not well understood. Therefore, this research aimed to assess the vegetative, chemical, and physiological status of grey mangrove (Avicennia marina (Forsk.) Vierh.) forests at different intensities of dieback in the Hormozgan Province, Iran. A total of 40 plots categorized into four dieback intensities (severe, medium, low, and control) were randomly selected for monitoring, and various parameters related to vegetative, chemical, and physiological status of grey mangrove forests were examined. The results revealed that the control group had the highest tree density, seedling density, vitality levels, aerial root density, and aerial root height. Generally, as dieback severity increased, a decrease in demographic and vegetative parameters of trees and seedlings was observed in the dieback treatments. The amounts of heavy metals (lead, cadmium, and nickel) in the sediment, roots, and leaves of grey mangrove trees at different dieback levels indicated that lead levels were the highest in the sediment, roots, and leaves in the severe dieback treatment. At the same time, the control had the lowest values. Cadmium concentrations in the sediment followed the pattern of severe dieback>moderate dieback>low dieback>control with no significant differences in the roots and leaves. Nickel amounts in all three parts, i.e., sediment, roots, and leaves showed the highest levels in the severe dieback treatment. Furthermore, metal level analysis in the organs of grey mangrove trees at different dieback levels revealed that lead and nickel were more abundant in the root organ compared with the leaves. In contrast, the leaf organ exhibited the highest cadmium levels. Dieback significantly impacted water electrical conductivity (EC), soil organic carbon (SOC), and chlorophyll a, b, and total chlorophyll contents, with the highest values observed in the severe dieback treatment. However, no significant differences were observed in acidity and carotenoid levels. In conclusion, sediment erosion and heavy metal accumulation were critical contributors to dieback of grey mangrove trees, affecting their physiological, vegetative, and plant production characteristics. As the ability of these plants to rehabilitate has diminished, effective management planning is imperative in dieback-affected areas.