Abstract
Mangrove ecosystems under tropical monsoon climates experience changes in environmental factors, especially seasonal variations in salinity. These changes might have direct influences on the mangrove root sphere, which plays an important role in carbon dynamics and supports mangrove growth. We aimed to elucidate how the soil properties including salinity and nutrient budget affect the mangrove roots in the wet and dry seasons across the mangrove zonation (Avicennia, Rhizophora, and Xylocarpus zones). This area is in a secondary forest at the Trat River estuary, eastern Thailand. Root mass was observed at 0–10 and 10–20 cm depths across all zones and the living roots were separated into diameter classes. The soil water salinity was measured at a 10 cm depth. We analyzed the nitrogen, phosphorus, and carbon contents in the roots and soil. Spatiotemporal changes occurred due to the vegetation zonation and the variations in salinity and the content of soil available phosphorus that caused different root sphere conditions along the distance from the river. The highest root biomass was found in the riverward Avicennia zone, which was 4.8 times higher than that of the inland Xylocarpus zone in the wet season. The root necromass distribution along the zonation showed an opposite trend to that of biomass. Among seasons, the root size-class proportion differed, with high fine roots observed during the wet season. We confirmed that the root sphere showed both spatial and temporal heterogeneity. Mangrove roots, especially fine roots, interacted with changing salinity, inundation regime, and biological processes evoked by microtopographic gradients as a consequence of mangrove zonation and seasonal rainfall. Our findings indicate how the root sphere differed by specific vegetation structure in this mangrove forest. Therefore, these might provide an ecological perspective for the mangrove rehabilitation plans to facilitate below-ground carbon stock.
Highlights
Mangrove ecosystems have been considered to be carbon-rich ecosystems with the capacity for potentially long-term carbon storage based on their high productivity [1,2,3].A considerable amount of carbon is allocated in mangrove root systems [4,5], which is known as the ‘bottom-heavy tree form’ [6], with a high ratio of root to shoot biomass [7,8,9]
In the Rhizophora zone, the fine roots contributed the highest percentage of total root biomass, which accounted for 62% and 73%
We found that different inundation, salinization, and decay processes according to the topographic gradient in the study site caused spatial heterogeneity in the root sphere in the mangrove forest with a distinct zonation
Summary
Mangrove ecosystems have been considered to be carbon-rich ecosystems with the capacity for potentially long-term carbon storage based on their high productivity [1,2,3].A considerable amount of carbon is allocated in mangrove root systems [4,5], which is known as the ‘bottom-heavy tree form’ [6], with a high ratio of root to shoot biomass [7,8,9]. A large amount of root biomass functions to support overall plant growth mechanically and physiologically in muddy and waterlogged soil conditions [10], maximizing water uptake [1], and increasing nutrient acquisition under saline and low-nutrient conditions [11,12,13]. Mangrove roots are an important source of organic matter in soils [14], with approximately 7% of the total organic carbon within 1 m-deep soil contributed by mangrove root biomass and necromass, as reported by Alongi [2]. A large fraction of root necromass and organic matter exists in the soil because of low decomposition and waterlogged conditions [15,16]. The mangrove forests showed resilience to changing environments, especially the response of below-ground components [18,19]
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