Abstract
Mangroves are among the world's most carbon-dense ecosystems, but have suffered extensive deforestation, prompting reforestation projects. The effects of mangrove reforestation on belowground carbon dynamics are poorly understood. In particular, we do not know how fine root production develops following mangrove reforestation, despite fine root production being a major carbon sink and an important control of mangrove soil accretion. Using minirhizotrons, we investigated fine root production and its depth variation along a chronosequence of mature Vietnamese mangroves. Our results showed that fine root production decreases strongly with stand age in the uppermost 32cm of our soil profiles. In younger mangrove stands, fine root production declines with depth, possibly due to a vertical gradient in soil nutrient availability; while root production in the oldest stand is low at all depths and exhibits no clear vertical pattern. A major fraction of fine root production occurs deeper than 30cm, depths that are commonly omitted from calculations of mangrove carbon budgets. Younger mangroves may accrue shallow soil organic matter faster than older mangroves. Therefore, root productivity and forest stand age should be accounted for when forecasting mangrove carbon budgets and resistance to sea-level rise.
Highlights
Background and rationaleMangroves provide ecosystem services, such as flood protection and carbon sequestration, that have been estimated to be worth US$194 000 per hectare per yr (Costanza et al, 2014, using data from 2011); these valuable coastal wetlands are being lost rapidly (Duke et al, 2007; Richards & Friess, 2016)
Our results showed that fine root production decreases strongly with stand age in the uppermost 32 cm of our soil profiles
Fine root production declines with depth, possibly due to a vertical gradient in soil nutrient availability; while root production in the oldest stand is low at all depths and exhibits no clear vertical pattern
Summary
Background and rationaleMangroves provide ecosystem services, such as flood protection and carbon sequestration, that have been estimated to be worth US$194 000 per hectare per yr (Costanza et al, 2014, using data from 2011); these valuable coastal wetlands are being lost rapidly (Duke et al, 2007; Richards & Friess, 2016). Mangrove reforestation has intensified since it was recognised that mangroves are among the world’s most carbondense ecosystems and protect the coast from storms (Bosire et al, 2008; Donato et al, 2011; Lee et al, 2014). Up to 90% of mangrove ecosystem carbon is stored in the soil, of which fine roots are one of the primary sources, along with riverine sediments (Middleton & McKee, 2001; Bouillon et al, 2008; McKee, 2011; Ezcurra et al, 2016). The resistance of reforested mangroves to sea-level rise, and the long-term trajectory of the carbon store in reforested mangroves, are unclear
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