Fine root production along a 40-year chronosequence of restored mangroves in Vietnam

Mangroves are hotspots of carbon sequestration, providing ecosystem services worth US$194 000 per hectare per year. In response to widespread mangrove losses, reforestation projects have been promoted. Monitoring and assessment of those projects have mainly focused on aboveground carbon stocks, but most of the carbon is found underground (as soil carbon and roots) and little is known about belowground carbon dynamics in mangroves. In particular, it is unknown how fine root production develops during the period following reforestation. A better understanding of fine root production is important, since fine root production is a major driver of soil organic matter accumulation, which allows mangroves to occupy vertical accommodation space to withstand sea-level rise. Using minirhizotrons, we characterised the variation of fine root production along a chronosequence of mangroves in the Mekong Delta in Vietnam replanted in 1978, 1986 and 1991. We found that fine root production declines with: i) mangrove age, as a result of the self-thinning processes associated with mangrove ageing; and ii) soil depth, likely due to a vertical gradient in soil nutrient availability. Our findings have important implications for understanding belowground carbon dynamics, and highlight the need to account for mangrove age when forecasting mangrove carbon dynamics and resistance to sea-level rise.