Abstract
Mangrove wetlands provide ecosystem services for millions of people, most prominently by providing storm protection, food and fodder. Mangrove wetlands are also valuable ecosystems for promoting carbon (C) sequestration and storage. However, loss of mangrove wetlands and these ecosystem services are a global concern, prompting the restoration and creation of mangrove wetlands as a potential solution. Here, we investigate soil surface elevation change, and its components, in created mangrove wetlands over a 25 year developmental gradient. All created mangrove wetlands were exceeding current relative sea-level rise rates (2.6 mm yr−1), with surface elevation change of 4.2–11.0 mm yr−1 compared with 1.5–7.2 mm yr−1 for nearby reference mangroves. While mangrove wetlands store C persistently in roots/soils, storage capacity is most valuable if maintained with future sea-level rise. Through empirical modeling, we discovered that properly designed creation projects may not only yield enhanced C storage, but also can facilitate wetland persistence perennially under current rates of sea-level rise and, for most sites, for over a century with projected medium accelerations in sea-level rise (IPCC RCP 6.0). Only the fastest projected accelerations in sea-level rise (IPCC RCP 8.5) led to widespread submergence and potential loss of stored C for created mangrove wetlands before 2100.
Highlights
Mangrove wetlands occupy 83,4951 to >137,000 km[2] of coastline[2, 3], providing ecosystem services for millions of people[4]
Mangrove wetlands are capable of adjusting their soil surface elevations adaptively with sea level to help influence their course along populated coastlines, as long as sea-level rise rates are not too high and the mangrove ecosystem itself remains relatively healthy[12]
Surface elevation change for natural mangrove wetlands ranges from −5.8 to 6.3 mm yr−1, a rate that reflects a balance between vertical accretion of sediments and sub-surface change from compaction, decomposition, and/or root zone expansion[12, 19, 20]
Summary
Drivers of surface elevation change in created mangrove wetlands. We suspected that root zone processes would be critical in influencing the surface elevation trajectories on created mangrove wetland sites. While vertical accretion of mineral sediments remained similar on created versus reference sites, root zone expansion was replaced by root zone compaction over time In keeping with this trend, bulk density decreased with created mangrove wetland age to approximate natural forests in 25 years[10], reflecting the greater compaction capacity over time as greater organic fractions contributed to soil structure. Surface elevation change is higher than current sea-level rise for Tampa Bay (Fig. 4); this comparison excludes IPCC projections of increased rates of sea-level rise into the future[45] and assumes a static elevation devoid of known biogenic feedbacks among vegetation productivity and vertical accretion of sediments as influenced by the elevation of the wetland relative to current sea level[46]. Landward migration of created and natural mangrove wetlands would generate additional opportunities for soil C storage as long as sea-level rise acceleration is not too fast and migration corridors are available[49,50,51]
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