Testing the Resiliency of Created Tidal Wetlands

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This article is part of the Fall 2017 issue of the Earth Science Matters Newsletter.

Extensive land use change, such as forest clearing for agriculture and urbanization, has taken place over recent decades and centuries in many coastal states of the United States, diminishing the distribution and extent of natural tidal wetlands. However, tidal wetlands provide many important goods and services to local communities that need to be maintained. As a result, scientists and community leaders have lauded different types of restoration activities – from large-scale wetland creation to simply changing local water management – to help offset losses of ecosystem services resulting from past destruction or degradation of tidal wetlands. However, the creation, restoration, or rehabilitation of tidal wetlands can be costly, and, the benefit gained from those efforts might be erased by sea-level rise if the wetlands cannot build surface elevation vertically at sufficient rates.

In a recent paper, USGS scientists and colleagues measured surface elevation change, vertical accretion of sediments, and subsurface change on nine created tidal wetlands and compared them with measurements from nine adjacent reference wetlands in Tampa Bay, Florida, USA. Measurements were taken over 5 years, but created tidal wetlands ranged in age from 2.4 to 20.2 years at the time that measurements began. Salt marsh grasses that were planted originally after sites were graded at an appropriate elevation were replaced by natural recruitment of mangrove trees over 7-25 years (Fig. 1).

comparison of mangrove seedlings to mangrove forest

Figure 1. (top) Large quantities of mangrove seedlings naturally colonize created tidal wetlands in Tampa Bay, Florida, USA to form (bottom) mangrove forests over 20-25 years, not only restoring the visual appearance of a forest but also considerable root growth in the soil which contributes to carbon storage and facilitates site resilience to sea-level rise.

(Public domain.)

One of the ecosystem services provided by tidal wetlands is carbon storage, which averaged 2.18 Mg C/ha/year in the upper 10 cm of soil on sites in Tampa Bay. Carbon storage is an important attribute of natural tidal wetlands, and signifies the health of the ecosystem not only for storing organic matter to help offset sea-level rise, but also for re-establishing other important ecosystem services. Initially, very high rates of surface elevation gain occurred (up to 11.2 mm/year). These high rates resulted from deposition of mineral matter from tides and upland erosion as well as high levels of root production by new seedlings, which displaced soil upward. In many cases, root volume expansion was the dominant process of surface elevation change during early phases of wetland development. In later stages, continued accumulation of minerals and organic matter caused compaction of the root zone, resulting in more modest rates of surface elevation gain of 4.0-4.2 mm/year at years 15-25. These rates were well within surface elevation change rates recorded in nearby reference forests

Based on recent rates of relative sea level rise, it appears that created wetlands would be self-sustaining if observed rates of elevation gain are maintained. However, many models of sea-level rise project an acceleration over the remainder of this century. The study also developed an empirical feedback model using surface elevation change versus absolute site elevation to determine sea-level rise susceptibility of the created and reference tidal wetlands under two IPCC scenarios: medium accelerations (RCP 6.0, 0.55 m rise by 2100) and fast accelerations (RCP 8.5, 0.74 m rise by 2100). Model output projected that only one reference mangrove site would submerge by 2100 with medium accelerations, but all sites (created and reference) were projected to submerge by 2100 under fast accelerations. Thus, only the fastest projected accelerations resulted in losses of these created tidal wetlands, suggesting future benefits to continuing national level programs dedicated to the creation, restoration, and rehabilitation of tidal wetlands in the US and abroad.

The paper, “Created mangrove wetlands store belowground carbon and surface elevation change enables them to adjust to sea-level rise”, was published in Scientific Reports. It is available at: https://www.nature.com/articles/s41598-017-01224-2.

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