Mangrove encroachment and climate warming influence belowground dynamics important for maintaining wetland resilience to inundation

Background/Question/Methods

Mangroves are encroaching into marsh-dominated ecosystems globally, representing one of the most dramatic plant range shifts occurring today.  While we are starting to capture the scope of mangrove encroachment, we lack on the ground and “in the ground” information required to understand its consequences. For instance, though plant range shifts can dramatically transform ecosystem structure, particularly when dominant plants in ecosystems shift from herbaceous to woody, no one knows how woody mangrove invasion into herbaceous salt marshes will affect the soil-accumulating processes that sustain these ecosystems in the face of sea level rise. The larger and more porous roots of mangroves are expected to increase oxygen delivery to anoxic wetland soils, potentially increasing decompostion rates, while at the same time building more soil volume.  In addition to plant range shift impacts on wetland soils, we do not yet understand how future warming will directly and indirectly impact soil elevation maintenance. Using a series of warming experiments, remote sensing products, and modeling exercises, we examined the importance of this wetland plant range shift for ecosystem processes such as soil carbon (C) storage, surface elevation maintenance and coastal protection. 

Results/Conclusions

In a mangrove-salt marsh ecotone in Florida, mangrove abundance increased by 69% in 7 years. Concomitant with this dramatic vegetation shift, C storage has increased at a rate of 2.7 Mg C ha^-1 yr^-1, which is double the C sequestration rate of many temperate wetlands. Wetland root biomass, which partially determines soil elevation, is three times higher in mangrove-dominated as compared to marsh-dominated plots and root ingrowth rates are significantly higher in mangrove-dominated plots. Despite these increases in organic matter due to this range shift, mangrove roots decompose more rapidly than marsh roots. After two years of warming, root biomass significantly decreased, particularly at depth, likely driven by an increase in above-ground biomass delivery of oxygen. As mangroves continue to encroach into higher latitudes, in the absence of deep freezes, mangrove-driven increases in organic matter accumulation will likely tend to increase soil elevation. However, as temperatures and sea levels continue to rise, the balance of root biomass accumulation and organic matter decomposition will be even more important in maintaining wetland resilience to inundation.