Review of the ecosystem service implications of mangrove encroachment into salt marshes

Abstract

Salt marsh and mangrove have been recognized as being among the most valuable ecosystem types globally in terms of their supply of ecosystem services and support for human livelihoods. These coastal ecosystems are also susceptible to the impacts of climate change and rising sea levels, with evidence of global shifts in the distribution of mangroves, including encroachment into salt marshes. The encroachment of woody mangrove shrubs and trees into herbaceous salt marshes may represent a substantial change in ecosystem structure, although resulting impacts on ecosystem functions and service provisions are largely unknown. In this review, we assess changes in ecosystem services associated with mangrove encroachment. While there is quantitative evidence to suggest that mangrove encroachment may enhance carbon storage and the capacity of a wetland to increase surface elevation in response to sea-level rise, for most services there has been no direct assessment of encroachment impact. On the basis of current understanding of ecosystem structure and function, we theorize that mangrove encroachment may increase nutrient storage and improve storm protection, but cause declines in habitat availability for fauna requiring open vegetation structure (such as migratory birds and foraging bats) as well as the recreational and cultural activities associated with this fauna (e.g., birdwatching and/or hunting). Changes to provisional services such as fisheries productivity and cultural services are likely to be site specific and dependent on the species involved. We discuss the need for explicit experimental testing of the effects of encroachment on ecosystem services in order to address key knowledge gaps, and present an overview of the options available to coastal resource managers during a time of environmental change.

Quantifying how changing mangrove cover affects ecosystem carbon storage in coastal wetlands

“…In this region, marsh vegetation is dominated by stress-tolerant succulent species (e.g., B. maritima), which is typical of other coastal marshes in the coastal bend region of Texas. Mangroves have greater potential to store aboveground biomass than marshes, but the extent of this difference likely depends on ecosystem conditions and developmental stage (Kelleway et al 2017). Mangroves have greater potential to store aboveground biomass than marshes, but the extent of this difference likely depends on ecosystem conditions and developmental stage (Kelleway et al 2017).…”

“…Although mangroves have the capacity to store more OC in aboveground biomass than marshes (Kelleway et al 2017), OC belowground has a much longer residence time (Souza Filho et al 2006) and therefore a greater impact on atmospheric C sequestration. Additionally, we found higher soil OC in mangrove patches than marsh patches, but small-scale differences did not translate to higher OC at the plot level.…”

“…Although mangroves have the capacity to store more OC in aboveground biomass than marshes (Kelleway et al 2017), OC belowground has a much longer residence time (Souza Filho et al 2006) and therefore a greater impact on atmospheric C sequestration. However, soil OC accumulation is a slow process and the impacts of mangrove encroachment on soil development can take decades in natural (Kelleway et al 2017) and restored ecosystems (Osland et al 2012). Over the short term, trade-offs in specific OC fluxes may minimize differences in soil OC storage in marsh and mangrove ecosystems (Perry and Mendelssohn 2009, Henry and Twilley 2013, Doughty et al 2016, Yando et al 2016).…”

The state of the world’s mangroves in the 21st century under climate change

“…Both mangroves and saltmarshes are foundational habitats that are independently valued for their contributions to coastal productivity, buffering capacity, and carbon storage (e.g., Mazumder & Saintilan, 2003;McKee & Rooth, 2008;Nagelkerken et al, 2008;Feller et al, 2010;Lee et al, 2014). Recent expansion of mangroves into saltmarshes is likely to have large impacts on the structure, function and service provisioning of coastal wetlands (Kelleway et al, 2017). Although mangrove encroachment may increase nutrient storage and improve storm protection (Sheng & Zou, 2017), Kelleway et al (2017) hypothesized that declines will occur in habitat availability for fauna requiring open vegetation structure, as well as in the recreational and cultural activities associated with this fauna.…”

“…Although mangrove encroachment may increase nutrient storage and improve storm protection (Sheng & Zou, 2017), Kelleway et al (2017) hypothesized that declines will occur in habitat availability for fauna requiring open vegetation structure, as well as in the recreational and cultural activities associated with this fauna. They further project that the impact on provisional services such as fisheries productivity and cultural services will be site-specific and dependent on the species involved (Kelleway et al, 2017).…”