Coastal regime shifts: rapid responses of coastal wetlands to changes in mangrove cover

Abstract

Global changes are causing broad-scale shifts in vegetation communities worldwide, including coastal habitats where the borders between mangroves and salt marsh are in flux. Coastal habitats provide numerous ecosystem services of high economic value, but the consequences of variation in mangrove cover are poorly known. We experimentally manipulated mangrove cover in large plots to test a set of linked hypotheses regarding the effects of changes in mangrove cover. We found that changes in mangrove cover had strong effects on microclimate, plant community, sediment accretion, soil organic content, and bird abundance within 2 yr. At higher mangrove cover, wind speed declined and light interception by vegetation increased. Air and soil temperatures had hump-shaped relationships with mangrove cover. The cover of salt marsh plants decreased at higher mangrove cover. Wrack cover, the distance that wrack was distributed from the water's edge, and sediment accretion decreased at higher mangrove cover. Soil organic content increased with mangrove cover. Wading bird abundance decreased at higher mangrove cover. Many of these relationships were non-linear, with the greatest effects when mangrove cover varied from zero to intermediate values, and lesser effects when mangrove cover varied from intermediate to high values. Temporal and spatial variation in measured variables often peaked at intermediate mangrove cover, with ecological consequences that are largely unexplored. Because different processes varied in different ways with mangrove cover, the "optimum" cover of mangroves from a societal point of view will depend on which ecosystem services are most desired.

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

“…Aerial photography indicates salt marsh dominated in the early 20th century and was largely replaced by mangroves by 1970 (Bianchi et al 2013). Before we manipulated vegetation cover, all plots had similar elevation, soil characteristics and vegetation communities dominated by black mangrove with 10% salt marsh vegetation that was predominantly Batis maritima, Salicornia depressa, Salicornia bigelovii, with small patches of Spartina alterniflora (Guo et al 2017). The 30-yr minimum air temperature in Port Aransas (À9.7°C), surpasses the minimum air temperature threshold (À6.7 to À8.9°C) that causes black mangrove mortality (Lonard and Judd 1991, Stevens et al 2006, Osland et al 2013.…”

“…We allowed marsh vegetation to recolonize areas where mangroves were removed (Appendix S1: Fig. Marsh vegetation (>80% B. maritima) recolonized for 2 yr (Guo et al 2017) before we conducted the research described in this paper. Treatments were maintained throughout the experiment by removing mangrove seedlings that recruited in marsh patches, and annually clipping mangroves encroaching into marsh patches.…”

“…Shifts in dominant vegetation can impact ecosystem structure and function (Van Auken 2000, Kominoski et al 2013), but many of the likely impacts resulting from vegetation shifts remain uncertain and untested. This is a widespread phenomenon, and such shifts in dominant species can alter microclimate, organic-matter sources, and biogeochemistry (Van Auken 2000, D'Odorico et al 2013, Guo et al 2017. This is a widespread phenomenon, and such shifts in dominant species can alter microclimate, organic-matter sources, and biogeochemistry (Van Auken 2000, D'Odorico et al 2013, Guo et al 2017.…”

“…Mangroves are most vulnerable to freeze events near the edge of their range, where prolonged periods of freezing temperatures can cause partial or complete mortality of mangroves and create gradients in mangrove and marsh cover (Guo et al 2013, Osland et al 2015. The impacts on ecosystem structure and function as the proportion in marsh and mangrove cover changes are largely unknown (Guo et al 2017). We quantified the impacts of marsh and mangrove plant species and percent cover on OC stocks (above and belowground biomass and soil OC) and fluxes (root ingrowth, surface accretion, and organicmatter processing rates).…”

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

“…Mangrove species, seedling age, salinity, and the presence/absence of marsh grass can influence mangrove survival outcomes to such events (Coldren & Proffitt, 2017). Mangroves are also expanding into coastal saltmarshes along the Gulf of Mexico (Comeaux et al, 2012;Osland et al, 2013;Guo et al, 2017;Yando et al, 2016) and throughout the Americas with historical evidence of similar largescale contractions in the past as a result of severe freeze events (Sherrod & McMillan, 1985;Everitt & Judd, 1989). In addition to sea-level rise, climate change is expected to result in increased frequency and intensity of rainfall and associated flooding that can discharge massive amounts of sediment into nearshore environments, which then provide favorable new substrate for rapid seaward expansion of mangroves, as has been observed in Northern Australia along the Gulf of Carpentaria (Ashbridge et al, 2016).…”