Seventy years of continuous encroachment substantially increases ‘blue carbon’ capacity as mangroves replace intertidal salt marshes

Study Number

100

Author

Jeffrey J. Kelleway, Neil Saintilan, Peter I. Macreadie, Charles G. Skilbeck, Atun Zawadzki, Peter J. Ralph

Abstract

Shifts in ecosystem structure have been observed over recent decades as woody plants encroach upon grasslands and wetlands globally. The migration of mangrove forests into salt marsh ecosystems is one such shift which could have important implications for global ‘blue carbon’ stocks. To date, attempts to quantify changes in ecosystem function are essentially constrained to climate-mediated pulses (30 years or less) of encroachment occurring at the thermal limits of mangroves. In this study, we track the continuous, lateral encroachment of mangroves into two south-eastern Australian salt marshes over a period of 70 years and quantify corresponding changes in biomass and belowground C stores. Substantial increases in biomass and belowground C stores have resulted as mangroves replaced salt marsh at both marine and estuarine sites. After 30 years, aboveground biomass was significantly higher than salt
marsh, with biomass continuing to increase with mangrove age. Biomass increased at the mesohaline river site by 130 ± 18 Mg biomass km-2 yr-1 (mean ± SE), a 2.5 times higher rate than the marine embayment site (52 ± 10 Mg biomass km-2 yr-1), suggesting local constraints on biomass production. At both sites, and across all vegetation categories, belowground C considerably outweighed aboveground biomass stocks, with belowground C stocks increasing at up to 230 ± 62 Mg C km-2 yr-1 (± SE) as mangrove forests developed. Over the past 70 years, we estimate mangrove encroachment may have already enhanced intertidal biomass by up to 283,097 Mg and belowground C stocks by over 500,000 Mg in the state of New South Wales alone. Under changing climatic conditions and rising sea levels, global blue carbon storage may be enhanced as mangrove encroachment becomes more widespread, thereby countering global warming.

Main Results and Conclusions

• There has been a trend in the last seventy years of mangrove encroachment on salt marshes.
  • “Over the past ~70 years, mangrove encroachment has resulted in the conversion of ~30% or more of salt marsh to mangrove forest across south-eastern Australia…” (1107)
  • “At a global scale, mangrove encroachment of salt marsh may be driven by a suite of changing environmental factors favouring mangrove, including rising sea level, elevated atmospheric CO2 and higher temperatures…” (1098)
  • “Such a change in ‘blue carbon’ habitats – that is, C-rich, marine habitats – could have significant implications for regional and global C pools, as mangroves (trees and shrubs) and salt marshes (communities of grasses, succulent herbs, rushes and low shrubs) are disproportionately important in sequestering C relative to their spatial extent…” (1097)
• Nutrient limitation has been linked to increases in biomass carbon storage.
  • “…mangrove biomass and forest architecture are known to vary substantially according to nutrient status, ranging from growth of vigorous tall trees in nutrient-rich riverine settings down to dwarf trees in nutrient-poor areas near the coastal fringe…-a trend generally consistent with biomass differences in our study.” (1104-1105)
• Rise in salinity levels have been linked to increases in biomass carbon storage.
  • “…growth studies have shown increased growth of A. marina seedlings under lower salinities (20–80% seawater concentrations)… and, more specifically, declines in photosynthetic capacities of both A. marina and A. corniculatum with increasing salinity.” (1104)
• Changed in sedimentation levels have been linked to increases in biomass carbon storage.
  • “…differences in sedimentation among sites may alter the sensitivity of mangrove growth to nutrient enrichment and alter nutrient and carbon cycling.” (1105)
• This encroachment has caused a rise in aboveground biomass accumulation.
  • “Aboveground biomass increase was seen at both study sites, as mangroves encroached areas previously vegetated by salt marsh species.” (1104)
  • “This may not be surprising, considering the low biomass of salt marshes relative to mangroves…” (1104)
  • “The increase in biomass measured aboveground as mangroves encroach salt marsh was also observed belowground in cores analysed for bulk C content.” (1106)
  • “…significant increases in belowground C coincided with fine root development of mangroves, with fine root biomass dominating sediment volume in surface layers of encroached areas.” (1106)
• This encroachment has caused a rise in belowground biomass accumulation.
  • “The results of two contrasting wetland settings in our study show that in the absence of extreme winter events, and given sufficient time, significant increases in belowground C stocks do become apparent under mangrove encroachment.” (1106)
• It is predicted that below and above ground biomass will continue to accumulate, which translates into increased C storage.
  • “While the exact timing and rates of change of mangrove encroachment cannot be determined at a regional scale with current data, extrapolation of findings from the present study suggests a significant increase in blue C stocks has already occurred.” (1107)
  • “Based on a 30% conversion of the current extent of salt marsh in New South Wales… biomass increases of 1618–4044 Mg yr-1 (113,239–283,097 Mg over 70 years) and belowground C increases of up to 7155 Mg C yr-1 (500,864 Mg C over 70 years; extrapolated from Georges River rate only) may have already occurred over the past 70 years in New South Wales alone.” (1107-1108)
  • “If mangroves continue to further encroach salt marshes in the region, as is predicted to happen, annual additions to above and belowground C stores of the magnitude reported here may continue.” (1108)