Mangrove regional feedback to sea level rise and drought intensity at the end of the 21st century
Mangrove ecosystems are threatened by climate change. We investigated the effects of expected future (year 2100) drought intensities and rising sea levels on the spatial extent and biomass production of mangroves located along the southern Iranian semi-desert coastal areas of the Persian Gulf (PG) and the Gulf of Oman (GO) under the projections of the RCP 8.5 climate change scenario. To do so, we first needed to establish a robust link of past drought intensities to spatial extents and biomass amounts of mangroves in the study region that would enable the prediction of biomass for the climatic conditions projected by the RCP 8.5 scenario for the year 2100. Large differences in drought intensities in the past pointed to a coordinated wet (1986–1998) and a dry (1998–2017) period throughout the study area and resulted in strong correlations of drought intensity to spatial extents and above- and below-ground biomass amounts. Whereas landward mangrove margins expanded modestly during the wet and contracted severely during the dry periods, leading to variable net areal gains and losses over time, seaward mangrove margins retreated during both periods, presumably due to rising sea levels. By the end of the 21st century, predicted values of biomass per hectare in the remaining mangroves exceeded current values by 47–64% (above-ground) and 41–48% (below-ground) due to a reduced drought intensity predicted for the region. Assuming no landward expansion, predicted mangrove areas declined between 4.9 and 7.2% for every 10 cm rise in sea levels, resulting in a net loss of total mangrove biomass between 18 and 56% throughout the study region at a sea level rise of 100 cm. Variability among sites at all times was partly due to differences in drought intensities, coastal topographies, and differential rates of sedimentation and subsidence/uplift, with greater adverse effects on the coastal areas of the GO than the PG. We conclude that adverse effects of rising sea levels on the extent of mangroves were only partly offset by the increased biomass in the remaining mangroves following reduced drought severities predicted for the end of the 21st century. It is still unclear to what degree mangroves can take advantage of lesser drought intensities predicted for the end of the 21st century and expand their landward margins.