Reconstructing extreme climatic and geochemical conditions during the largest natural mangrove dieback on record

Abstract

Abstract. A massive mangrove dieback event occurred in 2015–2016 along ∼1000 km of pristine coastline in the Gulf of Carpentaria, Australia. Here, we use sediment and wood chronologies to gain insights into geochemical and climatic changes related to this dieback. The unique combination of low rainfall and low sea level observed during the dieback event had been unprecedented in the preceding 3 decades. A combination of iron (Fe) chronologies in wood and sediment, wood density and estimates of mangrove water use efficiency all imply lower water availability within the dead mangrove forest. Wood and sediment chronologies suggest a rapid, large mobilization of sedimentary Fe, which is consistent with redox transitions promoted by changes in soil moisture content. Elemental analysis of wood cross sections revealed a 30- to 90-fold increase in Fe concentrations in dead mangroves just prior to their mortality. Mangrove wood uptake of Fe during the dieback is consistent with large apparent losses of Fe from sediments, which potentially caused an outwelling of Fe to the ocean. Although Fe toxicity may also have played a role in the dieback, this possibility requires further study. We suggest that differences in wood and sedimentary Fe between living and dead forest areas reflect sediment redox transitions that are, in turn, associated with regional variability in groundwater flows. Overall, our observations provide multiple lines of evidence that the forest dieback was driven by low water availability coinciding with a strong El Niño–Southern Oscillation (ENSO) event and was associated with climate change.

The Influence of 2015-16 El Niño On the Record-Breaking Mangrove Dieback Along Northern Australia Coast

“…As the global climate warms 46 , and features of ENSO may change 47 , mangroves are likely to experience further stress (e.g. Sippo et al 17 ). Future investigation could include an examination of changes in the large-scale climate stressors in climate model projections to determine if there is an increased likelihood of experiencing mangrove stress events similar to 2015 in the Gulf of Carpentaria in a warming climate.…”

“…Owing to their shallow roots, two mangrove species, particularly Avicennia marina and to a lesser extent Rhizophora stylosa are vulnerable to prolonged below normal sea-levels. Sippo et al 17 concluded that dry conditions combined with porewaters enriched in iron associated with unprecedented below-normal sea levels were the potential causes of the dieback. As a result of months-long episode or permanent sea level drop, a change in the ecosystem can be triggered, with a shift from mangrove forest to drier and more saline saltmarsh, where only a few specialised plants can only grow 18 .…”

Stable isotopes track the ecological and biogeochemical legacy of mass mangrove forest dieback in the Gulf of Carpentaria, Australia

“…The drought conditions most likely caused accumulative hydric, thermal and radiant stresses (Duke et al, 2017). In addition to low water availability, iron (Fe) toxicity due to a rapid mobilisation of sedimentary Fe and regional variability in groundwater flows may have also played a role in the dieback (Sippo et al, 2020a). The event led to the widespread death of mangrove trees in the region, providing an unfortunate yet unique opportunity to test tree mortality effects on the biogeochemical and ecological functioning of mangroves and capture recovery patterns.…”

“…In most cases, recovery of mangroves primarily relies on the recruitment of seedlings (Smith et al, 1994;Krauss and Osland, 2019). Disturbances in mangrove forests not only affect recruitment but can also change the cycling of C, N and S. Loss of mangrove trees and root structures can change organic matter inputs, sediment oxygenation and degradation of sediment organic matter, with consequences for benthic assemblages (Sweetman et al, 2010;Bernardino et al, 2018;Harada et al, 2019), the coastal carbon cycle (Jeffrey et al, 2019;Sippo et al, 2020b), sediment C and N stocks (Adame et al, 2018), microbial assemblages, and associated nutrient processes, e.g. nitrogen fixation and sulfate reduction (Sjöling et al, 2005).…”

Compound climate extremes driving recent sub-continental tree mortality in northern Australia have no precedent in recent centuries

“…These decades of above-average moisture conditions combined with increasing sea level over the same period effectively increased water availability to the mangrove ecosystem 9 , leading to a regional expansion of mangroves across much of northern Australia 7 , 9 , 10 . For Avicennia marina the increased water availability over the preceding decades facilitated a landward intrusion (sampled mangroves that died in 2015–16 in the Gulf of Carpentaria were 30–40 years old) 10 , 11 and hence higher tidal elevation 6 , 9 that left them vulnerable to a subsequent dry period, sudden fall in sea level, or both. Increased water supply to plants almost always results in an increased ratio of leaf to root area 12 , and mangroves with increased leaf to root area ratios are more vulnerable to drought stress 13 .…”