A project undertaken at the Hawkesbury Institute for the Environment, Western Sydney University, and supervised by Brendan Choat
In the latter part of 2015, an unprecedented die off of mangrove vegetation occurred along the Gulf of Carpentaria coast. This event caused the death of mangroves along a 1000 km swath of coastline, with around 7,400 hectares killed in a single month. Given the enormous value of mangroves to coastal biodiversity and to fisheries industries, it is essential to understand the factors underpinning this mass mortality event. Preliminary studies indicate that the event was driven by a range of environmental factors related to climatic change, most notably a period of severe drought combined with unprecedented high temperatures and a month-long drop in sea level of 20-30cm associated with the 2015-16 El Nino.
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However, we have only limited insight into the physiological underpinnings of mortality in these mangrove communities. This includes knowledge of specific physiological thresholds at which mortality occurs in different species or populations of mangroves. If we are to have any hope of predicting when these events will occur in future and how widespread they will be, a detailed knowledge of these physiological thresholds across relevant mangrove species is essential. In this project, we brought together extensive expertise in the ecology of mangroves and cutting edge plant physiology to investigate the mechanisms underpinning the die off event and determine the vulnerability of mangrove communities to future extreme climate events in northern Australia.
From a physiological perspective, mangroves are fascinating, with a range of adaptations that allow them to grow in saline, anoxic substrates and cope with frequent tidal inundation. An important consequence of growing in a high salinity substrate is an enormous osmotic pressure gradient that must be overcome in order to extract water necessary for transpiration and growth. At the salinity of seawater, this pressure exceeds 2.5 MPa, which is physiologically equivalent to growth in extremely dry soils. Thus, mangroves are effectively exposed to continuous water stress but still manage to thrive. However, the recent mass die off in Gulf of Carpentaria suggests that there are hard limits the level of water stress that mangroves can survive and that these limits may be more frequently encountered with increasing global temperatures.
We hypothesised that mangroves in the Gulf of Carpentaria were exposed to extreme water stress as a result of high temperatures, low inputs of fresh water, less frequent inundation by seawater, and hyper salinity in the root zone. The combination of hyper salinity in the root zone and high evaporative demand at the canopy would lead to the development of severe plant water stress and subsequent failure of the tree vascular transport system. Our study examined the hypothesis that mangroves were exposed to extreme water stress by quantifying levels of vascular impairment in surviving plants. High levels of water stress cause vessels in the plant water transport system (xylem) to fill with gas emboli, leading to declines in plant hydraulic conductivity. During periods prolonged and intense water stress, this can result in catastrophic hydraulic failure and whole plant mortality. However, when plants survive, gas emboli are retained in the xylem and provide a “memory” of the magnitude of water stress suffered by plants during periods of higher temperature and drought. We quantified levels of embolism in surviving individuals of one species, Avicennia marina, which is the dominant species in the Gulf and was most heavily impacted by the die off event.
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PhD student Alice Gauthey led fieldwork in Karumba (QLD) in 2018 and 2019 with colleagues from Southern Cross University and James Cook University. The Karumba site was heavily impacted by the die off event in 2016 with extensive areas of dead and heavily impacted vegetation (Fig 1). Four transects were established across the mangrove vegetation fringing the coast, with plots ranging from severely impacted (near the saltpan) to heathy (at the water edge). Three categories of canopy impact were defined: Severe, where most of the adult trees were dead, Intermediate, where half of the trees were dead, and Healthy, where canopies showed minimal signs of impact (Fig. 2). The results from this fieldwork showed that (i) branches from the severely impacted plots were exposed to greater levels of water stress than branches harvested from healthy plants, and (ii) the loss of hydraulic conductivity caused by xylem embolism was higher in branches from the severely impacted plots, compared with those from intermediate plots and healthy plots (Fig. 3). Vulnerability curves generated for A. marina showed that this species is exceptionally resistant to water stress when compared with other angiosperm plant species. This suggests that the magnitude of water stress imposed during the die off was in an extreme range (-11 MPa). This was confirmed by matching measurements of native embolism from branches measured across three zones (Fig 4). Overall, these results provide evidence to support our initial hypothesis, that trees impacted by 2016 die off event were exposed to extreme water stress and that this stress played an important role in the die off. This finding is important in understanding how Australian mangrove vegetation will cope with the combination of extreme heat and water stress and in predicting the risks posed to mangrove vegetation by rapid climate change.