In this post, Dana Bergstrom writes about her recent paper “Rapid collapse of a sub-Antarctic alpine ecosystem: the role of climate and pathogens”, which shows that the cushion plants, Azorella macquariensis, estimated to be hundreds of years old, are dying due to windier and drier conditions.
Early in the austral summer of 2008/09, I was beginning my field season on remote sub-Antarctic Macquarie Island, a tiny World Heritage site in the middle of the Southern Ocean. There were mixed emotions at the island’s research station: happiness because the biggest eradication operation for any island had just been approved and funded, but sadness because the feral rabbit population was booming and grazing coastal slopes to bare peat. So my reporting that something seemed amiss with many plants of the endemic cushion Azorella macquariensis was met by island managers with a heavy sense of weariness.
Azorella macquariensis is winter senescent (but not a deciduous plant species) with the cushion plant keeping its old leaves attached until they rot. So each summer new leaves extend above the previous season’s brown leaves. But that summer many cushions stayed brown with no new leaves. There were even grey cushions that were breaking up and blowing away, leaving exposed peat. In some localised areas, a yellow band moved across cushions, followed by death of tissue behind. This was of concern as Azorella macquariensis is both a keystone species and ecosystem engineer in the alpine fellfield that spans the island’s plateau uplands and bears the brunt of the winds from the furious fifties.
With so many plants suddenly dying the first causal idea was for a virulent disease perhaps spread by rabbits. Colleagues who managed the island had seen Phytophora cinamomii wreak havoc through the forests of Tasmania. As a precaution, the island was put into an instant disease quarantine lock-down with restricted access on the island and field wash downs for those who had to go off-track. But as well as cushions, many species of bryophytes were also dead or showing signs of stress. This lead to our second causal hypothesis that there had been some change in climate that was not favourable to many species in the fellfield.
In our first survey we recorded the presence of dieback in both cushions and bryophytes in over 80% of 115 sites across the island’s alpine plateau. But things were moving very quickly. Over three years (2009–2011) the extent of dieback (cover) in these sites expanded from a mean of around 20% to over 60% death in the cushion plant. With such speed of change and only annual opportunities to visit the island, we adopted a multipronged approach to identify cause. Parallel to this we worked with the island managers with regard to developing the best management response according to the rapidly changing state of knowledge.
Our plant pathologists used two approaches for identifying and isolating a pathogen: culturing and next generation sequencing. No putative pathogen was found; however, they identified a suite of bacterial, fungal and oomycete taxa that are generally saprophytes but with pathogenic potential suggesting that some stressed plants could be vulnerable. For our second hypothesis we used an inference approach. We looked for evidence of recent, biologically meaningful change in climate and then examined functional morphology of Azorella macquariensis to identify a mechanistic basis for any transformation.
The place this research has landed is a complex one. Associated with previously identified southward shifts of weather systems, Macquarie Island has experienced reduced summer water availability and this is despite an increase in annual precipitation. Mean windspeed, sunshine hours and evapotranspiration has increased over the last 40 years and plant available water (the running total in days of the difference between daily precipitation and evapotranspiration plus a 10 mm holding reserve) was negative for the majority of the growing season for 17 successive years leading up to the dieback (1992–2008). Looking at the functioning of the cushion we found the cushion plant to be very unusual: a species that was very slow-growing with minimal internal organisation (no fibres, lignified or other structural tissue, no vascular bundles, limited vascular tissue) and it relied solely on internal water supply to maintain turgor. Without water, the cushion structure is compromised. In summary, the functional morphology and anatomy was indicative of a species adapted to a cold, wet and misty environment and vulnerable to drought stress.
Increased sunshine hours resulted in a reduction of the time A. macquariensis and bryophytes were swathed in cloud and able to intercept cloud water or drizzle, while the increase in wind speed (and the modest increase in air temperature) enhanced water loss through evapotranspiration. With increased cyclonic activity, the alpine fellfield climate has changed from generally moist, to fluctuating conditions of wet frontal systems followed by drying winds and sunshine, resulting in suboptimal environmental conditions for many species. So we propose that plants were drying out with various feedback loops of community water loss and plants were getting stressed and then sick with some native microbes becoming pathogenic.
With so much death A. macquariensis was declared critically endangered in 2010. With dieback baseline data on Macquarie Island now established, future monitoring will determine whether this event represents a transient, decadal-level change in the ecosystem or the initiation of a climate-related, transformation away from an Azorella-dominated fellfield ecosystem. That mechanisms driving the ecosystem collapse appear complex with multiple stressors appeared to be impacting cumulatively may be relevant to other locations. This rapid ecosystem collapse on Macquarie Island is giving us a window into the potential impact of climate-induced environmental change on vulnerable ecosystems elsewhere.