Peatlands’ past suggests fast-changing future

As global climate change intensifies, many of the Earth’s ecosystems are threatened with irreversible change. Some will change gradually and predictably; others may undergo sudden, dramatic state shifts when one or more ecological parameters reach a tipping point, resulting in abrupt transitions from one ecosystem to another.

Predictions from peat
Such sudden state shifts are notoriously difficult to predict under conventional forward modelling approaches. However, rapid ecosystem transformations have occurred in the past and investigating these events may allow scientists to explore the conditions under which they happen, trace their ecological impacts, and estimate the likelihood of future shifts in similar ecosystems. Few relevant historical datasets have been explored so far, but Prof Sara Hotchkiss and Prof Robert Booth have identified one ecosystem that may be a useful model system for studying state shifts – the kettle hole-peatland mosaics of northern Wisconsin, US.

Peatlands cover only around 2% of the world’s land area, but punch far above their weight in terms of carbon capture, making them crucial weapons in the fight against climate change. Compared to other carbon sinks such as oil, which are held deep below ground, peat lies within a few metres of the atmosphere, making it highly responsive to changes in climate. Furthermore peat, comprising partially decayed vegetation, provides a uniquely high-resolution record of the past flora and climate of an area which can be extracted as sediment cores, often dating back tens of thousands of years. Such a record is well-suited to investigating past time periods of abrupt change.

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How bogs become
Kettle holes are created when buried blocks of ice melt as a glacier retreats, leaving lakes. Over time, the lakes are overgrown, and eventually filled, by floating mats of peat. Conventional wisdom suggests that the peat mats develop through gradual and inevitable inward encroachment of plants, independent of a lake’s shape, location, or climate. However, several recent studies suggest that much more rapid and discrete transitions from open water to peatland may have occurred, more characteristic of sudden state shifts.

Peatlands cover only 2% of the world’s land area, but punch far above their weight in carbon capture

Palaeoecologists Robert Booth of Lehigh University, Pennsylvania, and Sara Hotchkiss of the University of Wisconsin-Madison have used Wisconsin’s kettle holes to test the hypothesis that peatland develops suddenly, triggered by intermittent drought conditions. Prof Booth and his students had already documented similar transitions in a Pennsylvania bog system using radiocarbon dating and detailed sediment core analyses, finding discrete and rapid episodes of peatland formation, roughly coincident with independently-verified drought events.

Together, Booth, Hotchkiss, and their students have developed a new conceptual model, in which fluctuating water levels drive peatland establishment and expansion. For example, falling water levels during droughts cause rapid colonisation of kettle hole lakes by plants such as sedges. During subsequent wetter conditions, lake levels recover, uprooting the vegetation to produce floating mats of peat and plants, which are further colonised by typical peatland plants such as Sphagnum mosses. Beneath the mats, peat accumulates rapidly to infill the lake. They have been testing this model using sediment cores from the kettle hole peats.

Their projections could help anticipate and even mitigate the effects of climate change on these ecosystems

All kettle holes are not equal
In Hotchkiss and Booth’s model, climate is not the only factor determining how kettle hole lakes are converted to peatland: shallow kettle holes would be more susceptible to colonisation during short or low magnitude drought events, while deeper ones would require more prolonged periods of drought. It also follows that kettle lakes at higher altitude – which are generally rainfall-fed – would be more susceptible to colonisation than lower altitude lakes supplied by more reliable groundwater. If these hypotheses are correct, they may be able to predict which kettle holes are most susceptible to peat encroachment over coming decades. Integrated with regional climate forecasts, their projections could help anticipate and even mitigate the effects of climate change on these ecosystems.

This is important because kettle hole landscapes provide a range of ecosystem services including biogeochemical cycling, habitats for aquatic and wetland flora and fauna, areas for human recreation, and of course carbon sequestration. Although the end appearance of kettle holes is identical regardless of whether their creation was slow and steady or rapid and episodic, the contrasting paradigms have markedly different implications. As Hotchkiss and colleagues put it in a recent paper, “From an ecological perspective, the important question is, at what rate do these variables change? Are these lake systems responding linearly to peatland expansion over millennia or do they experience abrupt, threshold effects?” (Ireland et al., 2012: 996). For example, the aquatic communities supported by kettle hole lakes differ depending on whether or not they are peat-bordered because the presence of peat has substantial biogeochemical and hydrological effects on the lake itself. Under the slow and gradual model, lakes would be peat-bordered for a considerable time, and effects on the aquatic ecosystem would be gradual over many millennia. However, under the new model rapid changes would be expected.

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Hotchkiss and Booth’s research may also predict how sudden transitions from lake to peatland could alter the whole ecosystem’s carbon storage capacity. Peatlands accumulate carbon much faster than lake sediments do, but they occupy sensitive transition zones between aquatic and terrestrial ecosystems on the landscape. Although good at storing carbon, peatlands can also release carbon to the atmosphere if they begin to decompose. Thus, this research has implications for the total net carbon that can be fixed in kettle hole landscapes, an urgent parameter for those seeking to mitigate climate change through maximising carbon sequestration.

Even in regions of moderate climate, the future is expected to feature significantly more frequent and intense periods of drought. If the new model is correct, kettle hole ecosystems will therefore undergo increasing numbers of abrupt community changes over coming decades – posing new challenges to mitigating the effects of future climate change. And kettle holes are unlikely to be the only ecosystem prone to abrupt state shifts. Hotchkiss and Booth’s research provides a tantalising glimpse of our potential future and a long-term prediction of how our actions can trigger rapid and permanent changes in the natural world upon which we depend.