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By Dave Campbell

In 1769 Captain James Cook’s Endeavour anchored at the mouth of the Waihou River near the present-day town of Thames. Cook’s naturalist, Joseph Banks, was impressed by the evident resources within the vast swamp forest that covered the lower Hauraki Plains:

…The Noble timber, of which there is such an abundance, would furnish plenty of materials either for building defences, houses or Vessels.

…Swamps which might doubtless Easily be drained, and sufficiently evinced the richness of their soils by the great size of the plants that grew upon them, and more particularly of the timber trees which were the streightest (sic), cleanest, and I may say the largest I have ever seen…

Aerial view of the south eastern portion of Kopuatai bog, Mt Te Aroha in the distance. In the foreground is Empodisma robustum rush-land, the red-tinged vegetation in middle distance is the world’s largest remaining stand of Sporadanthus ferrugineus.

The great Kahikatea forests that so impressed Banks have now been replaced by farms where dairy cows graze on lush grass growing on the drained swamp soils. However, beyond the farms still lies a huge wetland that has somehow survived the early attempts to drain it. Kopuatai, at 90 km2 in area, is NZ’s largest remaining raised peat bog and one of our largest lowland wetlands.

Kopuatai bog has vegetation that is completely unique compared to the bogs of the northern hemisphere, where peat formation is dominated by mosses. In NZ, peat is predominantly formed from the remains of vascular plants of the southern hemisphere family Restionaceae. Two species, the wire rushes Empodisma robustum (north of 38oS) and E. minus (south of 38oS) are the main peat-formers[1], while Kopuatai bog is the last secure stronghold for the taller cane rush Sporadanthus ferrugineus.

Amid global concerns about rising atmospheric greenhouse gas concentrations and growing evidence that a warming climate is directly linked to human activities, the world’s peatlands are gaining increasing attention from scientists. Since the end of the last ice age, plants growing within peat wetlands (bogs and fens) have been taking up CO2 from the atmosphere and storing it underground as peat. Northern hemisphere peatlands alone store around half as much carbon as is presently in the atmosphere as CO2, so it’s understandable why scientists are concerned about what might happen to all of this stored carbon in the future[2].

While the world’s peatlands have been net “sinks” for CO2 for millennia (they take up more CO2 than they release), they also act as sources of methane (CH4), a greenhouse gas around 21 times more potent at trapping heat in the atmosphere than CO2. To find out whether peatlands have a net cooling or warming effect on the climate system, now and projected into the future, requires scientists to compile “net ecosystem carbon budgets” over annual and longer periods. This approach provides the opportunity to understand the environmental conditions and ecological properties that affect the components of these budgets, and how they might change in the future.

Kopuatai research site. On the tall tower are mounted eddy covariance instruments measuring CO2, CH4, water vapour and heat exchanges between the bog vegetation (predominantly E. robustum) and the atmosphere.

We have been slow to gain information on the carbon budgets of NZ peatland ecosystems and therefore we have been unable to predict how they might change in the future. To fill this knowledge gap we have established a research site deep in the heart of Kopuatai, where we are continuously measuring the exchanges of CO2 and CH4 between the peatland surface and the atmosphere, as well as a host of hydrological, ecological and weather variables. We are using the “eddy covariance” technique, which is employed at more than 500 sites worldwide across a wide range of ecosystems. Kopuatai is part of the “OzFlux” network of Australasian research sites. Our preliminary results suggest that Kopuatai bog has very high rates of CO2 uptake compared to analogous northern hemisphere peatlands, mainly because of the mild year-round growing conditions in the Waikato. Methane emissions are moderately low, and the amount of dissolved carbon leached out by water is in line with northern bogs. Combining these three carbon budget components at Kopuatai leads to a substantial overall sink for atmospheric carbon over the course of a year.

Our research will provide important baseline knowledge to support efforts to restore NZ wetlands and act as a yardstick against which to compare the greenhouse gas balances of other wetlands and former peatland areas that are now farmed.

References

[1] Wagstaff, S, Clarkson, B. 2012. Systematics and ecology of the Australasian genus Empodisma (Restionaceae) and description of a new species from peatlands in northern New Zealand. PhytoKeys 13: 39-79.

[2] Limpens, J, Berendse, F, Blodau, C, Canadell, JG, Freeman, C, Holden, J, Roulet, N, Rydin, H, Schaepman-Strub, G. 2008. Peatlands and the carbon cycle: from local processes to global implications–a synthesis. Biogeosciences Discussions 5: 1379-1419.


Dr Dave Campbell is a Senior Lecturer in the Department of Earth and Ocean Sciences at the University of Waikato. To see his research interests, visit www.waiber.com.