Research into fossilised tree-stumps unearthed during the development of new subdivisions and flood mitigation ponds yields a lot of information about the ecosystems of the past and can help determine the way Waitaha Canterbury approaches the restoration of native ecosystems and climate change mitigation. These trees were killed about 2500 years ago when a large river system of the Canterbury plains buried them in a huge amount of silt and clay.
“What those stumps are buried under is anywhere between half a metre to a metre of silt and clay that has been brought down by one of the large river systems of the Canterbury plains,” he said.
His research has been aided by a grant from the Brian Mason Scientific & Technical Trust.
“We know from other evidence throughout the city and the surrounds that the Waimakariri, which is the largest of the rivers to the north of the city had, at various times, avulsed, meaning it’s shifted from its current position, and we know that the last time it shifted, it shifted to the south of the city about 600 years ago.”
What causes a river to avulse is unclear, but evidence suggests that large seismic events such as earthquakes and long-term climate processes such as changes in rainfall and the volume of water flowing down those rivers could alter the course of river systems.
As well as stumps, the sites in Hoon Hay and Halswell also yielded the remains of invertebrates and large amounts of information about the sediment in which the stumps were buried. The insect remains found showed that there was a shift from forest-based animals to aquatic animals about 2500 years ago. In fact, the lower catchment of Hoon Hay Valley was a shallow lake for most of that time, up until the arrival of European settlers (including Dr Prebble’s ancestors, the Prebbles of Prebbleton) who then drained the valley as well as much of the city.
“Given what we know now about the changes that are occurring in the present climate, with changing rainfall patterns, but also with seismic events in our region, this tells us a lot about the dynamism of those river systems and how they could potentially affect our city into the future.
“What it does also suggest is that much of the Canterbury plains, instead of being under forest, was probably under more open wetland vegetation, which speaks to the kind of strategies that we should have in restoring different ecosystems and adapting to impending climatic changes.”
This research could also change the way we look at human interaction with the environment prior to the arrival of Europeans in Aotearoa. “Because there’s so few forests left and the early observations of European explorers found very little forest here, it’s been suggested that it was due to the firing of the landscape by Māori that led to this state of treelessness. But what I argue here is that, in fact, the river systems have been largely controlling the state of our forests for many thousands of years; far more so than the impact of fires.”
Dr Prebble is also conducting research on the islands of Wallis and Futuna, funded by the French Embassy, investigating the islands’ paleoecology, the influence of human settlement on the environment, and using that knowledge to build climate resilience for the future.
