The Ancient Treeline and the Carbon Cycle in the Siberian Arctic
Today the Siberian Arctic tundra appears to be a desolate endless expanse of green wetlands dotted with shallow lakes. But have the same species of herbs and shrubs been present over the last 10,000 years? When did trees first arrive at the present-day northern treeline? Is the Arctic tundra acting as a source or a sink of carbon to the atmosphere today?
|Arctic peatland at modern treeline, Western Siberia.|
US and Russian scientists joined together to answer these questions by digging up the tundra to find out what happened in the past. GISS staff scientist Dr. Dorothy Peteet and NRC postdoctoral-fellow Dr. Andrei Andreev spent a summer north of the Arctic Circle between the Pur and Taz Rivers of Siberia, east of the Ural Mountains (66°N, 79°E). From their analysis of a deep sequence of frozen peat, they found that the tundra has undergone a series of changes and that the peatland was initially a shallow lake. The environment changed from drier to wetter tundra and back again several times between 9,000 to around 4,500 years ago. This high-resolution, complex record of peatland changes demonstrates the high degree of variability in peatland growth over a span of 5,000 years in a permafrost environment. Different species of moss as well as seeds and leaves of higher plants tell us about the changes in hydrology, local nutrient supply, and temperature. Charcoal in the peat record tells us whether or not fire has played a big role in this landscape. There is very little charcoal in this peatland sequence. Thus, the role of fire appears to have been a minor one, in contrast to the fire history of Finnish peatlands, in which more than half of the carbon loss was attributed to fire (Tolonen et al., l992).
The migration of trees into the region is expressed at our site by the macrofossil pattern of larch (Larix siberica) and birch (Betula pubescens) arrival, followed by spruce (Picea obovata). About six thousand years ago, spruce trees moved even further northward. Climate at that time was warmer than today. Since that time, however, the treeline retreated to its present position, and tundra replaced the old trees. The redevelopment and spread of peatland resulted in increases in moisture and acidity. This vast spread of tundra within the last few millennia indicates that climate cooled after the mid-Holocene warming.
A major finding of this study is the surprisingly old ages of the uppermost peat in this part of Siberia. These results show a clear lack of peat accumulation in recent millennia, either due to very low net productivity, or alternatively, recent oxidation of fossil peats. Arctic and boreal peatlands are important components of climate change because they play a crucial role in the terrestrial/atmospheric carbon balance as sources and sinks of carbon dioxide and methane. Today, the role of the Arctic as either a carbon source or sink is highly controversial among researchers. Our study, along with similar studies in the Canadian Arctic suggests that Arctic peatlands are acting more as a source of carbon to the atmosphere than a sink, possibly due to many factors such as less nutrient mineralization, increased permafrost leading to drier peat, or oxidation from winds.
Peteet, D., A. Andreev, W. Bardeen, and F. Mistretta l998. Long-term Arctic peatland dynamics, vegetation and climate history of the Pur-Taz region, Western Siberia. Boreas 27, 115-126.
Tolonen, K., H. Vasander, A.W.H. Damman, and R.S. Clymo l992. Rate of apparent and true carbon accumulation in boreal peatlands. In Proceeding of 9th International Peat Congress, Uppsala, Sweden, 22-26 June l992. Volume 1, 319-333.
Please address all inquiries about this research to Dr. Dorothy Peteet.