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Large-Scale Treeline Advances in the Circumpolar Northern Hemisphere
Reference
Esper, J. and Schweingruber, F.H. 2004. Large-scale treeline changes recorded in Siberia. Geophysical Research Letters 31: 10.1029/2003GL019178.

Background
The authors note that "tree recruitment is related to some combination of temperature variations, micro-site conditions, insect outbreaks, winter-time snow and wind conditions and grazing pressure (Holtmeier, 2000; Payette, 1974; Payette and Filion, 1985; Stocklin and Korner, 1999)," but that growing season temperature alone is a major determinant of global treeline limits (Brockmann-Jerosch, 1919; Korner, 1998).

What was done
The two scientists from the Swiss Federal Research Institute analyzed treeline dynamics over western Siberia during the 20th century by comparing nine undisturbed polar sites located between 59 and 106E and 61 and 72N and merging information from nine sites in such a way that, in their words, "larger-scale patterns of treeline changes are demonstrated, and related to decadal-scale temperature variations." They also related current treeline positions to former treeline locations "by documenting in-situ remnants of relict stumps and logs."

What was learned
Two main pulses of northward treeline advance were detected in the mid and late 20th century. The first of these recruitment phases occurred between 1940 and 1960, while the second period of enhanced recruitment started around 1972 and lasted into the 1980s. These treeline advances corresponded closely to annual decadal-scale temperature increases; and Esper and Schweingruber remark that "the lack of germination events prior to the mid 20th century indicates this is an exceptional advance," but that "the relict stumps and logs found at most sites "show that this advance is part of a long-term reforestation [our italics] process of tundra environments." They note, for example, that "stumps and logs of Larix sibirica can be preserved for hundreds of years (Shiyatov, 1992)," and that "above the treeline in the Polar Urals such relict material from large, upright trees were sampled and dated, confirming the existence, around AD 1000, of a forest treeline 30 m above the late 20th century limit (Shiyatov, 2003)." They also note that "this previous forest limit receded around 1350, perhaps caused by a general cooling trend (Briffa, 2000; Esper et al., 2002."

What it means
"Synchronous with the advance shown from the western Siberian network," according to the authors, a mid 20th century tree recruitment period was occurring in "central Sweden (Kullmann, 1981), northern Finland (Kallio, 1975), northern Quebec (Morin and Payette, 1984) and the Polar Urals (Shiyatov, 1992)." Together with their own results from Asia, they conclude that "these findings from Europe and North America support a circumpolar trend, likely related to a global climate warming pattern." These data thus demonstrate the positive response of the biosphere to the warming that accompanied the demise of the Little Ice Age and the establishment of the Modern Warm Period. In addition, they demonstrate the existence of the warmer-than-present multi-century period centered around AD 1000 that we know today as the Medieval Warm Period.

References
Briffa, K.R. 2000. Annual climate variability in the Holocene: Interpreting the message of ancient trees. Quaternary Science Reviews 19: 87-105.

Brockmann-Jerosch, H. 1919. Beitrage zur Geobotanischen Landesauf-nahme, Vol. 6, Baumgrenze und Klimacharakter. Rascher, Zurich, Switzerland.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Holtmeier, F.K. 2000. Arbeiten aus dem Institut fur Landschaftsokologie, Vol. 8, Die Hohengrenze der Gebirgswalder, Institut fur Landschaftsokologie, Munster, Germany.

Kallio, P. 1975. Reflections on the adaptations of organisms to the northern forest limit in Fennoscandia. Paper Presented at the Circumpolar Conference on Northern Ecology, National Research Council, Ottawa, Canada.

Korner, C. 1998. A re-assessment of high elevation treeline positions and their explanation. Oecologia 115: 445-459.

Kullmann, L. 1981. Pattern and process of present tree-limits in the Tarna region, southern Swedish Lapland. Fennia 169: 25-38.

Morin, A. and Payette, S. 1984. Expansion recente du meleze a la limite des forets. (Quebec nordique). Canadian Journal of Botany 62: 1404-1408.

Payette, S. 1974. Classification ecologique des formes de croissance de Picea glauca (Moench) Voss et de Picea mariana (Mill.) BSP. en milieux subarctiques et subalpins. Nat. Can. 101: 893-903.

Payette, S. and Filion, L. 1985. White spruce expansion at the tree line and recent climatic change. Canadian Journal of Forest Research 15: 241-251.

Shiyatov, S.G. 1992. The upper timberline dynamics during the last 1100 years in the Polar Ural mountains. In: Frenzel, B. (Ed.) Oscillations of the Alpine and Polar Tree Limits in the Holocene. Fischer, Stuttgart, Germany, pp. 195-203.

Shiyatov, S.G. 2003. Rates of in the upper treeline ecotone in the Polar Ural Mountains. Pages Newsletter 11: 8-10.

Stocklin, J. and Korner, C. 1999. Recruitment and mortality of Pinus sylvestris near the Nordic treeline: The role of climatic change and herbivory. Ecological Bulletin 47: 168-177.


Reviewed 12 May 2004