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Tree-Line Evidence for the Medieval Warm Period in the Polar Ural Mountains
Reference
Mazepa, V.S. 2005. Stand density in the last millennium at the upper tree-line ecotone in the Polar Ural Mountains. Canadian Journal of Forest Research 35: 2082-2091.

Background
Noting that "dead trees located above the current tree-line ecotone provide evidence of the dynamic behavior in the location of the tree line in the recent past (Shiyatov, 1993, 2003)," Mazepa reports that "previous studies have concluded that increases in tree-line elevation, and associated increases in tree abundance within the transient tree-line ecotone, are associated with extended warm periods (Tranquillini, 1979; Kullman, 1986; Payette et al., 1989; Lloyde and Fastie, 2003; Lloyd et al., 2003; Grace et al., 2002; Helama et al., 2004)." Consequently, similar data were used by Mazepa to evaluate the uniqueness of Polar Ural tree-line and density response "to what is widely considered to be anomalous 20th-century warming."

What was done
The author's research, which was conducted over the period 1999-2001, extends the earlier (1960-1962) work of Shiyatov - who examined evidence of tree growth dynamics along a continuous altitudinal transect 860 meters long and 40-80 meters wide on the eastern slope of the Polar Ural Mountains (6648'57"N, 6534'09"E) - by repeating what Shiyatov had done four decades earlier.

What was learned
Most importantly, Mazepa reports that "a large number of well-preserved tree remains can be found up to 60-80 meters above the current tree line, some dating to as early as a maximum [our italics] of 1300 years ago," and that "the earliest distinct maximum in stand density [our italics] occurred in the 11th to 13th centuries, coincident with Medieval climatic warming."

What it means
Based on Mazepa's statement that "the vertical gradient of summer air temperature in the Polar Urals is 0.7C/100 m," the large number of tree remains found 60-80 meters above the current tree line suggests that either (1) there must have been an extended period of time when summer air temperatures were 0.42-0.56C warmer than they were over the last decades of the 20th century or (2) if late-20th-century warmth was as warm as it was as long ago as AD 700, it has not been maintained anywhere near long enough to produce the type of tree growth of that earlier period, which based upon Mazepa's stand density data likely continued to approximately AD 1300.

References
Grace, J. Berninger, F. and Nagy, L. 2002. Impacts of climate change on the tree line. Annals of Botany 90: 537-544.

Helama, S., Lindholm, M., Timonen, M. and Eronen, M. 2004. Dendrochronologically dated changes in the limit of pine in northernmost Finland during the past 7.5 millennia. Boreas 33: 250-259.

Kullman, L. 1986. Late Holocene reproductional patterns of Pinus sylvestris and Picea abies at the forest limit in central Sweden. Canadian Journal of Botany 64: 1682-1690.

Lloyd, A.H. and Fastie, C.L. 2003. Recent changes in treeline forest distribution and structure in interior Alaska. Ecoscience 10: 176-185.

Lloyd, A.H., Rupp, T.S., Fastie, C.L. and Starfield, A.M. 2003. Patterns and dynamics of tree line advance on the Seward Peninsula, Alaska. Journal of Geophysical Research 108: 10.1029/2001JD000852.

Payette, S., Filion, L., Delwaide, A. and Begin, C. 1989. Reconstruction of tree-line vegetation response to long-term climate change. Nature 341: 429-432.

Shiyatov, S.G. 1993. The upper timberline dynamics during the last 1100 years in the Polar Ural Mountains. European Palaeoclimate and Man 4: 195-203.

Shiyatov, S.G. 2003. Rates of change in the upper treeline ecotone in the Polar Ural Mountains. Pages News 11 (1): 8-10.

Tranquillini, W. 1979. Physiological Ecology of the Alpine Timberline. Springer-Verlag, Berlin, Germany.

Reviewed 6 December 2006