It has become traditional to divide the terrestrial North into two zones, the tundra  and the taiga, or boreal forest. It is usually accepted that the boundary between the two zones is the timberline, the line beyond which there are no significant forest or woodlands of coniferous trees. This idea was supported by a growing recognition that the timberline is related to climatic factors. Coniferous woodlands seldom extend into regions in which the mean (average) temperature for the whole of the warmest month of the year is less than 10ºC.

A line dividing these two zones on the basis of summer temperature is the 10°C July isotherm, which fairly closely approximates the location of the timberline. An isotherm is a line that joins places of similar temperature, in this case the places where the temperature averages 10ºC in July. It is important to realize that the line on the map actually represents a dynamic situation, and that the actual position of the isotherm fluctuates over time.

Map data source: Arctic Land-Surface Air Temperature:  1930-2004 Gridded Monthly Time Series, v1.03, by Matsuura and Willmott et al, University of Delaware:

Open in Google Earth/KML