The treeline is controlled by CLIMATE in interaction with SOIL. In the North, it is correlated generally with the modal (most common) position of the southern edge of the arctic front in summer, and with such temperature indices as the July 10°C isotherm. But wherever soils are deeper and warmer than normal, as in river valleys (eg, the THELON) or on upland, sandy ESKERS, ribbons of trees extend the treeline far into the tundra.
Trees of the Treeline
Climatic changes can cause abrupt shifts in the treeline, because seed production and survival of seedlings are precarious near the limit of trees. The conifers are also susceptible to fire during dry periods, and this and cutting by humans make today's treeline a diffuse and unstable boundary. The dotted lines and the label "Approximate Limit of Trees" that indicate the treeline on northern National Topographic Survey maps are appropriately vague, partly because of a lack of field surveys.
Author J. STAN ROWE and SERGE PAYETTE
Links to Other Sites
Flora of North America
The FNA website features information on the names, taxonomic relationships, continent-wide distributions, and morphological characteristics of all plants native and naturalized found in North America north of Mexico.
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