Collins
World English Dictionary
tree line
 
n
the zone, at high altitudes or high latitudes, beyond which no trees grow. Trees growing between the timberline and the tree line are typically stunted

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
tree line  
See timberline.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

tree line

upper limit of tree growth in mountainous regions or in high latitudes, as in the Arctic. Its location depends largely on temperature but also on soil, drainage, and other factors. The mountain timberline always would be higher near the Equator than near the poles if it were not for the abundant rainfall in equatorial mountainous regions, which lowers the air temperatures. The timberline in the central Rockies and Sierra Nevadas is around 3,500 metres (11,500 feet), whereas in the Peruvian and Ecuadorian Andes it is between 3,000 and 3,300 metres (10,000 and 11,000 feet). In much of the central and southern Rockies there is a double timberline: the usual high timberline below which there is a belt of normal tree growth; and then a low timberline below which no trees grow because of low precipitation and high temperatures.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
It disappeared over a tree line and into the forests beyond.
Teahouse keepers and porters now scurry ever farther down-valley to retrieve
  timber from the descending tree line.
Breeding: stunted boreal bogs and in open country near tree line, especially in
  willows and alders.
The zigzag relay is necessary to gain enough elevation for the signal to clear
  the tree line.
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