The central parts of the three northern territories are covered by the tundra.
Canada, in general can be easily divided by the biomes that cover its territory, by simply saying that the southern half of the country is covered by the taiga, and the northern half is covered by the tundra.
The three northern territories are Yukon Territory, Northwest Territories, and Nunavut. All of those are in close proximity to the Arctic, and the climate there is very cold for most of the year, with snow and ice dominating the landscape, and having only a very short fresh, relatively cold summer.
1st way is chopping trees for wood. 2nd would be pollution by fossil fuel.
It increases with distance from the ocean ridges.
A biggie: the transcontinental railroad. Because it would economically improve wherever it was, both the North and the South competed for the railroad to be in their respective territory, evidenced by the Kansas-Nebraska Act and the Gadsen Purchase and induction of California as a free state in 1860.
In addition: people could get land cheap because it was abundant and might leave to start afresh economically.
There's other stuff I can't remember as clearly, but there's a little and I hope it helps :)
