Subject: Treeline / Timberline

Friends from us are traveling around in the USA. Few days ago they ask 
us by e-mail, why trees in America are growing up to 2500m above 
sealevel even in regions with lots of snow during wintertime, whereas 
in Switzerland trees are just growing to an altitude up to 1800 m 
above sealevel.
Is it a question of different plant speecies, is the climate so 
different on the same altitude or are there other reasons to explain 
this phenomen?
Thanks for your efforts!
Sam Ginsburg

Re: Treeline / Timberline