..."tree-line"?

It has to do with regional differences in climate. I quote: "Generally speaking a change in latitude of 800 miles poleward is equal to an ascent of one mile in altitude. Also the average change in temperature due to change in altitude is about 1 degree for every 300 feet..."Source:http://www.nps.gov/mora/notes/vol13-3f.htmThere are other variables such as the differences in rainfall from one side of the mountain to another, differences in native species, and differences in soil development on steeper slopes, to name a few. Duration of the daily sunlight probably is another factor. The NPS website has some nice diagrams.