General Rules

[waldronate]

The first link that I posted above shows a diagram with a mountain leading down to a sea. Across the picture from left to right shows how temperature (and associated biomes) shift as the latitude increases. The same image shows the temperature decrease with altitude and the same corresponding march of biomes. The Wikipedia page for tree line has a nice table that shows how it varies from just a few hundred meters in extreme latitudes up to more than 5000 meters at near-equatorial areas. Similarly, the arctic tree line tends to hit around 60-70 degrees of latitude, depending on local conditions. Tree line also varies by species and you likely will encounter a succession of tree lines as you head up a mountainside. The 5200 meter treeline, for example, is for a local species that is very drought tolerant.

And the obligatory mapping wisdom: people don't go near the tree line without very good reason. Road paths tend to minimize the total energy required to traverse them. Rivers and cog railways cross contour lines at right angles; roads and animal paths rarely do.

[Azélor]

This is all new stuff to me and I have no idea. It doesn't help that I don't have much data.
That depends from which places you took the information. Some parts of central British Colombia are really dry, borderline desert.
The treeline is higher than 700m. In the south and central parts of the province at least. Not sure how high exactly but they still have trees around 1200m.

But the mountains closer to the coast don't have that problem. Vancouver receive twice as much rain than Calgary.
The wet season in Vancouver is in winter but it's summer for Calgary (generally).
Temperatures might be different.
One side might have more sunlight than the other.

It could be a lot of stuff.