A mt being forested depends on a lot of factors, but, usually the terrain is to harsh for trees to grow on mountain faces... For many mts the rocky terrain makes it difficult for trees to grow, so they will think out as they go up, where the sheer rock faces come up... Also the lower temperatures of higher altitutes will stunt and eventually stop tree growth... other factors such as wind/water/etc also contribute to the lack of trees...

As for mapping, most people illustrate the general makeup of the terrain, so that trees go up to mt areas and stop.. they don't mean the trees don't continue.. just that what is considered "forest" and what is considered "mt" separate here...