Vegetation in mountainous terrian occurs in distinct altitudinal zones becasue of the strong relationship among temperature and precipitation and elevation.
Figure: Generalized Zonal Sequence of Forest Vegetation in the Northern Rocky Mountains
This generalized zonation is affected by the aspect. The north-facing slopes are cooler and wetter, therefore, the vegetation zones dip lower in elevation. There also could be some zones missing. Thus, zones are driven by environmental gradients associated with elevation.
Along with aspect, the forest zonation also is affected by topography. Cooler air collects in and cools down drainages formed by streams, whereas the warmer air rises. This can cause reverse zonation.
Important concepts concerning the distribution of conifers within the Rocky Mountains:
1. Individual species have wide ecological amplitudes.
2. A species may be climax (tolerant) at one elevation and seral (intolerant) at another.
3. Some species are never 'climax' species & always dominate successional forests.
Figure: Distribution of Coniferous Trees of the Bitterroot Mountains in Northern Idaho
