Sky Island - New Mexico's Jemez Mountains

While watching and listening, please observe the variations in vegetation.  Recall the Orographic Effect, which leads to more precipitation in the mountains.  (The Orographic Effect takes place in more humid climates too, but its effects are more noticeable in the episodic precipitation of arid and semi-arid, temperate settings.)

With increased precipitation due to the Orographic Effect in arid and semi-arid climates, isolated and semi-isolated mountain ranges serve as "Islands of Diversity", due to variations in rock-types, elevation, slope-angles, slope-orientations, and valley-orientations in relation to daily and seasonal sunlight and soil-moisture retention.  Slope-angle also affects soil genesis and retention (greater slope-angle = more erosion).  

This increased micro-climate diversity also includes the "Morning vs. Afternoon Effect" (my terminology) that I have observed in the Eagle Mountains, the Franklin Mountains, and the Organ Mountains.  As the east side of the mountains receives the "cool morning sunlight", there is less soil (and plant-transpiration) moisture-loss due to evaporation.  In contrast, the west side of the mountains gets the "full brunt" of the afternoon sun and the dry air.

In the Guadalupe Mountains of SE New Mexico and West Texas, the southeast-orientation of McKittrick Canyon with its perennial (or semi-perennial) stream offers evidence of the "Morning vs. Afternoon Effect".  Though not within a canyon, one could make the same observation in regard to the small creek draining Aguirre Springs (Figure 1), on the east side of the Organ Mountains in southern New Mexico.

Figure 1.  Small creek draining Aguirre Springs (illustrating the segregation-by-density of "heavy minerals").

In these mountain settings, especially with the N-S orientation of the linear ranges of the Basin and Range Province, the Orographic Effect and the Morning vs. Afternoon Effect override the regional "Rain Shadow Effect".

Additionally, in mountain ranges derived from composite or caldera-type volcanoes, the presence of unconsolidated or "unwelded ash" can offer conditions more favorable to plant growth.  The shattered volcanic glass (as shards or pumice fragments) are chemically-unstable and in the presence of soil moisture and natural acids, the degrading ash provides a supply of nutrients to local plants.

So when watching videos referencing "Sky Islands", please keep these things in mind, as to the "Why?" in regards to vegetation (and micro-climate) diversity.