Look to the Forest
Elevation is the most important factor in defining a tree’s “neighborhood”
Many factors contribute to the relative suitability of a growing range for each species of tree. While soil composition, moisture, slope direction, inter-species competition, microclimate, and history of fire each play a role, elevation is the most important factor in defining a tree’s “neighborhood.” As elevation increases, temperatures decrease and moisture levels rise. Trees that require more water and can withstand colder temperatures tend to be found higher on the slopes. Other species that can withstand higher temperatures and drier soils grow at lower elevations.
Altitude, soil types, snow, rainfall and average temperatures all influence the evolution of a forest. Some trees will tolerate shade and grow best in full sun.
In Idaho, our forests fall into three basic categories. Historically, mixed conifer forests of north Idaho forests are fairly dense and have evolved in a wet, cool climate. These northern forests were subject to infrequent but massive fires.
The drier, warmer forests of southern Idaho had only 30-50 trees per acre in the past and were home to thick barked tree species like ponderosa pine that withstood frequent, low-burning ground fires. The Salmon River Valley is generally dividing point.
Much of southern and southeastern Idaho is rangeland with scattered juniper and pinyon-juniper dominated woodlands typical of the Great Basin. Idaho is the northern extent of the range and home to seven pinyon-juniper woodland communities that are recognized as occurring exclusively in the state.
The graphic below illustrates at which elevation certain trees thrive in Idaho.