Ecological Position and Plant Associations 



Ponderosa pine is the most drought tolerant of the major coniferous tree species 

 in the Intermountain Station territory. Therefore, it occupies the relatively warm and 

 dry lower slopes of the Rocky Mountains as they rise above the arid plains. In parts 

 of the territory, such as southern Idaho, Montana, and Utah, open stands of the pinyon- 

 juniper cover type form a transition zone between the prairies and forests in the 

 mountains. In northern Idaho and eastern Washington, however, the boundary between 

 ponderosa pine forests and prairie is abrupt. 



In the lowest zone of forest, ponderosa pine is the climax, and the only conif- 

 erous species able to survive in that droughty climate. As ponderosa pine extends 

 farther up the moisture-temperature gradient, it becomes a serai tree in stands where 

 more competitive species eventually become climax. 



Daubenmire and Daubenmire (1968) classified the forest vegetation of eastern 

 Washington and northern Idaho into ecological units termed habitat types. Ponderosa 

 pine occurs as a climax or serai species in 11 of their habitat types, listed below 

 in approximate order from most warm and dry to most cool and moist. 



Pinus ponderosa/Stipa eomata 

 Firms ponderosa/ Agropyron spioatum 

 Pinus ponderos a/Fes tuca idahoensis 

 Pinus ponderosa/Purshia tridentata 

 Pinus ponderosa/ 'Symphoricarpos albus 

 Pinus ponderos a/Physooavpus malvaceus 

 Pseudotsuga menziesii /Symphoricarpos albus 

 Pseudotsuga menziesii/Calamagrostis rubescens 



Pseudotsuga menziesii/Calamagrostis rubescens } Arctostaphylos phase 

 Pseudotsuga menziesii/Physooarpus malvaceus 

 Abies grandis /Pachistima myrsinites 



In the first six habitat types in the list, ponderosa pine is the climax dominant. 

 In the last five types it may be a temporary invader following burning or logging, but 

 it is replaced as the climax dominant by Douglas-fir or grand fir. Ponderosa pine 

 grows rapidly where it is serai if light is sufficient. In an understory position, it 

 cannot compete with the climax species and is eventually eliminated from the stand. 

 In the past, wildfires were largely responsible for maintaining ponderosa pine in mixed 

 stands by creating bare areas in which the pine could become established and compete 

 favorably with the slower-growing climax species. In recent years, logging and burning 

 of clearcut patches have created bare areas. 



Ponderosa pine is rated very resistant to ground fire, second only to western 

 larch among the coniferous species of the northern Rocky Mountains (Flint 1925) . This 

 rating applies to old trees with thick bark, but small trees are easily killed by 

 ground fires. 



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