Habitat Types

The Pinus ponderosa series Nearly everywhere in eastern Washington and northern Idaho as one leaves the steppe at the foot of the mountains and enters the forest, the first coniferous tree encountered is Pinus pondl!fosa. The ability of this species to endure dry climates · well exceeds that of our next most drouth-tolerant conifer, Pseudotsuga menziesii. Therefore, typically a belt of . climax p ine forest separates steppe from Pseudotsuga forest. Pinus pondet'osa does indeed extend farther up the moisturetemperature gradient than ·this marginal belt in which no other tree challenges its supremacy. But there it either perPetuates itself on slopes that are excessively dry for thOse altitudes and so spare it from competition or it is a temporary invader of logged or burned sites. After one generation, devastating competition from other trees completely eliminates it. In the present section, attention will be restricted to areas in which the pine is the climax dominant, i.e., the Pinus pondefosa Series. In Montana, southern Idaho, central Oregon, and southward in the Roclcy Mountafns, ~he transition from steppe on the basal plains to forest in the foothills is usually marked by associations having a savanna physiognomy. These associations are often dominated by trees considerably lower in stature . than Pinus pondefosa such as P. cembroides and Juniperus spp. (20) . But in easte~n Washington and northern Idaho, the transition from steppe lO Pinus pOndefosa forest is relatively abrupt. Only occasionally is the margin of the pine ~t open enough to give the appearance of savanna. · Soils are more varied in the Pinus ponderom series than anywhere else in the mountains. They include glacial till, glacio-fluvial sand and gravel, dune, basaltic rubble, colluvium and deep loess or volcanic ash. larson's statement (53) that soils supporting the "Pinus pondl!fosa type in the northern Rockies invariably contain much tock near the surface'' is not correct. A considerable share of the variation in herbace-