SOIL-PLANT COMMUNITY RELATIONSHIPS IN THE SELKIRK MOUNTAINS OF NORTHERN IDAHO

Soils and the native plant communities they support often exhibit a st rong mutual dependence within geographically similar areas. In mid- to high-elevation forests of northern Idaho, volcanic ash is an importan t soil parent material and has had a major impact on soil properties a nd the distribution of vegetation. As a result, there are several well -expressed soil-plant community type associations within this region. This study compares the properties and classification of soils and pla nt communities along a south-facing elevational gradient in the Selkir k Mountains of northern Idaho. Cold, moist soils of the higher elevati ons. corresponding to the Abies lasiocarpa (subalpine fir) series, are strongly influenced by volcanic ash. These soils have undergone inten se podzolization and are classified as Spodosols. The intermediate moi sture and temperature conditions associated with the middle elevations support plant communities of the Tsuga heterophylla (western hemlock) series. Associated soils exhibit progressively less influence of volc anic ash with decreasing elevation. Accordingly, soils of the upper en d of the western hemlock zone are classified as Andisols and those of the lower end are classified as intergrades to the Andisols. At relati vely low elevations, the warmest and driest soils of the climatic grad ient support communities of the Pseudotsuga menziesii (Douglas-fir) se ries. These soils contain little or no volcanic ash and are classified as non-andic Inceptisols and Entisols. Results from this study indica te that increased sensitivity of Soil Taxonomy to ash-influenced soils helps distinguish several of the important soil-vegetation interactio ns that have been operative in this region. As a result. interrelation ships between soils, their classification. and associated plant commun ities can be clearly demonstrated for these and, presumably, similar f orested ecosystems of the Pacific Northwest.