Soil Moisture in Relation to Vegetation Distribution in the Mountains of Northern Idaho

Soil drouth becomes intense in summer in the steppe (Festuca—Symphoricarpos) of the basal plain, decreasing progressively through Pinus ponderosa and Pseudotsuga menziesii forests on ascending the adjacent foothills. On mid—slopes where Abies grandis appears, and still higher in the mountains where Thuja plicata, Tsuga heterophylla, and Abies lasiocarpa occur, soil drouth appears to be of minor ecologic importance. In mesophytic forest areas early stages in secondary succession that are characterized by overly dense stands of slowgrowing individuals may develop intense soil drouth during this stagnation phase. On the upper parts of south—facing slopes, even high in the forested mountain, small parks with grassland climaxes containing Agropyron spicatum and Festuca idahoensis represent local areas of soil drouth comparable with that of steppe on the basal plain where the same grasses dominate. Climax vegetation types thus closely reflect soil moisture regimes, whereas altitude above sea level is very unreliable. Three grasses native to areas too dry for even the most xerophytic trees were found to germinate in autumn, develop slowly during the winter and so be sufficiently mature by early summer to endure several month of enforced aestivation. Trees, in contrast, do not germinate until spring and so are not far enough developed for their tap roots to descend more rapidly than the soil desiccates. Since they are incapable of enduring prolonged soil drouth they perish as the dry zone extends downward and overtakes their root tips.