Present boundaries of the giant sequoia groves of the Sierra Nevada appear to be remarkably stable. The maintenance of grove boundaries is controlled by an interaction of moisture availability, temperature and the tolerances of the seedling stage of giant sequoia. Measurements of soil-moisture stress and water-potential measurements of coniferous species within the Giant Forest, the largest extant giant sequoia grove, indicate that conditions of water availability for plant growth are more favorable inside the grove than outside. Although some soils outside the grove did not reach the permanent wilting point during the summer of 1968, pressure-bomb studies have shown that significant differences in water stress do occur between grove and nongrove habitats. High levels of soil moisture appear to be maintained within the Giant Forest during the dry summer months by the input of ground water whose hypothetical origin is summer thunderstorms in the High Sierra. This ground water percolates down to lower elevations where it appears in the soil profile of the grove. Outside of the grove, there is little input of ground water during the summer months and soil moisture remains low in late summer. This percolation of highelevation ground water into the groves during the dry summer months may be the key to the continued existence of the Giant Forest.