Radial changes were followed in 32 trees divided equally among four species of conifers growing on Thatuna Ridge in northern Idaho. Four trees in each species (A. grandis, L. occidentalis, P. ponderosa, and T. plicata) were located at relatively low altitude, and the other four at relatively high altitude. The period of observation extended from April 19 or May 15 until October 17, 1942. In spring the most apparent effect of the vertically stratified climates on tree growth consisted of a well-marked retardation of cambial activity at high altitudes, which was probably due to the temperature gradient. In summer and early autumn the water-balance gradient exerted the most evident influence, as shown by differences in the amount of shrinkage. At low altitudes the trunks invariably shrank in late summer. At high altitudes shrinkage was either less pronounced or not evident. The higher the altitude, the later the trees attained their maximal growth rates, and the later the effects of dry summer weather became...