Radial changes were followed in 32 trees divided equally among 4 spp. of conifers (Abies grandis, Larix occidentalis, Pinus ponderosa, Thuja plicata) growing in n. Idaho. Four trees of each sp. were located at low altitude and 4 at high. Observation extended from mid-April or mid-May until mid-Oct. In spring the most apparent effect of the vertically stratified climates on tree growth consisted of well-marked retardation of cambial activity at high altitudes, which was probably due to the temp. gradient. In summer and early autumn the water-balance gradient exerted the most evident influence, as shown by differences in the amt. 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 max. growth rates, and the later the effects of dry summer weather became evident. Pronounced differences in behavior were observed with as little as 227-m. difference in elevation. This was correlated with differences in location with respect to zones and suggests a possible practical application of a knowledge of zonation. The time when radial enlargement is completed varies widely among the 4 spp. and is strongly affected by altitude. In general, stability is attained at a later date at low altitude because the hydration of tissues formed in summer is deferred until after the rainy season begins in autumn.