- Twelve plots at six sites in southwestern Oregon were studied to determine the degree to which various soil characteristics are related to the occurrence and growth of Chamaecyparis lawsoniana and Thuja plicata. Soil profiles and vegetation were described in each plot, and measurements were made of insolation, soil and litter temperature, creek and groundwater characteristics, and litter accumulation. Growth was estimated by measurement of age, height, DBH, 10-year basal area increment, and foliage elongation between July, 1979, and January, 1980. In July and September, 1979, and January, 1980, mineral soils from the 0-10 cm level were analyzed for pH, moisture holding capacity, loss-on-ignition, and concentrations of nitrate, ammonium and total N. Nitrate and ammonium concentrations were also determined in stream and groundwater. In July and January, fine litter was analyzed for pH, nitrate and ammonium. On each sample date, soils and litter were incubated aerobically for five weeks at 28°C to determine their potential for ammonification and nitrification. Ammonium was added to some samples before incubation. Total N concentration was determined for individual foliage samples (collected in September) in most plots; foliage and mineral soil samples were composited for each plot and analyzed for P, Ca, K and Mg concentrations. The various soil and other measurements were related to basal area increment by multiple regression analysis. Occurrence of Thuja was associated with higher availability of N, particularly nitrate, and soils which were at or near field capacity in July and September. Within each locality Thuja also was most important on soils which were more basic and had higher Ca content than soils of sites dominated by Chamaecyparis. Litter under Thuja generally contained higher levels of extractable N and exhibited more vigorous N mineralization, particularly nitrification, than did litter under Chamaecyparis. Chamaecyparis was associated with higher soil C/N ratios and had significantly higher foliar K corresponding to higher soil K levels. No consistent relationship occurred between distribution of Thuja and Chamaecyparis and total soil N or loss-onignition. Variability in basal area growth was best explained for Chamaecyparis by soil nitrate concentration in July (R² = 0.30), and for Thuja by soil Ca concentration and clay content (R² = 0.37), after variability due to age had been removed. Growth of both Chamaecyparis and Thuja (and Pseudotsu:kg and Tsuga) was best where N (especially nitrate) was most available, and poorest on sites where N availability was lowest. Although growth at an early age appears lower, growth of older Thuja exceeded that of Chamaecyparis on most sites studied. It appears that water and N availability are the primary factors limiting distribution of Thuja in southwestern Oregon. Low availability of soil Ca and Ca/Mg ratio may also restrict Thuja from some soils. In contrast, Chamaecyparis tolerates a wider range in most of the soil characteristics measured. The sites studied seem to exhibit higher nutrient availability and better growth of Chamaecyparis than in most of its range; however, soil N appears more limited than in soils outside its range farther north. None of the factors measured here, with the possible exception of soil I availability, are correlated with Chamaecyparis' northern range boundary. Therefore, it seems likely that as conditions improve for growth farther north, growth of Chamaecyparis is less affected than that of Thuja or other species; and it may be eliminated by competitive exclusion. In general, distribution of pa is seems more limited by physical site characteristics, in contrast to Chamaecyparis, which is more subject to competitive stress. Chamaecyparis is often important on sites where soil conditions are not suitable for other species. It is doubtful whether Thula will replace Chamaecyparis over most of its range, as Chamaecyparis' importance declines due to disease and commercial harvest.