Soil‐Vegetation Relationships within the Ponderosa Pine Type in the Central Oregon Pumice Region

Six plant communities occurring within the ponderosa pine (Pinus ponderosa) and white fir (Abies concolor) zones were identified east of the Cascade Mountains in south—central Oregon. These units, listed in order of increasing effective moisture and increasing elevation, are as follows: (1) Pinus ponderosa/Purshia tridentata, (2) Pinus ponderosa/Purshia tridentat/Festuca idahoensis, (3) Pinus ponderosa/Purshia tridentata—Arctostaphylos parryana var. pinetorum, (4) Pinus ponderosa/Ceanothus velutinus—Purshia tridentata, (5) Pinus ponderosa/Ceanothus velutinus, and (6) Abies concolor/Ceanothus velutinus. The Pinus/Purshia/Festuca association is restricted to areas of Shanahan coarse study loam, while the remaining five units all occur on Lapine loamy coarse sand. Both series are Regosols developed on aeolian pumice deposits. Lapine profile characteristics influencing plant root distribution, such as thickness of the gravelly C1 horizon and amount of mixing of C2 material with the buried soil, showed some apparent correlations with plant communities. The Lapine soil under the Pinus/Purshia and Pinus/Ceanothus—Purshia communities had thickest C1 horizons and the smallest amounts of mixing in the C2. The soil associated with the Pinus/Ceanothus community had the thinnest C1 and a well—mixed C2 horizon. Roots were generally well distributed throughout the C2 horizon in the latter community, whereas in soils possessing a thick C1 and little mixing in the C2 roots are restricted largely to the A1, AC, and D horizons. Soil—moisture measurements indicated that depth and time of onset of soil drought are important factors in controlling the distribution of plant communities in the study area. The A1 horizon under Pinus/Ceanothus and Abies/Ceanothus communities contained appreciably greater quantities of available P, exchangeable Ca, and total N. Organic matter content of the Lapine A1 horizon was considerably higher under the more mesic communities. Carbon—nitrogen ratios of Lapine and Shanahan A1 and AC horizons were not correlated with plant groupings. C/N values were found to be very much higher than those encountered in surface horizons of zonal forested soils, probably due to slow rates of organic matter decomposition.