- Plantation failures on abandoned pasture lands and old clearcuts are associated frequently with heavy stands of herbaceous vegetation. The competitive influence of the vegetation has been described as the causative factor of mortality, but quantitative estimates of vegetation influence are lacking. The development of chemicals with promise for selectivity in conifer plantations has made it possible to create a range of vegetation conditions which makes quantitative study possible. This thesis undertakes to analyze the changes in some factors of the environment which occur as the function of vegetation manipulation, particularly moisture availability. Aerially-sprayed grassy south slopes near Corvallis, Oregon, supporting vegetation densities ranging from devegetated to fullystocked stands, were sampled intensively for moisture depletion in relation to the amount of vegetation surviving the herbicide effects. It was shown that the rate of depletion of moisture was the direct function of the amount of vegetation. Evaporation from the soil surface of devegetated plots accounted for moisture loss which was important in terms of tension only in the surface six inches. Abundant moisture remained within the root zones of planted seedlings below six-inch depth. Fully vegetated plots were completely depleted of available moisture in the surface 36 inches by June Z3, and depletion occurred at equal rates throughout the soil profile. Late spring rains contributed nothing to available moisture supplies after May 17. The year 1962, the seasonunder study, was later in this respect than average. Mathematical models were constructed which permitted prediction of moisture depletion rates on the basis of vegetation, climate and soil parameters. Models were derived for prediction of the amount of moisture available at any given time, on the basis of vegetation, drainage, and soil depth; and for prediction on a general basis of the moisture depletion rate which is likely to occur at any time, and under any conditions of vegetation and soil, when qualified by meteorological data. The former equation may be used on a given date during the period of rapid drying, and on a particular site, to predict the amount of moisture remaining as the function of vegetation. Qualification by soil depth was not as important as consideration of drainage, which was an indication of considerable unsaturated moisture flow into and away from devegetated areas, in particular. The latter equation is more complex, but affords a prediction of the moisture depletion pattern over the entire drying period as the function of vegetation, soils, potential evaporation, and date. Cubic expression of date gave a reasonable approximation of combined meteorological phenomena, and greatly simplified the equation. Calculation of the energy budget during the period of rapid vegetation development indicated that fully occupied stands of herbaceous vegetation were utilizing 82 percent of the net radiation in transpiration, and that 92 percent or more of the total moisture loss occurred through the transpiration process. Removal of the vegetation may divert substantial energy into sensible heat, which may cause localized soil surface heating during the drying period. The period of maximum soil-heating damage occurs during the season when little moisture is available for cooling in fully-vegetated sites, hence the significance of heat budget manipulation may be minor. In consideration of the amount of moisture conserved, it is likely that drought conditions can be avoided for at least one season with proper evaluation of vegetation conditions and proper chemical amelioration thereof. The habitat improvement was evidenced by the general initiation of lammas growth of Douglas-fir seedlings and by greatly increased survival and vigor on all coniferous species tested. The principles herein developed were verified by good survival and vigor on sites distributed throughout western Oregon where plantations had failed three or more times previously. While chemical control of drought conditions through vegetation manipulation is shown to be a powerful tool in reforestation, it must be recognized that solution of the drought problem will not overcome difficulties not related to vegetation.