On conifer plantations, competitive understory vegetation often retards growth and establishment of tree seedlings. Livestock grazing exemplifies a method of controlling the understory vegetation and increasing the availability of site resources to tree seedlings. We hypothesized that prescribed cattle grazing ameliorates water stress of young tree seedlings by reducing root growth of competing understory species. On a Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and ponderosa pine (Pinus ponderosa Dougl.) plantation in southwest Oregon planted in 1986, seedling water stress was evaluated with the pressure chamber technique and supplemented with gravimetric soil water determinations in 1986-1989. Root growth of orchardgrass (Dactylis glomerata L.), the major understory competing species, was quantified in 1988 and 1989 with the root periscope/mini-rhizotron technique. Seedling water stress levels during spring and summer were similar in a cattle-grazed vs. ungrazed area in 1986 through 1988, but in summer 1989, water stress was reduced significantly in the grazed area. Soil water content was higher in the grazed area in 1989, especially at the 10-20 cm soil depth. End of season (July) orchardgrass root growth was reduced 18% and 15% with grazing in 1988 and 1989, respectively. We conclude that repeated cattle grazing of orchardgrass reduced transpirational surface area and root growth sufficiently to increase soil water availability to seedlings. Thus, prescribed cattle grazing on conifer plantations can enhance seedling physiological status by acting as a regulator of above- and belowground competition. For. Sci. 39(3):405-418.