The objective of this research was to determine which environmental resources, light, water, and/or nutrients, control understory plant production in a Pinus ponderosa forest in northeastern Oregon. A split—plot experimental design, with three 5.0—ha blocks, four treatments, and 44 plots, was established in the summer of 1985. Twenty plots (4 X 4 m) were trenched (root—reduction treatment) °1 m in depth, and 24 non—trenched plots (root—control treatment) were used to assess the effects of root competition of overstory trees on understory plants. Trees were commercially thinned (canopy—reduction treatment) in half of each block (2.5 ha) during the winter and early spring of 1986, from a density of 345 to 148 trees/ha to increase light levels to the understory. Thinning significantly increased photosynthetically active radiation, decreased midday relative humidity, and increased midday air temperatures. Xylem potential of the dominant graminoid (Carex geyeri), soil water potential, mineralizable nitrogen, and pH were increased within the root—reduction vs. the root—control treatments. Micro— and macronutrients in C. geyeri and Symphoricarpos albus, the dominant shrub, were influenced in both treatments. Increasing light did not increase understory biomass production. Reducing root competition for soil water and nutrients increased understory aboveground biomass by 53 and 94% in 1986 and 1987, respectively. This research demonstrated that belowground resources were the primary controlling factors of understory production in P. ponderosa forests in northeastern Oregon.