Chiang, J-M., M. A. Arthur, and B. A. Blankenship (Department of Forestry, University of Kentucky, Lexington, KY 40546-0073). The effect of prescribed fire on gap fraction in an oak forest understory on the Cumberland Plateau. J. Torrey Bot. Soc. 132: 432–441. 2005.—Excessive shade in the understory has been widely cited as the most important factor causing poor oak regeneration in eastern deciduous forests. We examined the effect of single and multiple fires on forest stand structure and the consequent impacts on the gap fraction measured in the understory. Differences in midstory stem density (stems 2–15 cm diameter at breast height; DBH) before and after prescribed burning were highly correlated with fire frequency (R² = 0.86, P < 0.01). On average, each additional burn reduced midstory stem density by ∼550 stems per hectare. Hemispherical photography showed an increase of 3.8 % in understory gap fraction in the first growing season after burning. This increase in gap fraction, which was likely caused by the thinning of the midstory by prescribed burning, was quickly reduced in the next year. Decreases in gap fraction after the first year post-fire, which indicated progressive canopy closure, cannot be explained by the relatively stable midstory stem density in subsequent years after fire. Rather, the decreases in gap fraction after fire coincided with a rapid increase in stem density in the shrub stratum (> 50 cm in height and < 2 cm DBH) of more than two thousand stems/ha/year. The rapid flush of stump sprouts in the shrub stratum was the primary cause of the decrease in the understory gap fraction. Land managers whose goal is to improve oak regeneration by increasing the understory light environment through the use of prescribed burning will need to control the flush of stump sprouts to maintain the effectiveness of burning in increasing light availability. Multiple fires of the intensity used in this study failed to permanently reduce sprouts.