Oaks (Quercus spp.) have low or intermediate shade-tolerances and forest canopy gaps can promote their regeneration. Current fire frequencies and/or intensities in eastern North America are less than they were historically, thus changing gap dynamics and allowing mesic, fire-intolerant, shade-tolerant species to fill canopy gaps on many sites. Hence, many oak forests are changing in tree species composition. By contrast, it has been hypothesized that oak ecosystems are stable on xeric, nutrient-poor sites despite fire regime changes. We characterized the canopy gap regime and investigated changes in tree species composition in woodlands at the xeric edge of North American eastern deciduous forest. We used the line-intersect method to quantify canopy gap frequency, size, cause, and identities of gap replacement species in four young and four old post oak (Quercus stellata) woodlands in the Cross Timbers ecosystem of Kansas, USA. Woodland age did not affect gap characteristics. Canopy gap regime in these xeric woodlands was characterized by infrequent, small gaps that primarily form from snags or snapped boles. Through gap formation and filling, gradual change in the canopy composition of Kansas Cross Timbers woodlands is likely. Post oak dominates the canopy and forms the largest proportion of gaps, but it is the canopy replacement species in far-fewer gaps. By contrast, species that are minor components of the canopy, including hickories (Carya sp.), eastern redcedar (Juniperus virginiana) and more mesic-associated oaks, fill more gaps than they form. These xeric oak woodlands appear to be unstable under the current regional disturbance regime.