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In ecosystems where environments are extreme, such as deserts, adult plant species may facilitate the establishment and growth of seedlings and juveniles. Because high temperatures and evaporative demand characterize tall-grass prairies of the central United States (relative to forests), we predicted that the grassland-forest ecotone, by minimizing temperature extremes and moderating water stress, may function to facilitate the expansion of Quercus species into undisturbed tall-grass prairie. We assessed the carbon and water relations of juvenile Quercus macrocarpa and Q. muhlenbergii, the dominant tree species in gallery forests of northeast Kansas, in ecotone and prairie sites. To evaluate the potentially competitive effects of neighboring herbaceous biomass on these oaks, juveniles (<0.5 m tall) of both species also were subjected to either: (1) removal of surrounding above-ground herbaceous biomass, or (2) control (prairie community intact) treatments. Herbaceous biomass removal had no significant effect on gas exchange or water relations in these oak species in either the prairie or the ecotone environment. Although the ecotone did alleviate some environmental extremes, photosynthetic rates and stomatal conductance were ca. 20% higher (p<0.05) in both oaks in prairie sites vs. the ecotone. Moreover, although leaf temperatures on average were higher in oaks in the prairie, high leaf temperatures in the ecotone had a greater negative effect on photosynthesis. These data suggest that the grassland-forest ecotone did not facilitate the growth of Quercus juveniles expanding into this grassland. Moreover, the carbon and water relations of juvenile oaks in the prairie appeared to be unaffected by the presence of the dominant C-4 grasses.