Background/Question/Methods Understory plants in temperate deciduous forests are dependent on a spring window of high light availability between the time when warming begins and canopy leafout. Ohio buckeye (Aesculus glabra Willd., Hippocastanaceae or Sapindaceae) is a tree with unusual phenology, which produces leaves several weeks before other trees in the same habitat. Consequently, the presence of buckeye in the canopy is hypothesized to reduce or eliminate the spring high-light period, and therefore to have a negative effect on abundance of understory plants. This hypothesis was tested in a rich, mesic, secondary forest in northeastern Indiana at a time when buckeye had fully leafed out, but most other trees had not. Stratified random sampling was used to place transects in patches with and without buckeye canopy. Cover of understory vascular plants (herbs plus woody plants <0.5 m height) was measured in 88 quadrats on 8 transects. Light availability was measured with hemispherical canopy photography. Soil moisture was determined gravimetrically. Results/Conclusions   Microsites with buckeye-dominated canopy had significantly lower total cover of understory plants (26±19% vs. 43±20% for non-buckeye canopy; values are mean ± SD) and number of species per quadrat (9±2 vs. 13±3). Ordination indicated that buckeye significantly affected species composition. Of 66 understory species encountered in quadrats, 2 had significantly higher cover under buckeye canopy, and 11 were more common in microsites without buckeye. Light availability was significantly lower under buckeye canopy (28±10% of open vs. 50±8% for non-buckeye), but soil moisture did not differ (0.33±0.06, expressed as water/dry weight, vs. 0.33±0.07 for non-buckeye). Total understory cover and number of species per plot both were significantly correlated with light, but not with soil moisture. Possible confounding factors are differences in the timing of soil temperature increase, and allelopathic compounds potentially produced by buckeye.