Stomatal opening and closing was followed with a diffusion porometer in beech (Fagus grandi-folia Ehrh.), maple (Acer rubrum L.), oak (Quercus rubra L.) and yellow poplar (Liriodendron tulipifera L.) in a situation where illuminance could be changed by placing shades over the trees. In all four species the stomata opened quicker than they closed. The time to reach an equilibrium stomatal resistance was constant for a species but varied between species, for all changes in illuminance greater than 3000 ft-candles. To open to a constant leaf resistance took from 3 minutes in beech to 20 minutes in yellow poplar; oak and maple were intermediate at 12 minutes. Beech required 12 minutes to close to a constant leaf resistance, maple 18, oak 20, and yellow poplar 36 minutes. When the change in illuminance was less than 3000 ft-candles, the stomata took longer to reach a new equilibrium in all species except yellow poplar. Also beech stomata opened and closed in dimmer light than yellow poplar. The rates of stomatal opening and closing correlated well with the known shade tolerance of the species.