Ulmus alata and Diospyros virginiana are components of the shrubearly tree communities of old-field succession in several areas in the deciduous forests of eastern North America. In these habitats, the plants experience high insolation, high temperatures, and low soil moisture during the summer. They exhibit pronounced daily changes in water potential and usually develop more negative water potentials as the season progresses. The species light saturate at โผ1,150 ฮผE m sec with photosynthetic rates of 15 mg CO dm h for U. alata and 17 mg CO dm h for D. virginiana. The optimum temperatures for photosynthesis are โผ25ยฐC. Ulmus alata maintains maximum photosynthesis to water potentials of-14 bars and recovers from-20 bars to โผ60% of maximum photosynthesis within 10 hrs after watering. When they are deprived of water, twigs of D. virginiana exhibit faster decline in photosynthesis and leaf conductance than twigs of U. alata. The two species have somewhat different response to the environmental of high insolation and low water supply. Unlike Ulmus, Diospyros virginiana has some adaptations which may explain the persistence of a few individuals in mature forests.