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Setaria faberii, Abutilon theophrasti, and Polygonum pensylvanicum growing together in a 1-yr successional field exhibit differing daily patterns of leaf water potential; S. faberii develops and maintains more negative water potentials than A. theophrasti, and P. pensylvanicum maintains much less negative water potential than either species. Setaria faberii and P. pensylvanicum do not light-saturate at 1.1 X 10 lx irradiance while A. theophrasti approaches light saturation at 8 x 10' lx, indicating that the species are adapted to high light intensities of the open field. In the field, leaf temperatures are maintained appreciably below air temperatures and are in the range of temperatures consistent with near-maximal photo- synthetic rates. Under optimal conditions the rates of photosynthesis are 38, 24, and 18 mg C02 * dm2 * h1 respectively for S. faberii, A. theophrasti, and P. pensylvanicum. Setaria faberii maintains near-maximum photosynthesis down to water potential of -12 bars, photosynthesizes at 30% of maximum at -24 bars, and recovers from potentials of -50 bars. In A. theophrasti photosynthesis remains unchanged to -10 bars, and becomes negligible at -26 bars. In P. pensylvanicum photosynthesis begins to decline sharply at potentials of -6 bars; it becomes negligible at -18 bars; and the plants die at -20 bars. The root systems of the three species grow in different parts of the soil resource: the root system of S. faberii is fibrous and shallow; the tap root of A. theophrasti is sparsely branched and extends to intermediate depths; the tap root system of P. pensylvanicum is moderately branched in the upper soil layers and pro- liferates mostly below the rooting zone of the other two species. The three species have evolved differing intercompenstory mechanisms for habitat ex- ploitation. One apparent niche difference among the species that allows for their coexistence is the exploitation of various levels of the soil and consequently different moisture resources.