Soil water potentials were measured weekly by psychrometers at 20 cm depth during the dry season in a tropical moist forest on Barro Colorado Island, Panama. There was a persistent gradient of decreasing soil moisture from gap centre to gap edge to adjacent understorey at both a large and a small gap. On both a north-south and an east-west transect, the soil was drier (water potentials were more negative) on an upland surface than on moderate slopes. This trend was reflected in the predawn, total water potentials of shallow-rootedPsychotria horizontalisand deep-rootedTrichilia tuberculatameasured in the understorey during the last two months of the dry season.P. horizontalisfrom the wettest sites on the transects had higher osmotic potentials at full hydration and at zero turgor, indicating less drought resistance than for conspecifics from the driest sites.

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Seedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C species tested and in one CAM species, Aechmea magdalenae at high CO. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C species suggest that elevated CO may increase photosynthetic rate relative to ambient CO over a wide range of light conditions. The response of CO assimilation to internal C suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO. This experiment suggests that: (1) a global rise in CO may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO.

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