Photosynthesis and water relations of savanna tree species differing in leaf phenology.

Godmania macrocarpa Hemsley, a deciduous tree characteristic of fire protected areas of the savanna region of central Venezuela, was more drought tolerant, allocated more N to leaves and had consistently higher photosynthetic rates than Curatella americana L., a ubiquitous species growing successfully within the grasslands of tropical American savannas. Godmania macrocarpa maintained higher leaf conductance and photosynthesized at higher xylem water tensions than C. americana. As the dry season progressed, G. macrocarpa was more affected by water stress than C. americana, which may explain why G. macrocarpa shed its leaves before forming new leaves. For both species, leaf sap osmolality was strongly correlated with, but not completely accounted for by, soluble sugars. Integrated water-use efficiency, as measured by delta(13)C, was similar for both species, but young leaves were more efficient than old leaves. Water-use efficiency of adult leaves was similar in both species as a result of higher photosynthetic rates in G. macrocarpa and lower leaf conductances in C. americana. Compared to G. macrocarpa, instantaneous photosynthetic N-use efficiency was higher in C. americana despite its lower maximum photosynthetic rates. The absence of G. macrocarpa trees from open grasslands, despite their high productive capacity, is possibly the result of unfavorable conditions for germination, poor survival of seedlings, and lack of resistance against fire.