PHOTOSYNTHESIS AND WATER RELATIONS OF SAVANNA TREE SPECIES DIFFERING IN LEAF PHENOLOGY

Godmania macrocarpa Hemsley, a deciduous tree characteristic of fire p rotected areas of the savanna region of central Venezuela, was more dr ought tolerant, allocated more N to leaves and had consistently higher photosynthetic rates than Curatella americana L., a ubiquitous specie s growing successfully within the grasslands of tropical American sava nnas. Godmania macrocarpa maintained higher leaf conductance and photo synthesized at higher xylem water tensions than C. americana. As the d ry 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 C-13, w as 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 an d lower leaf conductances in C. americana. Compared to G. macrocarpa, instantaneous photosynthetic N-use efficiency was higher in C. america na despite its lower maximum photosynthetic rates. The absence of G. m acrocarpa trees from open grasslands, despite their high productive ca pacity, is possibly the result of unfavorable conditions for germinati on, poor survival of seedlings, and lack of resistance against fire.