Eucalypt plantations in Venezuela occupy 14% of land covered by savannas. Since water use by plantations in the savannas of Mapire (Venezuelan Llanos) was apparently similar to or lower than in the natural vegetation, evaluating how carbon fixation by plantations compares to the natural vegetation could help to elucidate whether plantations have higher water-use efficiency (WUE) than the savanna. We aimed to determine, using measurements of leaf gas exchange, the seasonal changes in photosynthetic rate (P N) and WUE of a seasonally dry savanna in Mapire, composed mainly of the grass Trachypogon vestitus and the tree Curatella americana, and trees of Eucalyptus urophylla growing in an experimental plantation. Daily integrated WUE (WUEint) was highest in C. americana and similar between T. vestitus and E. urophylla during both seasons. The Gross carbon fixation (GCF) at the leaf level in T. vestitus was similar to C. americana and lower than E. urophylla during the dry season, whereas during the rainy season it was nearly as high as in E. urophylla and both higher than C. americana. Gross carbon fixation (GCF) in T. vestitus was higher than in C. americana, but lower than in E. urophylla during the dry season, while it was much higher than the other two species during the rainy season. The contribution to savanna GCF of C. americana was small and constant throughout the seasons. The GCF in eucalypts was 15% higher than in the savanna during the dry season, but 45% lower during the rainy season, GCF averaging 85% of that in the savanna. Since water use by plantations is lower than in the savanna, they have a large potential for C sequestration without significantly altering ground water availability.

Vegetative as well as reproductive phenology of tropical trees frequently occur synchronously at the end of the dry season. It has been repeatedly observed on two individuals of the tropical species Ficus obtusifolia growing in Caracas that they lose their entire foliage in a period of 48 h during the dry season and renew it within the next 72 h. In order to gain knowledge on the mechanisms that govern leaf exchange in F. obtusifolia, we followed in these two individuals the seasonal changes in leaf and fruit phenology, water relations and environmental variables, and in another 25, phenology only. Although foliar exchange was always observed before the rainy season, it also occurred near the end of the rainy season. An increase in leaf turgor potential of mature leaves and a low sap flow velocity coincided with leaf fall. Foliar exchange in all 27 individuals, growing in locations with different elevation and, possibly, access to underground water, was bi-modal and occurred in a period of approximately one month around both equinoxes. The presence of fruits was also bi-modal, occurring after leaf renewal. The sequence of phenological events repeated itself in all the individuals studied, although with inter-annual and individual variations in date of foliar exchange. Leaf fall was independent of leaf water status and significantly correlated with, in decreasing order, day length, minimum air temperature, rainfall, maximum air temperature and total radiation. We conclude that in F. obtusifolia high nocturnal temperature, increasing daytime temperature and radiation, and low cloud cover apparently drove foliar exchange. Day length showed a strong correlation with leaf renewal possibly to due to the significant relationships between day length and environmental variables. Direct effects of day length alone were not evidenced, as foliar exchange took place during a long time-span around the equinoxes.