TRADE-OFF BETWEEN WATER TRANSPORT EFFICIENCY AND LEAF LIFE-SPAN IN A TROPICAL DRY FOREST

Drought-deciduous and evergreen species coexist in tropical dry forests. Drought-deciduous species must cope with greater seasonal leaf water-potential fluctuations than evergreen species and this may increase their susceptibility to drought-induced xylem embolism. The relationship between water transport efficiency and leaf life-span were determined for both groups. They differed in seasonal changes of both, wood water content ( W-c,) and wood specific gravity (G). During the dry season, the W-c in drought-deciduous species declined and the minimum value was recorded when leaf fall was complete. At this time, the volumetric fraction of gas (V-g) increased indicating air entry into xylem vessels. In contrast, W-c, G and V-g changed only slightly throughout the year for evergreen species. Maximum hydraulic conductivity of drought-deciduous species was 2-6 times that of the evergreen species, but was severely reduced at leaf fall. In the evergreen species, similar water conductivities were measured during wet and dry seasons. The trade-off between xylem water transport capacity and leaf lifespan found in species coexisting in this forest reveals the existence of contrasting but successful adaptations to this environment. Drought-deciduous species maximize production in the short term with higher water transport efficiency which leads to the seasonal occurrence of embolisms. Conversely, the behaviour of evergreen species with reduced maximum efficiency is conservative but safe in relation to xylem embolism.