Hydraulic and photosynthetic co-ordination in seasonally dry tropical forest trees

In the present study the linkage between hydraulic, photosynthetic and phenological properties of tropical dry forest trees were investigated. Seasonal patterns of stem-specific conductivity (K-SP) described from 12 species, including deciduous, brevi-deciduous and evergreen species, indicated that only evergreen species were consistent in their response to a dry-to-wet season transition. In contrast, K-SP in deciduous and brevi-deciduous species encompassed a range of responses, from an insignificant increase in K-SP following rains in some species, to a nine-fold increase in others. Amongst deciduous species, the minimum K-SP during the dry season ranged from 6 to 56% of wet season K-SP, indicating in the latter case that a significant portion of the xylem remained functional during the dry season. In all species and all seasons, leaf-specific stem conductivity (K-L) was strongly related to the photosynthetic capacity of the supported foliage, although leaf photosynthesis became saturated in species with high K-L. The strength of this correlation was surprising given that much of the whole-plant resistance appears to be in the leaves. Hydraulic capacity, defined as the product of K-L and the soil-leaf water potential difference, was strongly correlated with the photosynthetic rate of foliage in the dry season, but only weakly correlated in the wet season.