Physiological Significance of Hydraulic Segmentation, Nocturnal Transpiration and Capacitance in Tropical Trees: Paradigms Revisited

Abstract

Results from water relations and hydraulic architecture studies of trees from tropical savannas and humid tropical and subtropical forests were reanalyzed in view of paradigms related to the (i) physiological significance of hydraulic segmentation across trees with different life history traits and habitats, (ii) determinants of massive tree mortality, (iii) nocturnal transpiration, and (iv) the role of internal stem water storage. Stems and leaves of tropical and subtropical deciduous tree species are equally vulnerable to cavitation, whereas leaves of evergreen species are substantially more vulnerable than stems. Tree species from tropical ecosystems that do not experience seasonal droughts have stems and leaves with similar vulnerability to cavitation while trees from tropical ecosystems that experience seasonal droughts have leaves that are more vulnerable to drought induced cavitation compared to stems. Strong segmentation (whether hydraulic or vulnerability) during severe droughts may have an indirect negative impact on tree carbon balance. For example for Sclerolobium paniculatum, a widespread tree species in neotropical savannas and seasonally dry forests, the decrease in total leaf surface area per plant (which impact hydraulic architecture) during droughts help to maintain an adequate water balance but has large physiological costs: trees receive a lower return in carbon gain from their investment in stem and leaf biomass. Leaf hydraulic failure and carbon starvation may contribute to the massive, size-dependent mortality observed in this species. The functional significance of the widespread phenomenon of nocturnal transpiration in tropical trees is discussed. One of the most likely functions of nocturnal sap flow in savanna trees growing in nutrient poor soils appears to be enhanced nutrient acquisition from oligotrophic soils. Large capacitance plays a central role in the rapid growth patterns of tropical deciduous tree species facilitating rapid canopy access as these species are less shade tolerant than evergreen species. Higher growth rates in species with high capacitance could be achieved by keeping the stomata open for longer periods of time.