WATER RELATIONS AND HYDRAULIC ARCHITECTURE OF A TROPICAL TREE (SCHEFFLERA-MOROTOTONI) - DATA, MODELS, AND A COMPARISON WITH 2 TEMPERATE SPECIES (ACER-SACCHARUM AND THUJA-OCCIDENTALIS)

The water relations and hydraulic architecture of a tropical tree (Schefflera morototoni) and of two temperate species (Acer saccharum and Thuja occidentalis) are reported. Among the water relations parameters measured were leaf and stem water storage capacity, leaf water potential, transpiration, and vulnerability of stems to cavitation and loss of hydraulic conductivity by embolisms. Among the hydraulic architecture parameters measured were hydraulic conductivity per unit pressure gradient, specific conductivity, leaf-specific conductivity, and Huber value. In terms of vulnerability of stems to cavitation, stem and leaf capacitances, and leaf-specific conductivity, all three species followed the same sequence: Schefflera > Acer > Thuja. It is argued here that the high stem capacitance and high leaf-specific conductivity of Schefflera are necessary to compensate for its high vulnerability to cavitation. Extractable water storage per unit leaf area in Schefflera stems is > 100 times that of Acer and may permit the species to survive unusually long, dry seasons in Panama. Although Schefflera frequently grows > 20 meters, the biggest resistance to water flow in the shoots resides in the leaves.