STEM WATER STORAGE AND DIURNAL PATTERNS OF WATER-USE IN TROPICAL FOREST CANOPY TREES

Stem water storage capacity and diurnal patterns of water use were stu died in five canopy trees of a seasonal tropical forest in Panama, Sap flow was measured simultaneously at the top and at the base of each t ree using constant energy input thermal probes inserted in the sapwood , The daily stem storage capacity was calculated by comparing the diur nal patterns of basal and crown sap flow. The amount of water withdraw n from storage and subsequently replaced daily ranged from 4 kg d(-1) in a 0.20-m-diameter individual of Cecropia longipes to 54 kg d(-1) in a 1.02-m-diameter individual of Anacardium excelsum, representing 9-1 5% of the total daily water loss, respectively. Ficus insipida, Luehea seemannii and Spondias mombin had intermediate diurnal water storage capacities. Trees with greater storage capacity maintained maximum rat es of transpiration for a substantially longer fraction of the day tha n trees with smaller water storage capacity. All five trees conformed to a common linear relationship between diurnal storage capacity and b asal sapwood area, suggesting that this relationship was species-indep endent and size-specific for trees at the study site, According to thi s relationship there was an increment of 10 kg of diurnal water storag e capacity for every 0.1 m(2) increase in basal sapwood area, The diur nal withdrawal of water from, and refill of, internal stores was a dyn amic process, tightly coupled to fluctuations in environmental conditi ons. The variations in basal and crown sap flow were more synchronized after 1100 h when internal reserves were mostly depleted. Stem water storage may partially compensate for increases in axial hydraulic resi stance with tree size and thus play an important role in regulating th e water status of leaves exposed to the large diurnal variations in ev aporative demand that occur in the upper canopy of seasonal lowland tr opical forests.