Regulation of water flux through tropical forest canopy trees: Do universal rules apply?

Tropical moist forests are notable for their richness in tree species. The presence of such a diverse tree flora presents potential problems for scali ng up estimates of water use from individual trees Co entire stands and for drawing generalizations about physiological regulation of water use in tro pical trees. We measured sapwood area or sap flow, or both, in 27 co-occurr ing canopy species in a Panamanian forest to determine the extent to which relationships between tree size, sapwood area and sap flow were species-spe cific, or whether they were constrained by universal Functional relationshi ps between tree size, conducting xylem area, and water use. For the 24 spec ies in which active xylem area was estimated over a range of size classes, diameter at breast height (DBH) accounted for 98% of the variation in sapwo od area and 67% of the variation in sapwood depth when data for all species were combined. The DBH alone also accounted for greater than or equal to 9 0% of the variation in both maximum and total daily sap flux density in the outermost 2 cm of sapwood for all species taken together Maximum sap flux density measured near the base of the tree occurred at, about 1400 h in the largest trees and 1130 h in the smallest trees studied, and DBI-I accounte d fur 93% of the variation in the time of day at which maximum sap flow occ urred. The shared relationship between tree size and time of maximum sap fl ow at the base of the tree suggests that a common relationship between diur nal stem water storage capacity and tree size existed. These results are co nsistent with a recent hypothesis that allometric scaling of plant vascular systems, and therefore water use, is universal.