Temporal patterns of water flux in trees and lianas in a Panamanian moist forest

Using constant heat sap flow sensors, xylem water fluxes in ten tree species and two liana species were monitored for 5-10 days during the beginning of the wet season in May, 1993. For a subset of the trees, a branch was also monitored at the top of the crown for 5 days. Xylem flux (J(S)) was related diurnally in all plants to vapor pressure deficit (D) measured within the upper-third of the canopy, and to incoming shortwave radiation R-S above the canopy. Cross-correlation analysis was used to estimate time lags between diurnal patterns of J(S) and D or R-S, and between J(S) in stems and branches. The maximum correlation coefficient from cross-correlation of J(S) with R-S (range=0.57-0.92) was often higher than the maximum of J(S) with D (range=0.43-0.89), indicating that diurnal J(S) was more dependent on R-S than D. Time lags (lag corresponding to maximum correlation) of J(S) at stem-base with D was shorter (0-45 min) than with radiation (5-115 min), highly variable within a species, and uncorrelated to the height or exposure of tree crowns or liana in the canopy. On a stand level, not accounting for the diel lag between stem sap flux and canopy flux resulted in errors in estimated canopy transpiration of up to 30%.