Spatial and temporal properties of whole-tree and forest-level water transport

Investigations of spatial and temporal variation in whole-tree and forest-level water transport were conducted in young loblolly pine and mature hardwood stands in Duke Forest, NC, and a humid tropical forest site in Panama City, Panama. Five studies were conducted. Water transport within trees in each study was measured using constant-heat sap flow probes, with measurements of environmental driving variables. To quantify radial variation in sap flux with sapwood depth, an experiment was conducted on samples of trees wherein sap flow probes were inserted at two depth intervals within the xylem. A 59% reduced flow in inner relative to outer xylem was found.;Temporal variation in whole-tree flux was investigated by estimating the time constant for responses of water flux in stems of loblolly pine to canopy transpiration. While the time constants did not vary within the range of tree sizes studied, capacitance increased and resistance decreased with stem volume, indicating an inverse adjustment of resistance and capacitance to maintain a similar time constant over the range of tree sizes.;The third study compared temporal averaging procedures for estimating daily canopy conductance. An average of diurnal conductance values was compared to a daily conductance calculation based on daily constituent variables.;In the fourth study, fluxes of water in tree stems of 10 species and liana of two species were monitored. The xylem flux was related diurnally in all plants to vapor pressure deficit measured within the canopy, and to incoming shortwave radiation.;In the fifth study, an investigation into long-term water use of stands dominated by loblolly pine was conducted. The objective of the study was to detect changes through time in water use characteristics of the stands as functions of both vegetation growth parameters and external climatic influences. Stand transpiration, in conjunction with precipitation and soil drainage information, was successfully used to predict the time course of volumetric soil moisture content through one of the growing seasons. Furthermore, from scaled up sap flow data, we found that the species contributions to total stand flux were compensatory so as to result in a relatively uniform total stand water flux.