INFLUENCE OF SPATIAL VARIATION IN SAP FLUX DENSITY ON ESTIMATES OF WHOLE-TREE WATER USE IN AVICENNIA MARINA

Abstract

Sap flux density measurements are used worldwide as a relatively inexpensive means to provide an estimate of whole-tree and whole-stand water use in forest ecosystems. However, erroneous up-scaling from point measurements to the entire sapwood area remains an important issue, since sap flux density is hardly ever constant throughout the cross-section of a tree. In this study, two widely used sap flow methodologies (the Heat Ratio Method or HRM and the Heat Field Deformation method or HFD) are used to assess radial and azimuthal variations in sap flux density in the mangrove species Avicennia marina or grey mangrove, a species that is characterised by a secondary growth via successive cambia, resulting in an atypical sapwood pattern of xylem patches braided with phloem strings. Sap flux density values were scaled up to whole-tree water use in different ways. Initial estimates of sap flux were made from point measurements, ignoring spatial variations. We then adjusted these measurements by including radial and azimuthal variations assessed by measurements of radial profiles and multiple sensors around the circumference of the tree, respectively. Our results showed a discrepancy of 237+/-26% between the lowest and highest water use estimate, as a consequence of the high radial and azimuthal variations in sap flux density in this species. Our study indicates that caution is required when interpreting sap flux density values, hence it is crucial to account for radial and azimuthal variation when scaling up from point measurements to whole-tree water use.