Influence of temporospatial variation in sap flux density on estimates of whole-tree water use in Avicennia marina

Our study shows that sap flow in Avicennia marina varies significantly throughout the sapwood and that spatial patterns in sap flux density are dependent on meteorological conditions. Sap flux density measurements are used worldwide as a relatively inexpensive means to provide estimates of whole-tree and whole-stand water use in forest ecosystems. However, erroneous upscaling from point measurements to the entire sapwood area remains an issue, since sap flow is hardly ever constant throughout the tree. In this study, two widely used sap flow methodologies (the Heat Ratio or HR method and the Heat Field Deformation or HFD method) are used to assess radial and azimuthal variations in sap flux density in three mature trees of the mangrove species Avicennia marina in Brisbane, Australia. The genus Avicennia is characterised by secondary growth via successive cambia, resulting in an atypical sapwood pattern of xylem patches braided with phloem strings. Water use estimates were calculated in different ways. At first, spatial variation was ignored when upscaling from point measurements. Then, radial and azimuthal variations were incorporated subsequently by measuring at different depths and aspects around the tree. Ignoring azimuthal variation led to over- or underestimations of up to 102 %, while radial variation accounted for discrepancies of up to 25 %. Furthermore, the influence of changing meteorological conditions was assessed, which showed that radial profiles changed in shape during rain events, such that maximum sap flow rates occurred at different depths compared to dry periods. Our study thus indicates that spatial variation in sap flux density is highly unpredictable in A. marina due to its hydraulic architecture, and that changing meteorological conditions alter the pattern of this variation. These two factors should be accounted for when assessing whole-tree water use.