Measuring sap flow and stem water content in trees: a critical analysis and development of a new heat pulse method (Sapflow+)

Just as carbon, water is indispensable for plants to develop and grow. A lack of water causes turgor loss in plant cells which prevents further expansion of these cells and the coupled incorporation of carbon sources in the cell wall. This inhibits growth and, if this water scarcity continues, plant dimensions such as the stem diameter will start to decrease. Finally the plant will lose its vital functions and die. Worldwide, sap flow methods are applied to monitor plant water status and validate vegetation models. These methods determine flow direction as well as relative and absolute flow, forming the link between plant water uptake, release and storage. Hence, whether to assess the correct irrigation dose, to monitor forest vitality or to obtain trustworthy modelling results, reliable sap flow measurements are indispensable. The most commonly applied sap flow methods are based on heat dissipation in the sapwood. Within this group, a distinction can be made between those methods determining the total flow per time inside a stem or stem section and those assessing sap flux density, the flow per surface per time. While the former are widely applied in irrigation and other applications necessitating an estimation of total plant water use, the latter are applied to investigate specific hydraulic pathways and processes as they allow to distinguish spatial patterns in sap flow, both axially, radially and azimuthally. In this PhD study, the accuracy and applicability of the most important sap flux density methods were investigated. To this end, the underlying thermodynamic theory was studied, Finite Element Modelling (FEM) conducted and lab experiments on cut tree stem segments were undertaken, complemented with a field study on Avicennia marina (Forssk.) Vierh. and Rhizophora stylosa Griff. By investigating the thermodynamic interpretation of the thermal diffusivity as sapwood property, it became clear that the link between the Heat Field Deformation (HFD) temperature ratio and sap flux density, based on this thermal diffusivity, was incorrect. It was concluded that therefore, the continuous HFD method should be considered merely empirical, similar to the Thermal Dissipation method. Moreover, based on FEM, an improved empirical correlation between the HFD temperature ratio and sap flux density was proposed. Also for the methods based on the application of heat pulses, a flaw in the basic theory was noted. These methods are based on the isotropic heat conduction-convection equation for an ideal heater in an infinite medium. Sapwood, however, is known to be anisotropic. Fortunately, the Compensation Heat Pulse, Tmax as well as the Heat Ratio method are based on derivations of this basic equation in a way that is independent of the assumption of isotropy. Hence, for these methods the results are still theoretically correct. Nevertheless, attention should be paid to apply the correct anisotropic equation in modelling and method development, as recent examples show that by neglecting anisotropy, errors can be induced. Within the heat pulse sap flux density methods, the Heat Ratio method enables measurements of low and reverse flow, unlike the Compensation Heat Pulse and Tmax method. This method, however, is dependent on accurate estimations of axial thermal sapwood diffusivity. In this PhD, it was shown that in the currently applied method of mixtures to determine this diffusivity, no distinction was made between bound and unbound water, resulting in over- or underestimations of axial thermal diffusivity dependent on the dry sapwood density and sapwood water content. A correction to this method was proposed, differentiating between bound and unbound water based on the fibre saturation point. This correction has the disadvantage that fibre saturation point is a sapwood characteristic that is not measurable in-situ and, hence, has to be estimated based on dry sapwood density. In response to the difficulties encountered when studying the different sap flux density methods, a new method was developed: the Sapflow+ method. This method is based on a curve fitting procedure during which the anisotropic heat conduction-convection equation is directly fitted to measured temperature profiles located both axially and tangentially from the heater. As was shown by the conducted identifiability analysis and the lab experiments on stem segments of Fagus sylvatica L. , the Sapflow+ method enables simultaneous measurements of heat velocity, across the entire naturally occurring range, and thermal sapwood properties, from which sap flux density and sapwood water content can be derived. The applicability of the method to determine heat velocity was confirmed in a field experiment on Avicennia marina (Forssk.) Vierh. and Rhizophora stylosa Griff. For the determination of sapwood water content, further validation experiments and possible optimization of the method are needed. Next to providing an opportunity to test the Sapflow+ method in harsh field conditions, the experiments conducted on Avicennia and Rhizophora led to the remarkable finding that both species show a completely different pattern in stem diameter variation, despite being influenced by the same environmental conditions. This led to the hypothesis that endogenous control of stem diameter fluctuations and growth might be much more crucial than previously assumed and could play an important role in plant growth strategies. In conclusion, the presented PhD study has exposed some limitations of and inaccuracies in existing sap flux density methods and has provided an alternative based on correct theoretical principles. This Sapflow+ method is sensitive towards the entire naturally occurring sap flow range and holds the promise of accurately determining sapwood water content.