Álvaro López-Bernal

Thermal properties of sapwood of fruit trees as affected by anatomy and water potential: errors in sap flux density measurements based on heat pulse methods

Key messageThe accuracy of heat pulse methods is compromised both by changes in sapwood water content and by the methodology applied to determine its thermal properties.AbstractSapwood thermal properties and water content (Fw) natural variations affect the accuracy of heat pulse sap flow methods, as they are typically set as constants or calculated during zero flow conditions. In a first experiment, a characterisation of both thermal properties and some of their determining anatomical and functional factors was conducted on several fruit tree species assessing the reliability of different methodologies. Besides that, a second experiment was carried out to evaluate the errors of heat pulse methods arising from ignoring Fw variations. To do so, desorption curves were constructed, allowing the substitution of Fw changes with those of water potential (Ψ). Results of the first experiment showed considerable differences between species in both the thermal, anatomical and functional properties of sapwood. Apart from that, discrepancies between the methods applied to determine thermal properties were also found and their implications for some heat pulse methods are discussed. The analysis conducted for the second experiment indicated that large errors in sap flux density (J) determinations might occur when daily and seasonal variations of Ψ (and hence of Fw) are disregarded. The extent of these errors was influenced by the species and heat pulse technique. Thus, the heat ratio and Tmax were, respectively, the least and most vulnerable methods to errors in J determinations associated with changes in Fw.

Low winter temperatures induce a disturbance of water relations in field olive trees

Key messageLow winter temperatures induce an increase in the soil-to-trunk hydraulic resistance of field-grown olive trees resulting in a significant disturbance of their water relations.AbstractA disturbance of water relations in response to chilling have long been observed in potted plants growing under controlled conditions, but information is lacking for field plants. The aim of this study was to assess the effects of winter low temperatures on the water relations of mature olive trees. To this end, water potential, sap flux density, soil temperature and meteorological data were monitored in a hedgerow olive orchard near Córdoba, southern Spain, throughout two consecutive winters. Water stress symptoms were found in terms of midday Ψ, despite adequate water supply and low evaporative demand. These effects were associated with changes in the soil-to-trunk hydraulic resistance (Rroot), which increased by December–January to much higher values than those previously reported in the literature, particularly in the year of higher fruit load. The contribution of viscosity (η) to the observed Rroot dynamics was almost negligible as deduced from measurements of soil temperature, so the high winter values of Rroot were likely to have originated from other causes such as reductions in membrane permeability and root growth. The findings of this work raise new major issues that deserve further research such as the impact of the winter water stress on stomatal conductance and photosynthesis rates in mature olive trees.

Fractional Approach for Estimating Sap Velocity in Trees

Abstract In the context of fractional calculus (FC), this paper is devoted to model thermal processes in trees based on the measurement of the temperature difference (ΔT) between sensors located above and below a heater inserted in the tree trunk. By evaluating several temperature curves taken from real trees of different species, the current approach shows that the temperature in each probe can be successfully described by the two-parameter Mittag- Leffler function Eα,β. Then, a simple methodology is followed to derive a novel expression of the heat-pulse velocity (v) as a function of ΔT and the parameter α of the mentioned Eα,β function. Experimental results are given to validate the goodness of the current proposal.

Fractional models for measuring sap velocities in trees

The compensation heat-pulse (CHP) method is founded on the measurement of the temperature difference between sensors located above and below a heater inserted in tree trunks. This technique assumes that the temperature in each probe follows an exponential function and, as a result, an expression to calculate the heat-pulse velocity (v) is deduced. The current work investigates the application of recent theory of fractional calculus (FC) to model such thermal processes in trees. In particular, a generalisation of the mentioned exponential function, given by the two-parameter Mittag-Leffler function, is considered to model temperature dynamics. Several temperature curves, corresponding to zero and nonzero values of v, taken from real trees of different species are given to show that the proposed fractional order models are able to better describe such thermal flows, especially when velocities are different from zero.