Medium-term sap flux monitoring in a Scots pine stand: analysis of the operability of the heat dissipation method for hydrological purposes

Calibration of Thermal Dissipation Probes for Date Palm (Phoenix dactylifera L.)

The quantification of water flow through the stem is vital for date palm (Phoenix dactylifera L.) to promote a good water stress management. The thermal dissipation probe (TDP) method developed by Granier is widely used to evaluate transpiration of forest trees; however, there are contradictory reports regarding its reliability. Considerable errors in estimated sap flux density, which might be due to a lack ofspecies-specific calibrations. The TDP method uses a mathematical model that is based on an empirical equation to estimate sap flux density, which is claimed to be applicable to all tree species, independently of wood structure and anatomy. At the laboratory, we compared the rate of water uptake by cut stems with sap flux estimates derived from the TDP method to assess the validity of the method.Our calibration results were considerably different compared to the Granier’s original equation. Moreover, sap flux density was overestimated by 18.2±0.5% when the original calibration parameters of Granierare employed. However,using new calibration parameters improved the accuracy of sap flow measurements. Our results indicated that it is not appropriate to use a general equation for different species. Therefore, previous estimations of date palm’s water requirement through thermal dissipation probes should be revised.

Quantification of uncertainties in conifer sap flow measured with the thermal dissipation method

Trees play a key role in the global hydrological cycle and measurements performed with the thermal dissipation method (TDM) have been crucial in providing whole-tree water-use estimates. Yet, different data processing to calculate whole-tree water use encapsulates uncertainties that have not been systematically assessed. We quantified uncertainties in conifer sap flux density (F_d ) and stand water use caused by commonly applied methods for deriving zero-flow conditions, dampening and sensor calibration. Their contribution has been assessed using a stem segment calibration experiment and 4 yr of TDM measurements in Picea abies and Larix decidua growing in contrasting environments. Uncertainties were then projected on TDM data from different conifers across the northern hemisphere. Commonly applied methods mostly underestimated absolute F_d . Lacking a site- and species-specific calibrations reduced our stand water-use measurements by 37% and induced uncertainty in northern hemisphere F_d . Additionally, although the interdaily variability was maintained, disregarding dampening and/or applying zero-flow conditions that ignored night-time water use reduced the correlation between environment and F_d . The presented ensemble of calibration curves and proposed dampening correction, together with the systematic quantification of data-processing uncertainties, provide crucial steps in improving whole-tree water-use estimates across spatial and temporal scales.

Ecophysiological variation of transpiration of pine forests: synthesis of new and published results

Canopy transpiration (E_C ) is a large fraction of evapotranspiration, integrating physical and biological processes within the energy, water, and carbon cycles of forests. Quantifying E_C is of both scientific and practical importance, providing information relevant to questions ranging from energy partitioning to ecosystem services, such as primary productivity and water yield. We estimated E_C of four pine stands differing in age and growing on sandy soils. The stands consisted of two wide-ranging conifer species: Pinus taeda and Pinus sylvestris, in temperate and boreal zones, respectively. Combining results from these and published studies on all soil types, we derived an approach to estimate daily E_C of pine forests, representing a wide range of conditions from 35° S to 64° N latitude. During the growing season and under moist soils, maximum daily E_C (E_Cm ) at day-length normalized vapor pressure deficit of 1 kPa (E_Cm-ref ) increased by 0.55 ± 0.02 (mean ± SE) mm/d for each unit increase of leaf area index (L) up to L = ~5, showing no sign of saturation within this range of quickly rising mutual shading. The initial rise of E_Cm with atmospheric demand was linearly related to E_Cm-ref . Both relations were unaffected by soil type. Consistent with theoretical prediction, daily E_C was sensitive to decreasing soil moisture at an earlier point of relative extractable water in loamy than sandy soils. Our finding facilitates the estimation of daily E_C of wide-ranging pine forests using remotely sensed L and meteorological data. We advocate an assembly of worldwide sap flux database for further evaluation of this approach.

Comparing ∆Tmax Determination Approaches for Granier-Based Sapflow Estimations

Granier-type thermal dissipation probes are common instruments for quantifying tree water use in forest hydrological studies. Estimating sapflow using Granier-type sapflow sensors requires determining the maximum temperature gradient (∆T_max) between the heated probe and the reference probe below. ∆T_max represents a state of zero sap flux, which was originally assumed to occur each night leading to a ∆T_max determination on a daily basis. However, researchers have proven that, under certain conditions, sapflow may continue throughout the night. Therefore alternative approaches to determining ∆T_max have been developed. Multiple ∆T_max approaches are now in use; however, sapflow estimates remain imprecise because the empirical equation that transfers the raw temperature signal (∆T) to sap flux density (F_d) is strongly sensitive to ∆T_max. In this study, we analyze the effects of different ∆T_max determination approaches on sub-daily, daily and (intra-)seasonal F_d estimations. On this basis, we quantify the uncertainty of sapflow calculations, which is related to the raw signal processing. We show that the ∆T_max determination procedure has a major influence on absolute ∆T_max values and the respective sap flux density computations. Consequently, the choice of the ∆T_max determination approach may be a significant source of uncertainty in sapflow estimations.

Spatial sap flow and xylem anatomical characteristics in olive trees under different irrigation regimes

The compensation heat pulse (CHP) method is widely used to estimate sap flow and transpiration in conducting organs of woody plants. Previous studies have reported a natural azimuthal variability in sap flow, which could have practical implications in locating the CHP probes and integrating their output. Sap flow of several olive trees (Olea europaea L. cv. 'Arbequina') previously grown under different irrigation treatments were monitored by the CHP method, and their xylem anatomical characteristics were analyzed from wood samples taken at the same location in which the probes were installed. A significant azimuthal variability in the sap flow was found in a well-irrigated olive tree monitored by eight CHP probes. The azimuthal variability was well related to crown architecture, but poorly to azimuthal differences in the xylem anatomical characteristics. Well-irrigated and deficit-irrigated olive trees showed similar xylem anatomical characteristics, but they differed in xylem growth and in the ratio of nocturnal-to-diurnal sap flow (N/D index). The results of this work indicate that transpiration cannot be accurately estimated by the CHP method in olive trees if a small number of sensors are employed and that the N/D index could be used as a sensitive water status indicator.