Quantification of uncertainties introduced by data-processing procedures of sap flow measurements using the cut-tree method on a large mature tree

doi:10.1016/j.agrformet.2020.107926

Agricultural and Forest Meteorology

Volume 287, 15 June 2020, 107926

https://doi.org/10.1016/j.agrformet.2020.107926 Get rights and content

Highlights

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A validation of sap flow estimates based on sensors is lacking for large trees.
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A 20 m tall aspen tree was cut in water to compare sensor and gravimetric sap flow.
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Sensor accuracy was assessed across stem azimuths and data-processing approaches.
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Sensor accuracy was mainly affected by position, i.e. cardinal variation in sap flux.
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Sensor and gravimetric cumulative sap flow measures differed by up to 130 L in 5 days.

Abstract

Sap flow sensors are crucial instruments to understand whole-tree water use. The lack of direct calibration of the available methods on large trees and the application of several data-processing procedures may jeopardize our understanding of water uptake dynamics by increasing the uncertainties around sensor-based estimates. We directly compared the heat ratio method (HRM) sap flow measurements to water uptake measured gravimetrically using the cut-tree method on a large mature aspen tree to quantify those uncertainties for ten consecutive days. The influence of the azimuthal position of the sensors and the application of different data-processing procedures on the accuracy of the sap flow sensors’ estimates was assessed using different metrics. Overall, the sap flow measurements showed high temporal precision with the gravimetric data. Azimuthal and radial variability of measured sap flux density showed the most substantial effect on the accuracy of the sensors’ estimates of whole tree sap flow. The zero-flow corrections applied altered the accuracy and linearity of the sensors’ measurements at the hourly scale, while the sapwood area method used had a lesser impact. Across the ensemble of available data-processing procedures, the cumulative whole-tree water uptake estimates for five consecutive days from the sensor diverged from the gravimetric measurements by less than 1% to more than 50% depending on the sensors azimuthal position and data corrections applied. This study illustrates some of the uncertainties associated with the methodological approaches chosen when using sap flow sensors to estimate water uptake in tall and large diameter trees.

Introduction

Heat-based sap flow methods have become a principal means to non-destructively provide whole-tree water use estimates across spatiotemporal scales (Hassler et al., 2018), while also improving our understanding of the impact(s) of a changing climate (Berdanier and Clark, 2018; Zhang et al., 2016) and forest management practices (Doody et al., 2015) on forested landscapes. Several sap flow methods are available to measure tree-water use (Fernández et al., 2017) using heat as a tracer for sap flow. Calculating whole-tree sap flux rates using heat tracers requires several steps of data-processing (Looker et al., 2016; Peters et al., 2018). Proper estimation of the sapwood thermal diffusivity as well as corrections for probe misalignment and wounding need to be considered (Burgess et al., 2001; Burgess and Downey, 2018; Marshall, 1958). Zero-flow determination is an additional crucial step, for which a range of empirical, systematic (Granier, 1987; Lu et al., 2004), and physically-based (Burgess and Downey, 2018; Oishi et al., 2008) methods are available. Additionally, strong radial and circumferential variations (Van de Wal et al., 2015) are often ignored, potentially leading to over- or under-estimating whole-tree water use when extrapolating a single sensor measurement across the entire stem. A global assessment of the impact of the combination of those data-processing steps on sap flow studies is currently lacking (but see Looker et al., 2016; Peters et al., 2018; Rabbel et al., 2016; Van de Wal et al., 2015 among others, addressing individual data-processing approaches), challenging the interpretation, comparison, and integration of individual tree-based studies when used in large-scale ecohydrology models.

A general quantification of the uncertainties generated by both the sap flow sensor method itself and the combination of methodological decisions and data-processing procedures on large mature trees is lacking, but crucial (Flo et al., 2019), particularly when attempting to derive landscape-scale processes (e.g. ecosystem water and carbon fluxes, catchment hydrological and vegetation responses to climate and land-use changes; Bonan, 2008; Schlesinger and Jasechko, 2014). Calibration studies addressing the accuracy of the sap flow measurements in woody plants have been previously conducted (see the Supplementary Material in Flo et al. (2019) for a list of studies and calibration materials), however, many of them were mostly focused on smaller trees (less than 10 cm in diameter), shrubs, or branch segments, which are unlikely to provide reliable or appropriate estimates of water-use in larger diameter trees. To date, only a few studies have directly assessed the accuracy of sap flow sensors in estimating the water uptake of large mature trees (Olbrich, 1991; Roberts, 1977; Vertessy et al., 1997). However, the height (>10 m), diameter (>20 cm) and weight of mature forest trees presents experimental and logistical challenges for obtaining a direct measurement of water uptake to compare with the sap flow sensors’ estimates, and thus, these studies are very limited in scope. The anatomical, morphological, and physiological variability in the stem of large mature trees can result in potentially strong diverging patterns between and within-trees in sap flux densities (Cohen et al., 2008; Shinohara et al., 2013; Van de Wal et al., 2015). The choice of the method in correcting for zero-flows and extrapolating to whole-tree sap flux rates may increase the error in sensor-based estimates of water use of an individual tree. Ultimately, this can jeopardize the interpretation of climate-response analyses and stand-level water uptake estimates for large and mature trees across temporal and spatial scales.

In this study, we aimed to: (1) quantify the extent of uncertainty in sap flow measurements related to circumferential variability, the application of different sapwood area estimation methods and zero-flow corrections; and (2) assess the impact of each and the combination of these factors on the relationship with climate variables and estimates of cumulative whole-tree water uptake. We used heat ratio method (HRM) sap flow heat-pulse sensors (Burgess et al., 2001) which have shown relatively good performance in accuracy when compared to other sap flow methods (Flo et al., 2019). Furthermore, we used the cut-tree technique to directly measure trunk water uptake gravimetrically by immersing the stem of a large tree (20 m tall, 35 cm in diameter) in water (Roberts, 1977). Using this technique, we can directly compare the HRM sap flux estimates to the gravimetrically-measured water uptake, and then quantitatively assess the uncertainty and error associated with several data-processing methods applied to the sap flow data.

Section snippets

Study site and climate data

The study was carried out in Thorhild County (54°17′11.6″ N, 112°46′08.6″ W), approximately 120 km northeast of Edmonton, Alberta, Canada. The site is located within the dry mixedwood natural sub-region of the boreal region, composed of a mixture of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss), surrounded by agricultural lands. The selected trees for the study were part of a 60 - 70-year-old trembling aspen dominated stand (~ 600 stems per hectare).

Sap flow response to the cutting

Immediately after the cut and for the next 48 h, sap flux density (J_s) was similar to measurements taken before the cut and to those measured in the three control trees (Fig. 1a). Sap flux density then declined in the focus tree over the next eight days compared to the three control trees, which all maintained higher J_s. Interestingly the crown of our focus tree showed no sign of hydraulic damage for the duration of the study (i.e. wilting or color changes; Fig. 1b and c). Transpiration rate of

Discussion

We quantified the uncertainties introduced by the placement of sap flow sensors (cardinal directions) and the application of different data-processing approaches (radial integration method used, zero-flow correction, and sapwood area estimation methods) on sap flow estimates for large diameter trees. These estimates were compared to gravitational measurements using the cut-tree method. The temporal precision of sensor-derived fluxes with the gravimetrically-measured water uptake was high when

Conclusion

This cut-tree study of sap flux measurements on a large mature tree using the HRM sap flow sensors highlighted that uncertainties arise from several methodological steps from sensor installation to the calculation of hourly sap fluxes. Although the cut-tree method provides a unique way to directly obtain gravimetric measurement of water uptake at a fine temporal scale for large trees (Olbrich, 1991; Roberts, 1977; Smith, 1992; Vertessy et al., 1997), these studies are complex to set up and

CRediT authorship contribution statement

Morgane Merlin: Conceptualization, Methodology, Investigation, Formal analysis, Visualization, Writing - original draft. Kevin A. Solarik: Conceptualization, Methodology, Investigation, Writing - review & editing. Simon M. Landhäusser: Funding acquisition, Project administration, Supervision, Conceptualization, Methodology, Investigation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank Ian Jamieson for his invaluable help and expertise during planning and construction of the wooden structure, as well as Peter Lazowski for his help in the field. We thank all those who provided field and logistical support for this project (Frances Leishman, Robert Hetmanski, Natalie Scott, Ashley Hart, and Julie Zettl) as well as laboratory work (Pak Chow) and data-processing support (Newton Tran, ICT International Pty Ltd.). We thank Cinnamin Landhäusser for her support and

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Partitioning evapotranspiration (ET) into its components of canopy interception (Ic), transpiration (T) and soil evaporation (E) enhances understanding of hydrological cycle and plant water use strategy in drylands. However, variations in all three ET components and their fractions across multi-temporal scales (daily, monthly, annual), and their controlling factors remain poorly understood for shrub ecosystems. In this study, we aimed to address these issues by measuring daily rainfall partitioning, sap flow, soil evaporation and soil moisture in a xerophytic shrub (Salix psammophila) ecosystem in northern China during 2019–2021. We found T dominated ET with a mean ratio of T to ET ranging 61.7 %–71.1 %, which was consistently higher than the mean ratio of Ic and E to ET ranging 12.1 %-41.5 % and 24.5 %–32.2 %, respectively, from daily to annual scales (p < 0.05). There was no significant difference in mean ratio of T to ET among different temporal scales (p > 0.05), while mean ratios of Ic and E to ET were significantly higher at daily scale than those at monthly and annual scales (p < 0.05). Compared with monthly and annual scales, ET components and their ratios at daily scale were more responsive to bio-/abiotic factors. ET components were more susceptible to be affected by the influencing factors when compared to their ratios, regardless of the temporal scales. Specifically, daily ET components and their ratios were mainly controlled by precipitation and solar radiation. In addition, soil moisture also explained 10.2 % and 10.9 % of the variations of daily T and ET, respectively. At monthly scale, however, variations in T and ET were mainly controlled by LAI with explained variance of 26.6 % and 32.4 %, respectively. Our results highlighted the significant differences in ET partitioning and their different responses to bio- and abiotic factors at various temporal scales of the shrub ecosystems in drylands.
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Tree transpiration contributes to the forest water budget and its environmental responses differ between daytime and nighttime. Elucidating these divergent responses separately from their biophysical controls is crucial for ascertaining the forest-water relationship in changing environments. To investigate the biophysical controls on daytime and nighttime sap flow (Q_d and Q_n), the sap flux density, meteorological conditions, soil moisture, and canopy leaf area index (LAI) were simultaneously monitored in a Larix principis-rupprechtii plantation in the Liupan Mountains of Northwest China during the growing seasons (June to September) of three hydrological years (2019, 2021, and 2022). The results showed that the main control factors for Q_d and Q_n varied depending on the timescale. The variation in Q_d was mainly controlled by daytime solar radiation (R_d), vapour pressure deficit (VPD_d), relative extractable soil water (REW_d) and LAI at a daily scale, but only by R_d at a monthly scale and only by VPD_d at an interannual scale. The variation in Q_n was mainly controlled by the nighttime vapour pressure deficit (VPD_n) and relative extractable soil water (REW_n), LAI, and Q_d at a daily scale but only by VPD_n at monthly and interannual scales. The responses of Q_d and Q_n to soil water deficit were different and varied by timescale. At the daily scale, the effect of VPD_d on Q_d became weak, while the effect of VPD_n on Q_n became strong in drier years (2021 and 2022). At the monthly scale, as the main controlling factor of the monthly variation in Q_d (Q_n), R_d (VPD_n) played a decisive role in the wet (dry) months. At the interannual scale, VPD limited Q_d but drove Q_n, leading to a decrease in Q_d and an increase in Q_n with increasing soil water deficit. Overall, our findings revealed the different responses of Q_d and Q_n to biophysical factors and underscored that Q_d and Q_n should be explored both separately and synchronously; additionally, our results indicated that the proportion of Q_n might increase if soil water stress is increased due to future climate change.
Species with larger vessel area have higher bias for the original Granier equation in calculating sap flux density
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Accurate estimate of transpiration is crucial to understanding global water and carbon cycles. The thermal dissipation (TD) method with Granier’s equation, which is the most popular approach to measure whole-tree water use, requires species-specific calibration parameters to calculate transpiration. Calibrations require direct measurement of water uptake, often through gravimetric or potometric methods; therefore, it is often disregarded because of limited time and resources. It remains unclear which factor influences the variation of calibration parameters. In this study, we compiled published calibration experiments to elucidate the factors that could influence the variation in coefficients and attempted to predict the correction parameter. The results showed that: (1) only 14 % of the experiments provided accurate sap flux density estimates using the default parameter values in the Granier’s equation, and sap flux was underestimated to varying degrees in 72% of the experiments; (2) the regression slope of TD-measured and the actual sap flow with the y-axis intercept set to zero (hereinafter referred to as “Slope”) was significantly correlated to the mean vessel area (VA), i.e., species with larger VA consistently had higher Slope and greater bias. Stem diameter also had a positive influence on the Slope after accounting for covariation with the VA. Finally, a multiple linear regression model based on these two characteristics (VA and stem diameter) explained 89% of the variation of Slope. This study provides a simple calibration procedure for the users of TD-sensors with a linear correction model based on easily measurable vessel area and stem diameter. It may also provide a new development avenue for sensor manufacturers or others to improve the TD method to reduce its sensitivity to wood characteristics – or provide some “out-of-the-box” correction factors across a range of wood characteristics.
Characteristics of soil evaporation at two stages of growth in apple orchards with different ages in a semi-humid region
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Soil evaporation (E) accounts for a significant portion of the hydrological cycle especially under hot and dry conditions. The Loess Plateau of China contains the world’s largest area of apple orchards cultivated in a rather extensive way (720 trees ha⁻¹), while few studies were conducted to explore the characteristics of E at different stages of growth in apple orchards with different ages in water-limited regions. In this work, we measured E, sap flow, throughfall, stemflow and the leaf area index (LAI) in 7- and 17-year-old rainfed apple orchards in Changwu County on the Loess Plateau from May to September 2012–2015. We divided the stages of growth of the apple trees into stage I (rapid foliar growth) and stage II (rapid fruit enlargement and maturation) based on the seasonal variation of LAI and constructed empirical models for predicting E in each orchard at different growth stages. Tree age clearly affected the dynamics of daily E and the ratio of E to evapotranspiration (E:ET) for apple orchards. Daily E and E:ET during the four growing seasons were significantly higher for the 7-year-old orchard (1.3 $\pm$ 0.5 mm d⁻¹ and 55.1 $\pm$ 10.4%, respectively) than the 17-year-old orchard (1.1 $\pm$ 0.5 mm d⁻¹ and 44.1 $\pm$ 10.9%, respectively) (p < 0.05). Daily E and E:ET for both orchards also differed between the two growth stages and were significantly higher at growth stage I (p < 0.05) according with the higher solar radiation reaching the soil level. The stage of growth influenced the performance of empirical E models for apple orchards. Compared with measured daily E, simulated daily E was underestimated for the young orchard and overestimated for the old orchard at growth stage II. The relationships between daily E for each orchard and biotic/abiotic factors differed between the two growth stages. Daily E for both orchards was more likely to be affected by meteorological factors and soil temperature in the 0–20 cm layers at growth stage II. This study highlights the significant effects of tree age and growth stage on the characteristics of daily E and E:ET and the performance of empirical E models for apple orchards in a semi-humid region.
Increases in sap flow and storage can compensate for successively greater losses of conducting area in large trees
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Comparisons of three scaling up methods to estimate stand transpiration of a xerophytic shrub (Salix psammophila) in northern China
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Citation Excerpt :
In the S. psammophila stand, estimated daily mean stand T based on leaf area, cross-sectional area and number of branches was always higher than the actual value (p < 0.05), suggesting consistent overestimation. This result is not consistent with the general perception that SF measurements gave stand T values similar to those obtained by other T measurements (Granier, 1987; Yue et al., 2008; Merlin et al., 2020). In this study, the consistent overestimation between actual and estimated stand T based on different methods might be due to that SF measurements largely suffered from different potential sources of error related to intrinsic limitation of the SF methods and/or to how these methods are applied (Mitchell et al., 2009; Steppe et al., 2015; Vandegehuchte and Steppe, 2013).
Transpiration (T) is a key hydrological process, delivering water essential for plant metabolism and thus affecting productivity. A major challenge in estimating stand T is how to accurately scale sap flow data from individual trees to the stand. In shrub ecosystems, various scaling up methods have been used to extrapolate tree-level sap flow measurements to stand-level T, these include leaf area, cross-sectional area and number of branches. However, the performances of different scaling up methods have not been fully explored for shrubs. In this study, we measured sap flow of a xerophytic shrub (Salix psammophila) and scaled up using measures of leaf area, cross-sectional area and numbers of branches in order to estimate stand T during the rainy seasons in 2019 and 2020 on the northern Loess Plateau, China. In addition, we measured precipitation, throughfall, stemflow, soil evaporation, surface runoff and 0–200 cm soil water content to calculate actual stand T on the basis of soil water balance method. The results revealed that daily stand T differed according to the scaling up methods used for the estimation. Daily estimated stand T based on measures of leaf area (0.1–13.1 mm d⁻¹) was consistently higher than those based on cross-sectional area (0.2–8.6 mm d⁻¹) and number of branches (0.4–8.9 mm d⁻¹) (p < 0.05). During the two rainy seasons, the actual daily mean stand T (2.5 $\pm$ 1.6 mm d⁻¹) was significantly lower than the estimation by the three scaling up methods (p < 0.05). The method based on cross-sectional area appeared to be most suitable for scaling up because it had the lowest root mean square error and bias values (0.939 mm d⁻¹ and 0.633 mm d⁻¹, respectively). This study highlights the wide variations of stand T upon which scaling up method was chosen, and these differences need to be considered when converting tree-level sap flow to stand-level T in shrub and other ecosystems.

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Quantification of uncertainties introduced by data-processing procedures of sap flow measurements using the cut-tree method on a large mature tree

Highlights

Abstract

Introduction

Section snippets

Study site and climate data

Sap flow response to the cutting

Discussion

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

Agric. For. Meteorol.

Agric. For. Meteorol.

Agric. For. Meteorol.

SoftwareX

Agric. For. Meteorol.

Agric. For. Meteorol.

South African J. Bot.

Tree water balance drives temperate forest responses to drought

Ecology

Predictive models for radial sap flux variation in coniferous, diffuse-porous and ring-porous temperate trees

Tree Physiol.

A validation, comparison and error analysis of two heat-pulse methods for measuring sap flow in Eucalyptus marginata saplings

Funct. Plant Biol.

Forests and climate change: forcings, feedbacks, and the climate benefits of forests

Science

An improved heat pulse method to measure low and reverse rates of sap flow in woody plants

Tree Physiol.

SFM1 Sap Flow Meter Manual

Sap flow measurements with some thermodynamic methods, flow integration within trees and scaling up from sample trees to entire forest stands

Trees

Seasonal changes of azimuthal, radial, and tree-to-tree variations in sap flux affect stand transpiration estimates in a Cryptomeria japonica forest, central Taiwan

J. For. Res

Variations in the radial gradient of sap velocity in trunks of forest and fruit trees

Plant Soil

Quantifying water requirements of riparian river red gum (Eucalyptus camaldulensis) in the Murray-Darling basin, Australia - implications for the management of environmental flows

Ecohydrology

How reliable are heat pulse velocity methods for estimating tree transpiration?

Forests

Evaluation of transpiration in a Douglas-Fir stand by means of sap flow measurements

Tree Physiol.

Tree-, stand- and site-specific controls on landscape-scale patterns of transpiration

Hydrol. Earth Syst. Sci.

Integration of sapflow velocity to estimate plant water use

Tree Physiol.

Scaling from single-point sap velocity measurements to stand transpiration in a multispecies deciduous forest: uncertainty sources, stand structure effect, and future scenarios

Can. J. For. Res.