Sap Flow Measurements Research Articles

Estimating Canopy-Scale Evapotranspiration from Localized Sap Flow Measurements

The results reported in this work are based in part on measurements of sap flow in a few select trees on a representative riparian forest plot coupled with a forest-wide randomized sampling of tree sapwood area in a watershed located along the Pacific coast in Santa Cruz County, California. These measurements were upscaled to estimate evapotranspiration (ET) across the forest and to quantify groundwater usage by dominant phreatophyte vegetation. Canopy cover in the study area is dominated by red alder (Alnus rubra) and arroyo willow (Salix lasiolepis), deciduous phreatophyte trees from which a small sample was selected for instrumentation with sap flow sensors on a single forest plot. These localized sap flow measurements were then upscaled to the entire riparian forest to estimate forest ET using data from a survey of sapwood area on six plots scattered randomly across the entire forest. The estimated canopy-scale ET was compared to reference ET and NDVI based estimates. The results show positive correlation between sap flow based estimates and those of the other two methods, though over the winter months, sap flow-based ET values were found to significantly underestimate ET as predicted by the other two methods. The results illustrate the importance of ground-based measurements of sap flow for calibrating satellite based methods and for providing site-specific estimates and to better characterize the ET forcing in groundwater flow models.

Soil Moisture or ET-Based Smart Irrigation Scheduling: A Comparison for Sweet Corn with Sap Flow Measurements

Smart irrigation is one application of digital agriculture that can be used to achieve improved crop yields while saving water and energy. Many variations of smart irrigation can be applied, but there is no consensus among the scientific community as to which method is the best practice. The objective of this research was to evaluate two smart irrigation methods: a soil moisture–based method using sensors and an evapotranspiration- (ET-) based technique. The selected crop was sweet corn. Growth variables (biomass, yield, harvest index, and water productivity) were compared for each of the two methods. Three irrigation regimes were applied for each method: 60%, 90%, and 120% of crop evapotranspiration (ETc) for the ET-based method, and 25%, 30%, and 35% of the total soil moisture for the soil moisture–based method. Corn sap flow was measured in response to the three treatments in the soil moisture–based experiment to measure transpiration. The results showed that the ET-based method is easier to implement with less infrastructure, and it can result in similar yields compared to the soil moisture–based method. Although the fresh yield was 16% higher using the soil moisture–based method, grain yield can be sustained with the ET-based method using 8% less water. Selection of an appropriate irrigation scheduling method should be based on marketable yield: keeping the soil water content near field capacity will result in a higher fresh yield but will not translate into more dry matter. If grain yield is the target, the ET-based method is less expensive and more farmer friendly, but care should be taken to properly estimate irrigation losses to avoid underirrigation. An evaporative stress index (determined using sap flow measurements) of 70% will cause a 22% reduction in fresh sweet corn yield but only a 7% reduction in grain yield. Transpiration peaked when the soil moisture was above 85% of the available water (i.e., at 38% in the calcareous clay soil of the experiment).

Estimating oil palm water usage in peat soils using sap flow technique

Massive issues on significant water used by oil palm trees suggest further estimation of water exploited by oil palm trees. This research employed sap flow technique using thermal dissipation probes (TDP), compared to Penman-Monteith equation as the benchmark. Water utilization by plants was studied through evapotranspiration, to investigate their water footprint and water productivity. Field study was carried out in a 17 year old oil palm plantation in Siak, Riau, Indonesia. Sap flow measurement was carried out on the 17th frond over sapric peat soil during 7th January - 18th March 2019. Water used from 19th oil palm was 40.97±6.63 L day−1 equals to 0.51±0.083 mm day−1 (mean ±SD). While, estimation based on Penman-Monteith equation was 286.3 ± 64.078 L day−1 equals to 3.58±0.80 mm day−1 (mean ±SD). Total accumulated water from Penman-Monteith equation (286.3 L day−1) was considerably higher than the one from sap flow measurement (41 L day−1). The result showed that water footprint from 17 year old oil palm tree based on sap flow and Penman-Monteith equation was 0.204 and 1.43 m3 kg−1 fresh fruit bunches respectively. While water productivity estimated from sap flow was 0.70 kg m−3 while Penman-Monteith equation achieved 4.89 kg m−3.

Comparing sap flow calculations from Heat Field Deformation (HFD) and Linear Heat Balance (LHB) methods

Heat Field Deformation (HFD) is a widely used method to measure sap flow of trees based on empirical relationships between heat transfer within tree stems and the sap flow rates. As an alternative, the Linear Heat Balance (LHB) method implements the same instrumental configuration as HFD but calculates the sap flow rates using analytical equations that are derived from fundamental conduction-convection heat transfer theories. In this study, we systematically compared the sap flow calculated using the two methods based on data that were recorded using the same instrument. The measurements were conducted on four Norway spruce trees. We aimed to evaluate the discrepancies between the sap flow estimates from the two methods and determine the underlying causes. Diurnal and day-to-day patterns were consistent between the sap flow estimates from the two methods. However, the magnitudes of the estimated sap flow were different between them, where LHB resulted in much lower estimates in three trees and slightly higher estimates in one compared to HFD. We also observed larger discrepancies in negative (reversed flow) than in positive sap flow, where the LHB resulted in lower reversed flow than HFD. Consequently, the seasonal budget estimated by LHB can be as low as ∼20% of that estimated by HFD. The discrepancies can be mainly attributed to the low wood thermal conductivities for the studied trees that lead to substantial underestimations using the LHB method. In addition, the sap flow estimates were very sensitive to the value changes of the empirical parameters in the calculations and, thus, using a proper case-specific value is recommended, especially for the LHB method. Overall, we suggest that, despite the strong theoretical support, the correctness of LHB outputs depends largely on the tree individuals and should be carefully evaluated.

Water Use of Hybrid Poplar (Populus deltoides Bart. ex Marsh timesP. nigra L. “AF2”) Growing Across Contrasting Site and Groundwater Conditions in Western Slovakia

The water use by short rotation coppices (SRC) has been a focus of ongoing research in the last decades. Nevertheless, investigations that consider site factors and present long-term monitoring of the components of the water balance are rare. This research quantified the tree-based transpiration in the 4th growing season of uncoppiced 1st rotational hybrid poplar stands (Populus deltoides Bart. ex Marsh × P. nigra L. “AF2”) in western Slovakia. The aim of the study was to determine the influence of meteorological and soil-related site conditions on transpiration rates. Three experimental plots were located in the Morava River floodplains, on loamy sand-textured soils with different groundwater accessibilities: higher, low, and fluctuating groundwater level. We measured sap flow (Heat Ratio Method), volumetric water content, matric potential, groundwater level, and meteorological variables throughout the growing season in 2019. The results indicated that transpiration in the three sites was almost constant during that period, which was characterized by distinct conditions. The average cumulative transpiration at the site with a higher groundwater level (1105 mm) was larger than at the site with a lower groundwater level (632 mm) and the site with fluctuating groundwater (863 mm). A principal component analysis (PCA) and correlation analysis identified that the contribution of meteorological and soil-related site variables to transpiration differed among the sites. Soil water availability and groundwater accessibility are critical variables for the water use of poplar SRC. We concluded that the combination of site conditions needs to be reconsidered for the expansion of sustainable short rotation plantations in Europe.

Are calibrations of sap flow measurements based on thermal dissipation needed for each sample in Japanese cedar and cypress trees?

Are calibrations of sap flow measurements based on thermal dissipation needed for each sample in Japanese cedar and cypress trees?

Sap velocity, transpiration and water use efficiency of drip-irrigated cotton in response to chemical topping and row spacing

Directly measuring plant transpiration of field crops and determining water use efficiency are difficult but essential to understand plant-water relations. In this study, we aimed to quantify plant transpiration and water use efficiency at diurnal and daily bases using sap flow measurements in cotton growing under plastic film cover and drip irrigation in relation to row configurations and chemical topping. Field experiment was carried out in 2020–2021 in Xinjiang, China. The experiment included two topping treatments: chemical topping using heavy amount of mepiquat chloride and traditional manual topping; and two typical row spacing for machine-harvesting: equal row spacing (76 cm) and narrow-wide row spacing (10 cm + 66 cm). Sap flow was measured using a heat ratio method after cotton first flowering stage and then calculated to transpiration per plant and per unit ground area. Chemical topping increased cotton plant height by 12%, leaf area index by 13%, and stem diameter by 9% but did not affect cotton lint yield compared with manual topping across two years and row configurations. The sap velocity of drip-irrigated cotton ranged overall from 20 to 45 cm hr−1 at the daytime and close to zero at nighttime. Across two years, the daily transpiration in chemical topping after flowering was 5.57 mm d−1 and 14.8% higher than in manual topping. That in narrow-wide row spacing was higher than in equal rows. However, the water use efficiency did not differ between topping and row spacing treatments, being 5.64 kg m−3 on average for aboveground dry matter. This knowledge would be useful to optimize cotton irrigation managements and to improve crop models by knowing exact plant transpiration at both plant and system levels.

Effects of variable retention harvesting on canopy transpiration in a red pine plantation forest

BackgroundVariable Retention Harvesting (VRH) is a forest management practice applied to enhance forest growth, improve biodiversity, preserve ecosystem function and provide economic revenue from harvested timber. There are many different forms and compositions in which VRH is applied in forest ecosystems. In this study, the impacts of four different VRH treatments on transpiration were evaluated in an 83-year-old red pine (Pinus Pinus resinosa) plantation forest in the Great Lakes region in Canada. These VRH treatments included 55% aggregated crown retention (55A), 55% dispersed crown retention (55D), 33% aggregated crown retention (33A), 33% dispersed crown retention (33D) and unharvested control (CN) plot. These VRH treatments were implemented in 1-ha plots in the winter of 2014, while sap flow measurements were conducted from 2018 to 2020.ResultsStudy results showed that tree-level transpiration was highest among trees in the 55D treatment, followed by 33D, 55A, 33A and CN plots. We found that photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) were major controls or drivers of transpiration in all VRH treatments. Our study suggests that dispersed or distributed retention of 55% basal area (55D) is the ideal forest management technique to enhance transpiration and forest growth.ConclusionsThis study will help researchers, forest managers and decision-makers to improve their understanding of water cycling in forest ecosystem and adopt the best forest management regimes to enhance forest growth, health and resiliency to climate change.

Estimating Bulk Stomatal Conductance in Grapevine Canopies

In response to changes in their environments, grapevines regulate transpiration using various physiological mechanisms that alter conductance of water through the soil-plant-atmosphere continuum. Expressed as bulk stomatal conductance at the canopy scale, it varies diurnally in response to changes in vapor pressure deficit and net radiation, and over the season to changes in soil water deficits and hydraulic conductivity of both the soil and plant. To help with future characterization of this dynamic response, a simplified method is presented for determining bulk stomatal conductance based on the crop canopy energy flux model by Shuttleworth and Wallace using measurements of individual vine sap flow, temperature and humidity within the vine canopy, and estimates of net radiation absorbed by the vine canopy. The methodology presented respects the energy flux dynamics of vineyards with open canopies, while avoiding problematic measurements of soil heat flux and boundary layer conductance needed by other methods, which might otherwise interfere with ongoing vineyard management practices. Based on this method and measurements taken on several vines in a non-irrigated vineyard in Bordeaux France, bulk stomatal conductance was estimated on 15-minute intervals from July to mid-September 2020 producing values similar to those presented for vineyards in the literature. Time-series plots of this conductance show significant diurnal variation and seasonal decreases in conductance associated with increased vine water stress as measured by predawn leaf water potential. Global sensitivity analysis using non-parametric regression found transpiration flux and vapor pressure deficit to be the most important input variables to the calculation of bulk stomatal conductance, with absorbed net radiation and bulk boundary layer conductance being much less important. Conversely, bulk stomatal conductance was one of the most important inputs when calculating vine transpiration, emphasizing the usefulness of characterizing its dynamic response for the purpose of estimating vine canopy transpiration in water use models.

Comparisons of three scaling up methods to estimate stand transpiration of a xerophytic shrub (Salix psammophila) in northern China

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 ± 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.

The effect of soil type, fruit load and shaded area on ‘Hass’ avocado (Persea americana Mill.) water use and crop coefficients

‘Hass’ avocado tree water use was quantified within orchards located in the three main avocado growing regions of New Zealand, from 2016 to 2019. The three regions, the Bay of Plenty, the Whangarei District and the Aupouri Peninsula in the Far North District, differ in climate and soil type (allophanic, clay, and sandy soils, respectively). At each site, local meteorological conditions were monitored, avocado tree water use was quantified using heat-pulse sap flow measurement, total leaf area was obtained periodically, fruit load was recorded at harvest, and shaded area was measured in mid-summer. Reference evapotranspiration (ETo) calculated by the FAO-56 Penman-Montieth equation was highest in January at 3.8 mm (Whangarei) to 4.3 mm (Bay of Plenty) and lowest in June at 0.7 mm (Bay of Plenty) to 0.8 mm (Whangarei and Far North). The three sites had similar patterns of tree water use relative to ETo, regardless of the differences in soil type, and tree water use was highest in January at 2.7 mm d−1 and lowest in June at 1.2–1.4 mm d−1. Across the three sites, individual tree water use was related to variation in tree spacing and leaf area per tree, but on an orchard area basis was influenced more by fruit load, resulting in variation in estimated crop coefficients (Kc) among individual trees and seasons at all sites. Average monthly Kc was similar across all sites with a value of 0.60–0.65 in mid-summer, as all three orchards were considered mature with fully grown canopies and > 75% orchard shaded area. However, tree water use also increased from light to heavy fruit load. Therefore, when irrigation is applied to avocado, Kc values should be adjusted for variation in fruit load, with a range from 0.50 to 0.85 in mid-summer.

Negligible Response of Transpiration to Late-Summer Nitrogen Fertilization in Japanese Oak (Quercus crispula)

Increased atmospheric nitrogen (N) deposition, caused by anthropogenic activities, has various effects on forest ecosystems. Some reports have investigated the responses in tree transpiration to N addition, but few studies have measured the short-term response of mature tree transpiration to N fertilization. This study aimed to clarify the short-term transpiration response in 27-year-old deciduous hardwood trees to an increase in N availability. We established two plot types (control and N-fertilized plots) in Quercus crispula plantation stands in Hokkaido, Northern Japan. We measured sap flow density (SFD; cm3 m−2 s−1) using a thermal dissipation method for three months during the growing season. In the N-fertilized plot, we added 50 kg N ha−1 yr−1 of ammonium nitrate (NH4NO3) to the forest floor in the middle of the measurement periods. For daily mean SFD, we did not find a significant difference between the control and the N-fertilized plots. Leaf N contents did not differ between treatments, implying a negligible difference in physiological responses and transpiration rates. The slight difference between treatments could be because the trees had already foliated before applying the N fertilizer to our deciduous hardwood trees. The present results indicate that the potential increase in N deposition during the growing season does not immediately alter tree transpiration.

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.

Sap flow monitoring of two Australian native tree species in a suburban setting: Implications for tree selection and management

Sap flow, the transport of fluid in the water-conducting xylem tissues of plants, is commonly measured in studies of plant-water relationships by the heat pulse velocity method. Publications have been rare of long-term sap flow measurements for individual trees in a suburban environment. Plant-water relations in urban settings are essential for promoting urban greening where there is a perceived danger to infrastructure and buildings from planting trees in streets on clay sites. The function of residential houses built on reactive clays can be significantly impaired and walls of buildings cracked if considerable amounts of water are extracted from the soil by the root system of a tree or a group of trees in close proximity, leading to localised soil shrinkage settlement. This part of the wider study aimed to monitor sap flow of eight individual Australian native trees from two species using the heat ratio method (HRM) in the field over 12 months. The analysis of monthly sap flow volume showed a similar pattern for all monitored trees, although daily water demand varied substantially. Methods for estimating tree leaf surface area, crown shape and crown volume were investigated and the equation for calculating thermal diffusivity (k) and sap flow velocity on the basis of the HRM was reviewed. It has been proposed that k may vary substantially depending on how thermal conductivity (K) is estimated, which could lead to significant discrepancies for estimations of plant transpiration. Two K models (KHog and KVan) were investigated and it was found that the impact on mean daily sap volume was negligible for the trees in this study.

Swiss stone pine growth benefits less from recent warming than European larch at a dry-inner alpine forest line as it reacts more sensitive to humidity

High elevation mountain forests in the European Alps are strongly affected by climate change. In this study we aimed to investigate the long- and short-term effects of climate on radial stem growth and tree hydraulics of the two dominant tree species at the forest line of the Eastern central Alps, European larch (Larix decidua Mill. and Swiss stone pine (Pinus cembra L). To this end, we analyzed tree ring widths from stem cores from five sites between 1990 and 2320 m above sea level (a.s.l.) and measured sap flow for three years and radial stem variation for four years at three sites between 1990 and 2100 m a.s.l. in an inner alpine dry valley in Northern Italy. We found that tree ring width of larch responded stronger to temperature and increased more with warming in the last decades than in Swiss stone pine. In the short term, low soil moisture reduced sap flow during the summer in Swiss stone pine but not in larch. Additionally, air vapor pressure deficit clearly reduced the number of days with radial stem growth in Swiss stone pine, leading to lower annual growth than in larch. Consequently, larch at high elevation might actually benefit from climate change at the expense of Swiss stone pine.

Measuring and modelling nitrate fluxes in a mature commercial apple orchard

Leaching of nitrogen from intensive agriculture to groundwater and waterways is a source of environmental harm. Nitrate leaching beneath a commercial 10-year-old ‘Galaxy’ apple orchard was investigated using a combined measurement and modelling approach. Measurement of sapflow, climate, soil moisture, deep drainage and nitrate leaching beneath the root zone were monitored for 30 months in order to parameterise and calibrate the Soil Plant Atmosphere System Model (SPASMO). Sap flow peaked at around 15 L tree−1 day−1 in late January with average sap flow for the 2013/14 season of 8.58, 6.39 and 6.47 L tree−1 day−1 for the high (4.0 L ha−1), medium (2.3 L ha−1) and low (1.6 L ha−1) irrigation treatments, respectively. Deep drainage and nitrate leaching were highly variable and did not appear to be influenced by irrigation rate. Over the 30-month monitoring period, drainage beneath the rootzone averaged 89 mm yr−1, whilst nitrate leaching averaged 33.2 kg-NO3 ha−1 yr−1, equivalent to 55% of the applied fertilizer. Average nitrate concentration in the leachate exceeded the World Health Organisation threshold for drinking water of 50 mg L−1 in 41 out of 279 samples. Long-term modelling indicated the average amount of drainage and nitrate leaching were 136 mm yr−1 and 11.4 kg-NO3 ha−1 yr−1, respectively. Scenario analysis indicated that few options existed to reduce nitrate leaching at the site through irrigation and fertigation management.

Use of gravimetric measurements to calibrate thermal dissipation probes with stem segments

AbstractTree transpiration is an important part of the global water cycle, and it is frequently represented by sap flow measurements at the individual plant scale. The thermal dissipation probes (TDPs) are commonly employed to measure and calculate sap flows, for which Granier equation is generally used. However, the original Granier equation has been found to underestimate sap flow rates. Calibration methods using different materials (i.e., whole trees or stem segments) have been proposed to recalibrate TDPs; however, these methods are often either expensive or complicated. This study proposes a new device that uses the gravimetric measurement method (GMM) to recalibrate TDPs. The GMM is relatively simple and easy to conduct. The proposed device uses a tube and siphon to maintain a constant pressure; an electronic balance is used to measure the actual sap flow. Four plant species with different stem porosity types, Pyrus pyrifolia (diffuse‐porous), Quercus fabri (ring‐porous), Platycarya strobilacea (ring‐porous) and Kalopanax septemlobus (ring‐porous), were selected to examine the performance of the GMM. The results showed that: (1) good correlations were obtained between the actual sap flow (from gravimetric measurements) and sap flow estimates obtained using the original TDP technique; (2) the results of the GMM‐recalibrated TDP measurement equation greatly differed from those of the original Granier calibration equation; and (3) the recalibration equation derived from the GMM outperformed the original Granier equation. This study suggests that the developed GMM can be used to recalibrate TDPs, and that using this method can help to improve sap flow measurements, thereby improving estimates of tree water consumption and transpiration.

Quantifying Stomatal Canopy Conductance in a Pine Forest During Drought from Combined Sap Flow and Canopy Surface Temperature Measurements

Quantifying Stomatal Canopy Conductance in a Pine Forest During Drought from Combined Sap Flow and Canopy Surface Temperature Measurements

Water Sap flow and Soil Moisture Measurements for an Automatic Irrigation Control System

A sound water management requires accurate field data inquiry that can eventually allow for a proper control of the field’s irrigation. This work pro-poses an automatic irrigation control system that aims at optimizing the quantity of water needed for plants’ growth. The system consists of three key blocks: first, the continuous inquiry of the physical characteristics of the plant; second, the control unit to decide upon the actions to launch by the actuation unit; and third the actuation unit that sets parameters for pumps and valves. Conventional smart irrigation control systems use soil moister sensors which neglect the plant’s physiology as a monitoring factor, so this paper suggests the use of a sap flow sensor that accurately evaluates the plant’s condition; hence, decide upon the proper water distribution, illustrated with preliminary experimental results.

A Temperature Ratio Heat Pulse (Trhp) Method for Measuring Sap Flow

A Temperature Ratio Heat Pulse (Trhp) Method for Measuring Sap Flow