Sap Flow Measurements Research Articles

Multi-species test and calibration of an improved transient thermal dissipation system of sap flow measurement with a single probe.

Transient thermal dissipation (TTD) systems provide a simple way to measure xylem sap flow with dual or single Granier-type probe, which gives lower energy consumption and higher accuracy due to its lessor sensitivity to thermal interferences. A new system, TTD heat within 5 min (TTD5), proposed on the reduction of the heating duration. This evolution captured interest through decreased energy consumption and increased temporal resolution. Within our study, the first objective was to test and calibrate this new system with a single probe for young rubber tree - Hevea brasiliensis. The second objective was to explore the sources of variability in calibration such as species, individual cut-stems and probe-wood contact. The complementary species consisted of two diffuse-porous species (mango tree - Mangifera indica, eucalyptus tree - Eucalyptus camaldulensis) and one ring-porous species (teak tree - Tectonia grandis). Twenty-eight response curves were assessed over a large range of flux densities from 0.5 to 10 l dm-2 h-1. The incremental rise of temperature from 30 to 300 s (T300-30) after commencement of heating was sensitive to flux density over the complete range. Compared with the full signal at 300 s, the incremental signal markedly reduced the variability between response curves within species and between species. Moreover, a new index K2, defined as (T0 - Tu)/T0, normalized the responses between 0 and 1. However, the responses had a non-linear trend above 5 l dm-2 h-1. Within diffuse-porous wood type, the species did not differ in calibration, whereas the ring-porous species was markedly different. A sigmoid function provided the best fit for the diffuse-porous species. Individual stems were identified as the main source of within-species variability in calibration. The normalizing K2 index removed the influence of probe-wood contacts, controlled through drilling difference; however, there was still an effect of individual stems interacting with flux density (P = 0.019). Replications of cut-stems and response curves are necessary to assess a reliable averaged calibration. In conclusion, the applicability of the TTD5 system with a single probe has been confirmed and several sources of variability in calibration have been evaluated.

Leaf water relations in Diospyros kaki during a mild water deficit exposure

The resistance mechanisms (stress avoidance and stress tolerance) developed by persimmon plants (Diospyros kaki L. f. grafted on Diospyros lotus L.) in response to mild water stress and the sensitivity of continuously (on a whole-day basis) and discretely (at predawn and midday) measured indicators of the plant water status were investigated in 3-year old ‘Rojo Brillante’ persimmon plants. Control (T0) plants were drip irrigated in order to maintain soil water content at levels slightly above soil field capacity (102.3% of soil field capacity) and T1 plants were drip irrigated for 33 days in order to maintain the soil water content at around 80% of soil field capacity. The results indicated persimmon plants confront a mild water stress situation by gradually developing stomata control (stress avoidance mechanism) and exhibiting some xeromorphic characteristic such as high leaf relative apoplastic water content, which could contribute to the retention of water at low leaf water potentials. In addition, sap flow measurements made by the heat-pulse technique were seen to be the most suitable method for estimating persimmon water status, because it provided the highest signal intensity (actual value/reference value):noise (coefficient of variation) ratio in almost all intervals of time considered and provides continuous and automated registers of the persimmon water status in real time.

Crop Coefficients and Transpiration of a Super Intensive Arbequina Olive Orchard using the Dual Kc Approach and the Kcb Computation with the Fraction of Ground Cover and Height

The SIMDualKc model was used to simulate crop water requirements for a super high density olive orchard in the region of Alentejo, Portugal. This model uses the dual crop coefficient approach to estimate and partitioning the actual crop evapotranspiration (ETc act) and therefore to perform the soil water balance. The model was calibrated with 2011 tree transpiration using trunk sap flow measurements and was validated using similar data from 2012 and tested with 2013 data. Low root mean square errors (RMSE < 0.53 mm·d−1) and acceptable modelling efficiency indicators (EF > 0.25) were obtained. Further validation was performed comparing modelled ETc act with eddy covariance measurements. These indicators support the appropriateness of using SIMDualKc to guide irrigation management. The basal crop coefficient (Kcb) curves obtained with SIMDualKc for those 3 years were compared with the Kcb values computed with the Allen and Pereira approach (A&P approach) where Kcb is estimated from the fraction of ground cover and plant height considering an adjustment factor for crop stomatal control (Fr). Fr values were obtained through a trial and error procedure through comparing the Kcb estimated with this approach and with SIMDualKc. The Kcb curves obtained by both methods resulted highly correlated, which indicates that the A&P approach may be used in the irrigation management practice to estimate crop water requirements. Results of performing the soil water balance with SIMDualKc have shown that soil evaporation is a large fraction of ETc act, varying between 41% and 45% for the 3 years under study. Irrigation, applied with a drip system, represented 39 to 56% of ETc act, which shows the great importance of irrigation to achieve the water requirements of super intensive olive orchards. Nevertheless, the analysis has shown that the irrigation management adopted at the orchard produces a water deficit larger than desirable, with a ratio of ETc act to non-stressed crop evapotranspiration (ETc) varying from 70% to 94% during the mid-season, when that ratio for a eustress irrigation management could be around 90%.

Drought Differentially Affects Growth, Transpiration, and Water Use Efficiency of Mixed and Monospecific Planted Forests

Drought conditions may have differential impacts on growth, transpiration, and water use efficiency (WUE) in mixed species and monospecific planted forests. Understanding the resistance (i.e., the capacity to maintain processes unchanged) of different tree species to drought, and how resistance is affected by complementary interactions within species mixtures, is particularly important in the seasonally dry tropics where projected increases in the frequency and severity of drought threaten tree planting efforts and water resources. Complementary interactions between species may lead to more resistant stands if complementarity leads to greater buffering capacity during drought. We examined growth, transpiration, and WUE of mixtures and monocultures of Terminalia amazonia (J.F. Gmel.) Exell and Dalbergia retusa Hemsl. before and during a prolonged drought using intensive measurements of tree sap flow and growth. Tree sapwood area growth was highest for T. amazonia in mixtures during normal (6.78 ± 4.08 mm2 yr−1) and drought (7.12 ± 4.85 mm2 yr−1) conditions compared to the other treatments. However, stand sapwood area growth was greatest for T. amazonia monocultures, followed by mixtures, and finally, D. retusa monocultures. There was a significant decrease in stand transpiration during drought for both mixtures and T. amazonia monocultures, while Dalbergia retusa monocultures were most water use efficient at both the tree and stand level. Treatments showed different levels of resistance to drought, with D. retusa monocultures being the most resistant, with non-significant changes of growth and transpiration before and during drought. Combining species with complementary traits and avoiding combinations where one species dominates the other, may maximize complementary interactions and reduce competitive interactions, leading to greater resistance to drought conditions.

Incorporating a root water uptake model based on the hydraulic architecture approach in terrestrial systems simulations

A detailed representation of plant hydraulic traits and stomatal closure in land surface models (LSMs) is a prerequisite for improved predictions of ecosystem drought response. This work presents the integration of a macroscopic root water uptake (RWU) model based on the hydraulic architecture approach in the LSM of the Terrestrial Systems Modeling Platform. The novel RWU approach is based on three parameters derived from first principles that describe the root system equivalent conductance, the compensatory RWU conductance, and the leaf water potential at stomatal closure, which defines the water stress condition for the plants. The developed RWU model intrinsically accounts for changes in the root density as well as for the simulation of the hydraulic lift process. The standard and the new RWU approach are compared by performing point-scale simulations for cropland over a sheltered minirhizotron facility in Selhausen, Germany, and validated against transpiration fluxes estimated from sap flow and soil water content measurements at different depths. Numerical sensitivity experiments are carried out using different soil textures and root distributions in order to evaluate the interplay between soil hydrodynamics and plant characteristics, and the impact of assuming time-constant plant physiological properties. Results show a good agreement between simulated and observed transpiration fluxes for both RWU models, with a more distinct response under water stress conditions and with uncertainty in the soil parameterization prevailing to the differences due to changes in the model formulation. The hydraulic RWU model exhibits also a lower sensitivity to the root distributions when simulating the onset of the water stress period. Finally, an analysis of variability across the soil and root scenarios indicates that differences in soil water content are mainly influenced by the root distribution, while the transpiration flux in both RWU models is additionally determined by the soil characteristics.

Seasonality of the Transpiration Fraction and Its Controls Across Typical Ecosystems Within the Heihe River Basin

AbstractUnderstanding the seasonality of the transpiration fraction (T/ET) of total terrestrial evapotranspiration (ET) is vital for coupling ecological and hydrological systems and quantifying the heterogeneity among various ecosystems. In this study, a two‐source model was used to estimate T/ET in five ecosystems over the Heihe River Basin. In situ measurements of daily energy flux, sap flow, and surface soil temperature were compared with model outputs for 2014 and 2015. Agreement between model predictions and observations demonstrates good performance in capturing the ecosystem seasonality of T/ET. In addition, sensitivity analysis indicated that the model is insensitive to errors in measured input variables and parameters. T/ET among the five sites showed only slight interannual fluctuations while exhibited significant seasonality. All the ecosystems presented a single‐peak trend, reaching the maximum value in July and fluctuating day to day. During the growing season, average T/ET was the highest for the cropland ecosystem (0.80 ± 0.13), followed by the alpine meadow ecosystem (0.79 ± 0.12), the desert riparian forest Populus euphratica (0.67 ± 0.07), the Tamarix ramosissima Ledeb desert riparian shrub ecosystem (0.67 ± 0.06), and the alpine swamp meadow (0.55 ± 0.23). Leaf area index exerted a first‐order control on T/ET and showed divergence among the five ecosystems because of different vegetation dynamics and environmental conditions (e.g., water availability or vapor pressure deficits). This study quantified transpiration fraction across diverse ecosystems within the same water basin and emphasized the biotic controls on the seasonality of the transpiration fraction.

Contrasting stomatal sensitivity to temperature and soil drought in mature alpine conifers.

Conifers growing at high elevations need to optimize their stomatal conductance (gs ) for maximizing photosynthetic yield while minimizing water loss under less favourable thermal conditions. Yet the ability of high-elevation conifers to adjust their gs sensitivity to environmental drivers remains largely unexplored. We used 4years of sap flow measurements to elucidate intraspecific and interspecific variability of gs in Larix decidua Mill. and Picea abies (L.) Karst along an elevational gradient and contrasting soil moisture conditions. Site- and species-specific gs response to main environmental drivers were examined, including vapour pressure deficit, air temperature, solar irradiance, and soil water potential. Our results indicate that maximum gs of L.decidua is >2 times higher, shows a more plastic response to temperature, and down-regulates gs stronger during atmospheric drought compared to P.abies. These differences allow L.decidua to exert more efficient water use, adjust to site-specific thermal conditions, and reduce water loss during drought episodes. The stronger plasticity of gs sensitivity to temperature and higher conductance of L.decidua compared to P.abies provide new insights into species-specific water use strategies, which affect species' performance and should be considered when predicting terrestrial water dynamics under future climatic change.

The role of decoupling factor on sugarcane crop water use under tropical conditions

AbstractThe expansion of sugarcane crop to regions with lower water supply in Brazil has increased the importance of correct estimation of crop water requirements. Currently, the irrigation management is generally done using the crop coefficient (Kc) based on the FAO 56 bulletin. Kc is used to determine the potential water demand of the crop for a given period of time and is considered constant for each crop stage. However, some recent studies have shown that Kc can be significantly variable under different evapotranspiration (ETo) rates. This paper aimed to analyse sugarcane water consumption at different scales: plant (sap flow measurements by energy balance method); canopy (Bowen ratio energy balance method); and plant–atmosphere coupling (infrared gas analyser) to reduce the uncertainties on the irrigation practices. Measurements were taken at two experimental sites, where a modern Brazilian cultivar CTC 12 was grown under drip irrigation and an old main Brazilian cultivar (RB867515) was grown under sprinkler irrigation by a central pivot. The mean crop evapotranspiration (ETc) values by the Bowen ratio energy balance method were 2.92 and 3.68 mm d−1 for RB867515 and CTC 12, respectively, resulting in a mean Kc of 0.99 at the full vegetative growth stage. Kc values were dependent on ETo and varied between 0.2 and 1.7 for both cultivars. This occurred in a crop coupled to the atmosphere (Ω = 0.37) and was the same found in other coupled crops such as coffee and citrus. In conclusion, the sugarcane Kc for southeast Brazil presented temporal variability due to coupling conditions according to reference evapotranspiration, and this should be considered in irrigation management.

The Dual Method Approach (DMA) Resolves Measurement Range Limitations of Heat Pulse Velocity Sap Flow Sensors

Sap flow, the movement of fluid in the xylem of plants, is commonly measured with the heat pulse velocity (Vh) family of methods. The observable range of Vh in plants is ~−10 to ~+270 cm/h. However, most Vh methods only measure a limited portion of this range, which restricts their utility. Previous research attempted to extend the range of Vh methods, yet these approaches were analytically intensive or impractical to implement. The Dual Method Approach (DMA), which is derived from the optimal measurement ranges of two Vh methods, the Tmax and the heat ratio method (HRM), also known as the “slow rates of flow” method (SRFM), is proposed to measure the full range of sap flow observable in plants. The DMA adopts an algorithm to dynamically choose the optimal Vh measurement via the Tmax or HRM/SRFM. The DMA was tested by measuring sap flux density (Js) on Tecoma capensis (Thunb.) Lindl., stems and comparing the results against Js measured gravimetrically. The DMA successfully measured the entire range of Vh observed in the experiment from 0.020 to 168.578 cm/h, whereas the HRM/SRFM range was between 0.020 and 45.063 cm/h, and the Tmax range was between 2.049 cm/h and 168.578 cm/h. A linear regression of DMA Js against gravimetric Js found an R2 of 0.918 and error of 1.2%, whereas the HRM had an R2 of 0.458 and an error of 49.1%, and the Tmax had an R2 of 0.826 and an error of 0.5%. Different methods to calculate sapwood thermal diffusivity (k) were also compared with the kVand method showing better accuracy. This study demonstrates that the DMA can measure the entire range of Vh in plants and improve the accuracy of sap flow measurements.

Design and implementation of a low-cost device for measuring sap flow in woody species

Transpiration is a key variable in water balance. Sap flow measurement methods allow to estimate transpiration at plant level, but scalable at the stand, river basin or ecosystem level. This requires recording data continuously, for which there are commercial devices, but they are specialized and expensive. This document shows that it is possible to integrate a data capture, management and storage system, as a counterpart to a commercial system such as Campbell's CR-100, for measuring and recording environmental variables, with a notable reduction in costs. The methodology used to develop a device for the estimation of sap flow using the heat ratio method, the design of the Arduino-based electronic circuit and the software, with register 13-64-171 of the Dirección Nacional de Derechos de Autor, is presented.

Relations among Sap Flow, Soil Moisture, Weather, and Soybean Plant Parameters in High Water Demand and Final Growth Stages

The dynamics of sap flow in relation to plant morphology and weather conditions during reproductive growth of soybean (Glycine max. L. Merr.) influence decisions pertaining to efficient irrigation management and other inputs for high yields. Field studies began in 2017 at Marianna, Arkansas to measure moisture dynamics of soybeans during seed fill (R5 to R7) using heat balance stem flow gauges. Sap flow was highly correlated to solar radiation with maximum rates observed during beginning seed fill (R5). A solar radiation efficiency (SRE) value, calculated as hourly sap flow rate per Watt-hour of solar radiation (g/Wh2), is proposed. The SRE relates to crop water demand and hydraulic resistance of the soil-root-stem-leaf-pod-seed pathway. SRE values ranged from 0 - 1.2 g/Wh2. Soil moisture, growth stage, time of day, and weather conditions influenced the SRE, with higher values observed in the morning, late afternoon, and during R5 growth. Peak sap flows of 39 g/h at R5, 25 g/h at R6, and 3 g/h at R7 occurred. The ratio of measured sap flow to estimated crop evapotranspiration was 0.9 to 1.3 during R5 to R6.9 (maximum dry matter), but dropped to 0.2 at R7. Further research is needed to better understand late season reproductive moisture dynamics in soybeans.

Application and evaluation of Stanghellini model in the determination of crop evapotranspiration in a naturally ventilated greenhouse

Stanghellini model is one of the few models primarily developed to predict the evapotranspiration of crops (ETc) in naturally ventilated greenhouses. However, there are insufficient data on the model regarding its use, particularly in China where solar greenhouses without heating systems are fast spreading for vegetable growth and production. The application of Stanghellini model and the evaluation of its performance using meteorological and tomato plant data generated inside an unheated and naturally ventilated multi-span Venlo-type greenhouse is exploited in this study. Model capability was evaluated by utilizing data from sap flow measurements, meteorological and crop data. Measured meteorological data included solar radiation (Rs), air temperature (Ta), relative humidity (RH) and net radiation (Rn). Average leaf area index (LAI) values measured during the experimental period were 1.00, 3.30, 4.05 and 2.93; while determined crop coefficients (Kc) changed from 0.40, 0.62, 1.12 to 0.83 for the initial stage, development stage, mid-season stage and late-season stage, respectively. Results from the study indicated that the average hourly ETc values of tomato plants using sap flow measurements were 0.165 mm/h, 0.148 mm/h, 0.192 mm/h and 0.154 mm/h for the initial stage, development stage, mid-season stage and late-season stage, respectively. Meanwhile, the ETc values obtained from calculation using Stanghellini model were 0.158 mm/h, 0.152 mm/h, 0.202 mm/h and 0.162 mm/h for the initial stage, development stage, mid-season stage and late-season stage, respectively. These ETc values calculated by the Stanghellini model were close to the measured values within the same period. The coefficients of correlation (R2) based on hourly ETc for the calibration data was 0.94 and that of the validation dataset was 0.90. Scatter plots of the estimated and measured hourly ETc revealed that the R2 and the slope of the regression line for May, June and July were 0.94, 0.90, 0.96 and 1.15, 0.97, 1.10 respectively. These data were well represented around the 1:1 regression line. A model sensitivity analysis carried out illustrates how the changes in Rs and Ta affect greenhouse ETc. Stanghellini model was therefore proven to be suitable for ETc estimation with acceptable accuracy in unheated and naturally ventilated greenhouses in the Northeast region of China. Keywords: calibration, verification, crop evapotranspiration, naturally ventilated greenhouse, sap flow DOI: 10.25165/j.ijabe.20181106.3972 Citation: Acquah S J, Yan H F, Zhang C, Wang G Q, Zhao B S, Wu H M, et al. Application and evaluation of Stanghellini model in the determination of crop evapotranspiration in a naturally ventilated greenhouse. Int J Agric & Biol Eng, 2018; 11(6): 95–103.

Measurement of water-use by <i>Jatropha curcas</i> L. using the heat-pulse velocity technique

In response to the proposed introduction of the potential bio-diesel species Jatropha curcas (Linnaeus) to South Africa, field experiments were conducted to investigate its likely water-use impacts relative to other forms of vegetative land use. As no existing water-use data could be found for this species worldwide, sap flow in Jatropha curcas trees was measured continuously for a 17-month period at two sites in eastern South Africa. These consisted of young (4-year-old) trees at a relatively wet site and mature (12-year-old) trees at a dry site. The heat-ratio method of the heat-pulse technique was utilised, together with measurements of meteorological variables and soil water. Sap- flow rates varied according to tree age, season, prevailing meteorological conditions, and soil moisture levels. Peak sap-flow rates occurred during the warm wet summer months, but due to the deciduous nature of the species, water use was negligible during winter. Scaled-up sap-flow measurements resulted in estimates of total annual transpiration of 1 983 l (147 mm) for a 4-year-old J. curcas tree, and 4 884 l (362 mm) for a 12-year-old J. curcas tree. The study concluded that the J. curcas trees studied were conservative in their water use, and were unlikely to transpire more water than indigenous vegetation types of the area.

Partitioning evapotranspiration of a drip-irrigated wheat crop: Inter-comparing eddy covariance-, sap flow-, lysimeter- and FAO-based methods

A precise estimate of the evapotranspiration (ET) partitioning is fundamental for determining the crop water needs and optimizing irrigation management. The plant transpiration (T) is generally considered to be the most desirable component, while reducing the soil evaporation (E) could be one of the most important water-saving actions in semi-arid agricultural regions. Given the lack of reference method to estimate the E/T partitioning of wheat crop, this study inter-compares four different methods based on eddy covariance, sap flow and lysimetry measurements and FAO modeling. The objectives are: i) to quantify the systematic and random uncertainty in E and T observations, ii) to evaluate the partitioning ratio (T/ET) at the daily/field scale and iii) to assess the performance of the FAO model over two drip irrigated wheat fields. Results indicate that despite the small surface sensed by mini-lysimeters, the partitioning ratio is evaluated more precisely (19% relative error) with lysimetry than with the other systems (any combination of eddy covariance, lysimetry and sap flow measurements). Moreover, stem-scale T measurements from sap flow sensors are subject to representativeness issues at the field scale, and to systematic errors during water-stress and senescence periods. The lysimeter-derived partitioning ratio increases from about 0.50 to 0.85 during the growth stage and rapidly drops towards 0 during senescence. Its dynamics is found to be significantly correlated (R>0.7) with the 5-cm soil moisture. By comparing FAO simulations with observations, it is found that the FAO method overestimates T and underestimates E, while keeping satisfying ET estimates for drip irrigated wheat. This study suggests that different independent measurement techniques should be implemented to both quantify and reduce uncertainties in the T/ET ratio, and that accurate observations are still needed to improve the modeling of E/T components.

Water consumption of Populus alba trees in tree shelterbelt systems in Central Asia – a case study in the Chui Valley, South Eastern Kazakhstan

Agriculture in Central Asia largely relies on irrigation. The water is withdrawn from the rivers of the region, which predominantly originate from snowfields and glaciers. Due to the global warming these water resources are expected to decline substantially, resulting in an aggravation of the already existing water scarcity. Tree shelterbelt systems as the most prominent practice of agroforestry in Central Asia are reported to help to reduce water consumption in irrigated agriculture. Though, studies about water consumption of the shelterbelts are lacking. Therefore, the objectives of this study are to investigate (1) the water consumption of Populus alba trees in a shelterbelt system in Central Asia and (2) analyze the influence of the local climatic conditions on the sap flow of such a shelterbelt. P. alba is one of the most important shelterbelt trees in Central Asia. Tree water consumption was assessed through sap flow measurements on three trees in a crop shelterbelt system in the Chui Valley in SE Kazakhstan during June and July 2016. The average daily water consumption were 187.6 l/d, 44.8 l/d, and 160 l/d for the trees, respectively. These results were extrapolated for a representative shelterbelt section. Water consumption of that section was 7.8 mm/d, while average ETo was 5.3 mm/d. Considerable influences of water vapor saturation deficit, air temperature and relative humidity on the sap flow could be observed. Solar radiation played a role, too, whereas little or no influence for wind speed was found.

Water transport secrets of the dragon’s blood trees revealed through sap flow measurements following partial stem incision

Dragon’s blood trees have long been prized for their red resin, and are consequently subjected to traditional practices for its harvesting which involves inflicting numerous, deep stem wounds. Nevertheless, these iconic monocots have continued to survive such drastic treatments. Partial stem incision (PSI) treatment effectively imitates stem wounding, which when combined with sap flow measurements monitored in a close distance of few centimeters above PSI, may significantly improve our understanding of water transport efficiency after stem damage in monocots. As it is impossible to conduct these experiments on protected adult Dracaena species native to remote sites, we demonstrated this approach on young Dracaena draco L. and Dracaena cinnabari Balfour f. plants ex-situ. The goal of this study was to simulate the effects of serious stem wounding on water transport of these woody monocots by applying PSI under permanent sap flow monitoring by the heat field deformation (HFD) method using multi-point sensors. This allowed us to simultaneously monitor sap flow in both wounded and intact stem parts. It was observed that PSI applied in stems up to one-half of the total stem diameter did not reduce sap flow in D. species in any of the monitored stem part indicating efficient bypassing interrupted axial xylem transport by lateral water movement. The dense three dimensional network of interconnected vascular bundles and abundant ground tissue with large simple pits could play an important role in such high transport effectivity of Dracaena species. Synchronous sap flow and nuclear magnetic resonance measurements would be especially useful for the revealing of water flow complexity of Dracaena species in future.

Determining mid-day stem water potential from sap flow measurements

Trees tend to stabilize mid-day sap velocity through apparently coordinated active canopy conductance (Gc) responses to atmospheric evaporative demand. We recently reported a model describing this stabilization based on the linear relationship between canopy resistance (Rc) and vapor pressure deficit (VPD). The model was able to accurately reproduce the daily and seasonal courses of sap flow in nectarine trees for different leaf water potentials. Here, we studied inversion of the model to determine mid-day stem water potential, Ψmd, from sap flow measurements. The study was in a nectarine orchard in the hot Hula valley of northern Israel. Measurements included sap flux, weather variables, mid-day stem water potential and other parameters. Twelve trees were measured, where four were well-watered and irrigation was withheld from eight trees for eight weeks in the hot and dry summer. Ψmd declined from -1.05 to -2 MPa, and pre-dawn Ψ from -0.42 to -0.89 MPa. Similar to previous results, we found that the slope of the linear relationship between canopy resistance and VPD increased as Ψmd decreased in a monotonic manner, which was described by a linear relationship. Slopes of the relationship of canopy resistance to VPD varied linearly from 1.2 to 2.2 s mm-1 kPa-1 over the range of Ψmd values. Re-calculation of Ψmd using the relationship was very successful, giving an r2 of 0.83 between calculated and measured values.

Measurement of sap flow in young apple trees using the average gradient heat-pulse method

The average gradient heat-pulse method (CHMP-AG) has been used to monitor sap flow in very young (1-year-old) apple trees, from budburst through until leaf fall. Data are presented to illustrate how CHPM-AG can resolve the full range of sap flux densities in these small trees. The methodology has been implemented by making a very simple software change to existing CHPM instrumentation.

Assessment of olive transpiration derived from a remote sensing energy balance model compared with sap flow measurements

A remote sensing energy balance (R(SEB)) algorithm was used to estimate transpiration (TRSEB) from a high density and drip-irrigated olive (Olea europaea L. 'Arbequina') orchard, located in Pencahue Valley, Maule Region of Chile (35°23'S; 71°44'W). Meteorological variables over the orchard such as solar radiation, air temperature, relative humidity, wind speed and canopy temperature from zenith were recorded during the southern hemisphere summer, from January through February 2015. The RSEB model was evaluated using sap flow measurements obtained from compensation heat-pulse sensors. In 30-min intervals, the results showed that RSEB model estimated the olive transpiration with errors of 16%, presenting a root mean square error (RMSE) of 0.2 L h-1 and a mean absolute error (MAE) of 0.1 L h-1. On a daily basis, the RSEB model was able to predict the olive transpiration with RMSE and MAE values of 0.68 and 0.57 mm day-1, respectively. The overall analysis suggested that the model underestimates transpiration with errors of 10% on a daily basis.

Transpiration at leaf and tree level in a poplar short-rotation coppice culture: seasonal and genotypic differences

Poplar (Populus spp.) is one of the most commonly cultivated genera in experimental and commercial short-rotation coppice (SRC) plantations. The genus is among the fastest growing in temperate latitudes, but the success of highly productive poplar SRC plantations strongly depends on soil water availability. We examined the transpiration at the leaf and the individual tree levels of four different poplar genotypes under an SRC regime. Measurements were performed for the entire growing season of 2016, during the seventh growth year of the plantation and before the third coppice on four poplar genotypes (three individual multiple-stem trees per genotype) belonging to different species and from a different genetic background. The experimental site was a commercial scale multi-genotype SRC plantation, established in Flanders (Belgium). Measurements at the leaf level were performed on specific days of the growing season that differed in evaporative demand, temperature and incoming radiation. To determine the transpiration at the stem level, we measured single-stem sap flow using the stem heat balance (SHB) method and daily stem diameter variation measurements. The whole-tree transpiration was estimated by summing the sap flow rates from all stems. Measurements at the stem level were continuously monitored during the entire growing season. Sap flow was tightly connected to the phenological stage of the trees, thus onset of spring (leaf area development) and late autumn (leaf fall) were easily identifiable from sap flow measurements, showing differences among the four genotypes. The dynamics of transpiration at the leaf and tree level were driven by photosynthetic photon flux density (PPFD), but the sap flow intensity was controlled by vapour pressure deficit (VPD). The four poplar genotypes showed different water use strategies, based on determination of transpiration and other plant water status indicators.