Heat Ratio Method Research Articles

Leaf gas exchange, water use, and yield of Marula (Sclerocarya birrea) and Kei Apple (Dovyalis caffra): South African indigenous fruit trees with domestication potential

Conventional crop production is expected to decline in the future in sub-Saharan Africa due to the rising costs of inputs, soil degradation, and increasing water scarcity related to climate variability and change. South Africa has more than 30 species of indigenous fruit trees that are adapted to grow under harsh conditions, yet these species are currently underutilized. Their cultivation is constrained by the lack of detailed ecophysiological information required to develop effective management strategies that optimize tree performance. In this study we investigated how two species with domestication potential namely Marula (Sclerocarya birrea) and Kei apple (Dovyalis caffra) responded to climate and soil factors over a period of two years (2019–2021). Photosynthesis, transpiration, and yield were quantified for mature trees growing in experimental orchards in the Mpumalanga Province of South Africa. Leaf gas exchange data were collected at selected intervals during the growing season using an infrared gas analyser while whole tree transpiration rates were quantified using the heat ratio method of monitoring sap flow. These data were used to parameterize a big leaf Penman-Monteith (PM) model to estimate the transpiration dynamics of individual trees at the daily time step. S. birrea's transpiration responded strongly to both climatic and soil water content variations while the water use of D. caffra was driven mostly by climatic factors. Transpiration rates averaged 0.30 L/m2 of leaf area for S. birrea while that of D. caffra was around 0.45 L/m2. Both species exhibited significant alternate bearing with yield of individual trees fluctuating by more than 50 % between successive years. For the 2019/20 season, water productivity (grams of fruit per litre of water transpired) varied between 6.2 and 17.4 g/L for S. birrea and between 24.3 and 41.7 g/L for D. caffra. Whole tree transpiration by both species were accurately predicted using the PM model albeit with different parameters for each species. This study demonstrates that a simple PM model can accurately estimate the water requirements of these species in other growing regions.

Estimating crop coefficients and water use of a full-bearing mango orchard in north-eastern South Africa using the fraction of vegetation cover and a dual source evapotranspiration model

In semi-arid countries like South Africa, commercially produced mango (Mangifera Indica L.) are grown entirely under irrigation. However, there is little accurate quantitative information on the water use of mango orchards, and few accurate water use models currently exist. In this study, we evaluated the performance of the FAO 56 method of estimating crop evapotranspiration (ETc) and a dual source water use model against measurements of actual consumptive water use of a commercial mango orchard. The FAO approach calculates ETc as the product of a crop coefficient (Kc) and the reference evapotranspiration (ETo). We derived Kc for well-watered orchards from readily available data including the fraction of vegetation cover, average tree height, and a stomatal control coefficient. The dual-source model calculated orchard evapotranspiration as the sum of tree transpiration and orchard floor evaporation derived from the modified Shuttleworth and Wallace model. Actual evapotranspiration was measured using an open path eddy covariance system, while tree transpiration was measured using the heat ratio method of monitoring sap flow. Data were collected in a mature ‘Tommy Atkins’ mango orchard grown with micro-sprinkler irrigation in north-eastern South Africa. Results show that the reference evapotranspiration explained most of the variation in orchard transpiration (R2 ∼ 0.78) compared to the solar radiation (R2 ∼ 0.66) and the vapour pressure deficit of the air (R2 ∼ 0.66). Compared with field measured water use data, the FAO method gave relatively accurate estimates of transpiration (R2 = 0.74, NRMSE = ±18.00 %, NMAE = ±15.00 %, NSE = 0.70) and evapotranspiration (R2 = 0.58, NRMSE = ±19.00 %, NMAE = ±16.00 %, NSE = 0.55). NSE is the Nash-Sutcliffe Efficiency. The dual-source model yielded slightly less accurate estimates of transpiration (R2 = 0.61, NRMSE = ±22.00 %, NMAE = ±20.00 %, NSE = 0.39) and evapotranspiration (R2 = 0.52, NRMSE = ±23.00 %, NMAE = ±25.00 %, NSE = 0.51). The basal crop coefficient (Kcb) showed small seasonal fluctuations, varying between 0.40 and 0.60 throughout the year. In contrast, Kc showed clear seasonality varying between 0.59–0.64 at flowering and fruit set and rising to 0.81–0.90 during the fruit growing phase. Estimates of the annual total transpiration (686 mm) from the FAO method were within 5 % of the actual measured values (677 mm) while the dual source model estimated 612 mm of transpiration, which was almost 10 % lower than the measured values. The results highlight that even though the FAO method requires fewer input parameters, it is potentially more accurate in estimating water use in mango orchards than the dual-source model.

Testing transpiration rates of juvenile Aleppo pine trees using the heat ratio method under laboratory conditions

AbstractTree transpiration considerably contributes to evaporative fluxes to the atmosphere in terrestrial ecosystems. Accurate transpiration quantification provides relevant information about forest water use and may benefit adaptive forest management, especially in a global change context. Tree transpiration can be measured by several methods, and sap flow measurements are one of the most valued. However, species‐specific validations of these techniques are required to avoid undesirable bias. This is especially relevant in species with low transpiration rates where errors may be relevant, such as Aleppo pine trees (Pinus halepensis Mill.). Moreover, another significant source of uncertainty in sap flow measurements is probe misalignment. Hence, the aim of this study was to correlate transpiration rates estimated by sap flow probes using the heat ratio method (THRM) and load cells to independently monitor water transpiration in juvenile Aleppo pine trees. The corrections to improve transpiration measures, including misalignment correction, were applied to THRM results to test if the accuracy of results improved. These measurements were recorded in greenhouse under controlled conditions to implement different environmental conditions. The environmental variables that ruled the experiment, mainly vapour pressure deficit and soil water availability, spanned in a wide range of values. The results showed an accurate linear correspondence between TOBS and THRM for low and medium values, but moderate underestimations at high transpiration rates were observed. These underestimations were partly removed when applying probe misalignment correction. This study supports the notion that HRM offers accurate Aleppo pine transpiration estimations with low and medium values under a variety of abiotic conditions, which also has implications for HRM application in other isohydric species. The results also support the interest in the use of probe misalignment correction to estimate transpiration, mainly when high transpiration values are recorded. The results of this study can be considered as a preliminary approach for future research in order to improve the estimates of the transpiration rates of the Aleppo pine under the limiting conditions of the Mediterranean.

Deriving crop coefficients for evergreen and deciduous fruit orchards in South Africa using the fraction of vegetation cover and tree height data

Inaccurate crop coefficients are major contributing sources of uncertainty that lead to inefficient use of limited available water resources. Understanding the need to improve water use efficiency in South Africa’s fruit industry, this study evaluated the method of deriving crop coefficients developed by Allen and Pereira (2009) over a variety of irrigated fruit tree crops. Detailed data of transpiration, evapotranspiration and weather variables measured using the heat ratio method, eddy covariance method and automatic weather stations, were collected from a water research funding body established by the South African government. This study adjusted the stomatal sensitivity function (Fr) in the model by replacing the ratio of the leaf resistance (rl) to the standard leaf resistance of a reference crop (100 sm−1) with rl/α where α is a resistance parameter for the specific crop. The resistance parameter was solved accordingly for each fruit type. Respective unique α values were obtained as: macadamia nuts (200 sm−1), citrus (50 s m−1), peaches (20 s m−1) and pecans (20 s m−1). These unique values were used to simulate basal and single crop coefficients that produced satisfactory results when compared to the actual measured values. Overly, no unique standard α value exists for most tree crops although a value close to 20 sm−1 may give reasonable estimates for pome and stone fruit. Crop coefficients derived using locally measured data were standardised and tabulated in a format that facilitates their transferability between sites. However, there is still a need to acquire crop specific information to parameterize α and improve accuracies.

Comparing dual heat pulse methods with Péclet's number as universal switch to measure sap flow across a wide range.

Accurate determination of sap flow over a wide measurement range is important for assessing tree transpiration. However, this is difficult to achieve by using a single heat pulse method. Recent attempts have been made to combine multiple heat pulse methods and have successfully increased the sap flow measurement range. However, relative performance of different dual methods has not yet been addressed, and selection of the numerical threshold used to switch between methods has not been verified among different dual methods. This paper evaluates three different dual methods with respect to measurement range, precision and sources of uncertainty: (method 1) the heat ratio (HR) and compensation heat pulse method; (method 2) the HR and T-max method; and (method 3) the HR and double ratio method. Field experiments showed that methods 1, 2 with three needles and 3 compare well with the benchmark Sapflow+ method, having root mean square deviations of 4.7cmh-1, 3.0cmh-1 and 2.4cmh-1, respectively. The three dual methods are equivalent in accuracy (P>0.05). Moreover, all dual methods can satisfactorily measure reverse, low and medium heat pulse velocities. However, for high velocities (>100cmh-1), the HR+T-max (method 2) performed better than the other methods. Another advantage is that this method has a three- instead of four-needle probe configuration, making it less error prone to probe misalignment and plant wounding. All dual methods in this study use the HR method for calculating low to medium flow and a different method for calculating high flow. The optimal threshold for switching from HR to another method is HR's maximum flow, which can be accurately determined from the Péclet number. This study therefore provides guidance for an optimal selection of methods for quantification of sap flow over a wide measurement range.

Effect of different irrigation systems on water use partitioning and plant water relations of apple trees growing on deep sandy soils in the Mediterranean climatic conditions, South Africa

All commercial apple fruit (Malus Domestica (Borkh) exported from semi-arid regions are grown under irrigation with drip and micro sprinkler systems being the most widely used. Few studies have directly compared the physiological responses of fruit trees to these systems in detail leading to uncertainties around their performance. This study investigated variations in transpiration rates, tree water status, growth, water use partitioning, fruit yield and quality for trees growing on deep sandy soils under these two systems. Data were collected in a mature Royal Gala orchard in South Africa over three growing seasons. Tree transpiration was quantified using the heat ratio method of monitoring sap flow while the soil water balance approach was used to derive the evapotranspiration (ET) rates. Leaf level results showed that one day after irrigation on hot dry days, the stomatal conductance was, on average, almost double for trees under micro than those under drip irrigation. There was more stress under drip with the minimum midday leaf water potential dropping to under -1.80 MPa compared to only around -1.20 MPa under micro sprinklers. Consequently, the tree transpiration per unit leaf area was substantially higher under micro sprinkler (2.9 L/m2/d) compared to 2.3 L/m2/d under drip (P ≤ 0.05). Canopy growth was slower under drip with peak leaf area index (LAI) around 2.1 compared to 2.7 under the micro sprinkler system. The micro sprinkler system had a more active ground cover than the drip. At peak canopy cover in summer, up to 28% of ET was derived from the orchard floor under micro compared to only 15% under drip. However, fruit size and fruit quality were lower under drip compared to micro sprinkler irrigated trees. The study highlights that while water savings are high under drip irrigated orchards on sandy soils, trees tend to experience considerable water stress culminating in smaller fruit of compromised quality.

Spatial and temporal variation in forest transpiration across a forested boreal peatland complex

AbstractTranspiration is a globally important component of evapotranspiration. Careful upscaling of transpiration from point measurements is thus crucial for quantifying water and energy fluxes. In spatially heterogeneous landscapes common across the boreal biome, upscaled transpiration estimates are difficult to determine due to variation in local environmental conditions (e.g., basal area, soil moisture, permafrost). Here, we sought to determine stand‐level attributes that influence transpiration scalars for a forested boreal peatland complex consisting of sparsely treed wetlands and densely treed permafrost plateaus as land cover types. The objectives were to quantify spatial and temporal variability in stand‐level transpiration, and to identify sources of uncertainty when scaling point measurements to the stand‐level. Using heat ratio method sap flow sensors, we determined sap velocity for black spruce and tamarack for 2‐week periods during peak growing season in 2013, 2017 and 2018. We found greater basal area, drier soils, and the presence of permafrost increased daily sap velocity in individual trees, suggesting that local environmental conditions are important in dictating sap velocity. When sap velocity was scaled to stand‐level transpiration using gridded 20 × 20 m resolution data across the ~10 ha Scotty Creek ForestGEO plot, we observed significant differences in daily plot transpiration among years (0.17–0.30 mm), and across land cover types. Daily transpiration was lowest in grid‐cells with sparsely treed wetlands compared to grid‐cells with well‐drained and densely treed permafrost plateaus, where daily transpiration reached 0.80 mm, or 30% of the daily evapotranspiration. When transpiration scalars (i.e., sap velocity) were not specific to the different land cover types (i.e., permafrost plateaus and wetlands), scaled stand‐level transpiration was overestimated by 42%. To quantify the relative contribution of tree transpiration to ecosystem evapotranspiration, we recommend that sampling designs stratify across local environmental conditions to accurately represent variation associated with land cover types, especially with different hydrological functioning as encountered in rapidly thawing boreal peatland complexes.

Water abstraction of invasive Prosopis juliflora and native Senegalia senegal trees: A comparative study in the Great Rift Valley Area, Ethiopia

Besides direct water abstraction, natural water scarcity in semi-arid and arid regions may be further exacerbated by human-assisted changes in vegetation composition, including the invasion by non-native plant species. Water abstraction by the invasive tree Prosopis juliflora and by the native Senegalia senegal was compared in the dry Great Rift Valley, Ethiopia. Transpiration rates were quantified using the heat ratio method on six trees each of P. julifora and S. senegal, growing adjacent to each other in the same environment. Water use for P. juliflora trees ranges from 1 to 26 L/day (an average of 4.74 ± 1.97), and that of S. senegal trees from 1 to 38 L/day (an average of 5.48 ± 5.29 during two study years). For both species, soil heat, latent heat, and soil moisture status influenced the rates of sap flow of trees; in addition, water use by P. juliflora trees was related to vapor pressure deficit; the higher the vapor pressure deficit, the higher the water abstraction by P. juliflora. Stand densities of pure P. juliflora and S. senegal were 1200–1600 trees and 400–600 trees per ha, respectively. At the stand scale, P. juliflora consumed approximately 6636 L/day/ha (transpiration: 242 mm per year) and S. senegal stands consumed 2723 L/day/ha (transpiration: 87 mm per year). That is, P. juliflora stands consumed three times more water than S. senegal stands, because of two reasons: (1) P. juliflora stands are denser than S. senegal stands, and denser stands consume more water than less dense stands, and (2) P. juliflora is evergreen and uses water all year-round, while S. senegal sheds its leaves during the peak dry seasons. Our findings suggest that, compared to S. senegal, P. juliflora invasion results in severe impacts on groundwater resources of the drylands of Ethiopia, with direct and indirect consequences to ecosystem services and rural livelihoods.

Influence of Environmental Factors on the Sap Flow Activity of the Golden Pear in the Growth Period of Karst Area in Southern China

Under extreme drought and climate change, golden pear trees have experienced problems such as yield reduction, dryness and death. This suggests that we know very little about the mechanisms regulating pear tree growth, assuming that meteorological factors positively influence plant sap flow. Based on this, we used the heat ratio method to monitor the sap flow of pear trees from June to December 2020, and recorded the changes in various environmental factors. The results showed that: (1) Sap flow velocity has obvious radial variability in tree sections; the sap flow velocity during the day was significantly higher than that at night (p < 0.05) and was higher in the growing season than in the non-growing season. (2) All environmental factors, except relative humidity and precipitation, were positively correlated with sap flow, vapor pressure deficit and photosynthetically active radiation, which are the key factors affecting daytime flow, and vapor pressure deficit and plant water potential are the key factors affecting nighttime flow. The linear regression results also showed that the daytime sap flow had a significant positive effect on the nighttime sap flow (p < 0.05). (3) The contribution of night flow to total daily flow varied from 17.3% to 50.7%, and most of the non-growing season values were above 40%. The results show that nighttime sap flow accounts for a significant portion of the pear tree’s water budget. Continuous irrigation during fruit enlargement and non-growing seasons will increase fruit yield and maintain plant sap flow activity to avoid death due to drought.

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.

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.

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.

On the Use of Sap Flow Measurements to Assess the Water Requirements of Three Australian Native Tree Species

The measurement of sap movement in xylem sapwood tissue using heat pulse velocity sap flow instruments has been commonly used to estimate plant transpiration. In this study, sap flow sensors (SFM1) based on the heat ratio method (HRM) were used to assess the sap flow performance of three different tree species located in the eastern suburbs of Melbourne, Australia over a 12-month period. A soil moisture budget profile featuring potential evapotranspiration and precipitation was developed to indicate soil moisture balance while the soil-plant-atmosphere continuum was examined at the study site using data obtained from different monitoring instruments. The comparison of sap flow volume for the three species clearly showed that the water demand of Corymbia maculata was the highest when compared to Melaleuca styphelioides and Lophostemon confertus and the daily sap flow volume on the north side of the tree on average was 63% greater than that of the south side. By analysing the optimal temperature and vapour pressure deficit (VPD) for transpiration for all sampled trees, it was concluded that the Melaleuca styphelioides could better cope with hotter and drier weather conditions.

Partitioning tree water usage into storage and transpiration in a mixed forest

BackgroundWater migration and use are important processes in trees. However, it is possible to overestimate transpiration by equating the water absorbed through the plant roots to that diffused back to the atmosphere through stomatal transpiration. Therefore, it is necessary to quantify the water transpired and stored in plants.MethodThe δ2H/δ18O technique and heat ratio method were used to explore the water usage of coniferous and broad-leaved tree species, including the proportions of water used for transpiration and water storage.ResultsPlatycladus orientalis and Quercus variabilis had strong plasticity in their water usage from different sources. Platycladus orientalis primarily used groundwater (30.5%) and the 60–100-cm soil layer (21.6%) throughout the experimental period and was sensitive to precipitation, absorbing water from the 0–20-cm layer (26.6%) during the rainy season. Quercus variabilis absorbed water from all sources (15.7%–36.5%) except from the 40–60-cm soil layer during the dry season. In addition, it did not change its water source but increased its groundwater uptake during the rainy season. The annual mean water fluxes of P. orientalis and Q. variabilis were 374.69 and 469.50 mm·year− 1, with 93.49% and 93.91% of the water used for transpiration, respectively. However, nocturnal sap flow in P. orientalis and Q. variabilis was mainly used for water storage in the trunk rather than transpiration, which effectively alleviated drought stress and facilitated the transport of nutrients.ConclusionsThe water stored in both species comprised 6%–7% of the total water fluxes and, therefore, should be considered in water balance models.

Measurement of low sap flux density in plants using the single needle heat pulse probe

Heat pulse probes are used worldwide to quantify sap flow in plants. Probes consisting of a single needle inserted into a plant have a simple physical configuration relative to multi-probe methods. The single probe also minimizes physical damage to plant tissue. Single probe methods are therefore increasingly used for sap flux density measurements. However, at low sap flux density values, existing single probe heat pulse (SPHP) methods cannot be used for accurate measurements. To increase the sensitivity of SPHP to low sap flux densities, this paper describes and validates a simplified expression of sap flow and heat transport under various heat inputs and finite heat pulse time intervals. To test this theoretical development, measurements were collected from SPHP probes inserted into mature poplar forest trees (Populus tomentosa Carrière) during the 2020 growing season. The results show that the new method can successfully monitor low sap flux density: the lowest measured heat pulse velocity was 3 cm h−1, which is an improvement from the original SPHP method (20 cm h−1). The improved SPHP method had a root mean square deviation (RMSD) of 1.63 cm h−1 in comparison with the heat ratio (HR) method. The proposed SPHP method is therefore suitable for quantifying both low and high sap flux densities.

Water use of Prosopis juliflora and its impacts on catchment water budget and rural livelihoods in Afar Region, Ethiopia

Dense impenetrable thickets of invasive trees and shrubs compete with other water users and thus disrupt ecosystem functioning and services. This study assessed water use by the evergreen Prosopis juliflora, one of the dominant invasive tree species in semi-arid and arid ecosystems in the tropical regions of Eastern Africa. The objectives of the study were to (1) analyze the seasonal water use patterns of P. juliflora in various locations in Afar Region, Ethiopia, (2) up-scale the water use from individual tree transpiration and stand evapotranspiration (ET) to the entire invaded area, and 3) estimate the monetary value of water lost due to the invasion. The sap flow rates of individual P. juliflora trees were measured using the heat ratio method while stand ET was quantified using the eddy covariance method. Transpiration by individual trees ranged from 1–36 L/day, with an average of 7 L of water per tree per day. The daily average transpiration of a Prosopis tree was about 3.4 (± 0.5) mm and the daily average ET of a dense Prosopis stand was about 3.7 (± 1.6) mm. Using a fractional cover map of P. juliflora (over an area of 1.18 million ha), water use of P. juliflora in Afar Region was estimated to be approximately 3.1–3.3 billion m3/yr. This volume of water would be sufficient to irrigate about 460,000 ha of cotton or 330,000 ha of sugar cane, the main crops in the area, which would generate an estimated net benefit of approximately US$ 320 million and US$ 470 million per growing season from cotton and sugarcane, respectively. Hence, P. juliflora invasion in the Afar Region has serious impacts on water availability and on the provision of other ecosystem services and ultimately on rural livelihoods.

Calibration of the heat ratio method by direct measurements of transpiration with the weighing root-ball method for Michelia figo

Calibration of the heat ratio method by direct measurements of transpiration with the weighing root-ball method for <i>Michelia figo</i>

Contribution of understorey vegetation to evapotranspiration partitioning in apple orchards under Mediterranean climatic conditions in South Africa

Orchard evapotranspiration (ET) is a complex flux which has been the subject of many studies. It often includes transpiration from the trees, cover crops and weeds, evaporation from the soil, mulches and other orchard artefacts. In this study we investigated the contribution of the orchard floor evaporative fluxes to whole orchard ET focusing on the transpiration dynamics of understorey vegetation which is currently not well known. Data on the partitioning of ET into its constituent components were collected in apple (Malus Domestica Bork) orchards with varying fractional canopy cover. The study orchards were in the prime apple growing regions in South Africa. The orchards were planted to the Golden Delicious/Reinders and the red cultivars (i.e. Cripps’ Pink/ Royal Gala/Fuji). Tree transpiration was quantified using the heat ratio method and the thermal dissipation sap flow techniques. Understorey transpiration was measured at selected intervals using micro stem heat balance sap flow gauges calibrated against infrared gas analyser readings. Orchard ET was measured using an open path eddy covariance system while the microclimate, radiation interception, and soil evaporation were also monitored. Orchard floor evaporative fluxes accounted for as much as 80% of the measured ET in young orchards with dense understorey vegetation that covered most of the orchard floor. In these orchards the understorey transpiration was of the same order of magnitude as the bare moist soil evaporation suggesting that water use by the understorey vegetation was substantial. However, in mature orchards with a high canopy cover (>55% fractional cover), orchard floor water losses were less than 30% of the measured ET. Understorey transpiration rates were much lower, contributing less than 10% of the whole orchard ET. Significant volumes of water can be saved, especially in young orchards, by keeping the orchard floor vegetation short, reducing the area occupied by understorey vegetation, and by reducing the wetted ground surface area.

Uncertainty in sap flow of Brazilian mahogany determined by the heat ratio method

The tropical arboreal species Brazilian mahogany (Swietenia macrophylla) is very important economically and ecologically, for which understanding ecophysiological variables such as sap flow will improve understanding of the species and its cultivation. This paper aims to measure uncertainties (U) involved in the application of the heat ratio method for determining sap flow in Brazilian mahogany using sets of heating probes and thermometers installed on plants of 18 months of age, cultivated in Yellow Latosol, under a weighing lysimeter and located in a protected environment. The uncertainty in sap flow was calculated as the combination of uncertainty in the thermal diffusivity (Uk), conductive section (USc) and corrected sap velocity (UVc). Uk had greater weight in determining the flow of sap in Brazilian mahogany, when compared to USc and UVc. The thermal diffusivity during the cycle, or period evaluated, must be adjusted to improve the accuracy of the heat ratio method because the sap flow overestimated transpiration by 15.0%. When soil water was optimal In addition, the vapor pressure deficit linearly and indirectly influenced the SF with a difference of 14.6%.

Characteristics and processes of reverse sap flow of Platycladus orientalis based on stable isotope technique and heat ratio method

Plants could maintain growth by foliar water uptake and reverse sap flow under certain conditions, particularly in regions with seasonal drought. This physiological activity is often overlooked, however, leaving a gap in quantitatively understanding the processes and mechanisms underlying water utilization of forest vegetation under drought stress. In this study, with both field comparison experiments and pot experiments, we used heat ratio method with stable isotope technique to monitor a typical plantation tree species, Platycladus orientalis, in the Beijing mountainous area. We aimed to analyze the patterns and the influencing factors of the reverse sap flow occurrence in P. orientalis, to quantify the amount and the replenishment rate of reverse sap flow, and to examine the characteristics and processes of reverse sap flow at different parts of plants. In the field comparison experiment, reverse sap flow was detected at the breast height of stem and in the root in the controlled plot (drought plot) after rainfall. The reverse sap flow of root system was detected later than that in the stem. By contrast, no reverse sap flow was observed in the natural plot. In the pot experiments, the recharge rate of all the groups reached the peak value two hours after the rainfall treatment. Except for the groups of severe and moderate drought, recovery of δD to the original level was observed eight hours after rainfall, and the reverse sap flow on plants generally lasted no more than 24 h. The amount of foliar water uptake and the reverse sap flow to the branches and rhizosphere soil had a negative relationship with the initial soil moisture. The maximum recharge rates for leaves, branches, and rhizosphere soil were (9.5±0.1)%, (5.9±0.3)% and (5.7%±0.6)%, respectively. Different rates and timing of the reverse sap flow were observed at different parts of P. orientalis. Under complex and variable conditions of water supply, it is of great significance to examine the process and mechanism of reverse water movement of plants to better understand its survival and competitive strategies.