Tree Transpiration Research Articles

Mitigation strategies in densely urbanized contexts: the case study of a historical district in Naples

The design of our cities, their planning, and management must be increasingly oriented towards sustainability, identifying solutions that can systemically address problems on a city scale, namely urban heat islands, rainwater runoff, storms, and acoustic issues. This study begins by evaluating the impact of a green area on pedestrian comfort in the historical center of Naples, employing an experimental and numerical approach. Temperature, relative humidity, and CO2 concentration were measured at three significant points in a historical quarter in “Chiaia” district (Naples): a) in the center of the square “Piazza del Martiri” in the sun, b) in the center of the same square in the shade, and c) in the shade of the vegetation, under the effect of transpiration of deep trees. Then, these measures are used for the calibration of a numerical model developed in an environmental analysis software. The numerical approach has been used to predict the impact of possible mitigation scenarios, by increasing the vegetation and varying the density of the canopy, using non-white cool materials for the urban surfaces and for the facades and roofs of the surrounding historical buildings, and adding fountains. These strategies have a positive effect on urban temperatures and outdoor comfort, in densely urbanized contexts. The findings are finally used to identify the best design solutions, to improve pedestrian comfort and reduce the urban heat, without affecting the historical value of the urban district.

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.

Do urban tree hydraulics limit their transpirational cooling? A comparison between temperate and hot arid climates

Evaporative cooling due to transpiration of urban trees in two contrasting climates is the subject of this study. Transpiration was studied experimentally on local tree species at ‘tree lab’ sites in Munich, Germany (temperate climate) and in Beer Sheva, Israel (hot arid climate), within various settings (park, street, square) with natural and sealed surface conditions. The comparison was based on linear relationship of midday canopy resistance to atmospheric vapor pressure deficit (VPD), where the slope is proportional to tree hydraulic conductance and midday stem water potential. Sap flux densities in all trees were similar but crown projected area (CPA) was larger in Beer Sheva, resulting in higher hydraulic resistance that limits transpirational cooling (regression slopes of 0.25–0.44 in Beer Sheva vs. 0.10–0.18 in Munich). The contribution of the transpirational cooling, studied as the proportion of latent heat to total available energy was about 40% less significant at noon hours in summer in Beer Sheva than in Munich. Results suggested that shading should be the main consideration in planting local trees in hot dry climates. The linear relationship of midday canopy resistance to VPD is proposed as an operative tool for comparing evaporative cooling between local trees in different climatic regions.

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.

Three-dimensional multimodal porous graphene-carbonized wood for highly efficient solar steam generation

Recently, solar steam generation systems have become a promising solar energy technology that can alleviate the energy crisis and reduce water pollution. However, because of weak solar radiation energy, rapid heat loss, and negative environmental impacts, there are still numerous challenges that must be overcome before solar steam generation systems can be used for practical energy utilization. Inspired by natural tree transpiration, this paper reports a 3D carbonized wood (hydrophilic)-based photothermal system coated with 2D porous reduced graphene oxide (hydrophobic) to promote solar steam generation. Wood with vertically aligned microchannels and high hydrophilicity is used to efficiently pump water, which is then spread to a 2D porous reduced graphene oxide layer with a sufficient evaporation surface. The unique 3D micro-sized water reservoirs formed between the wood and reduced graphene oxide maximize the thermal energy use of the graphene layer by preventing heat loss from its transference to the water and reducing light reflection. The evaporation rate and solar energy conversion efficiency were 1.492 kg⋅m−2⋅h−1 and 94.7% under 1 sun illumination, respectively. Moreover, our solar steam evaporator maintained its original evaporation rate after 20 h. This work provides new insights into the structural design for highly efficient solar steam generation devices.

Partitioning and sourcing of evapotranspiration using coupled MARMITES-MODFLOW model, La Mata catchment (Spain)

The new, two-way coupled, distributed and transient MARMITES-MODFLOW (MM-MF) model, coupling land surface and soil zone domains with groundwater, is presented. It implements model-based partitioning and sourcing of subsurface evapotranspiration (ETss) as part of spatio-temporal water balance (WB). The partitioning of ETss involves its separation into evaporation (E) and transpiration (T), while the sourcing of E and T involves separation of each of the two into soil zone (Esoil and Tsoil) and groundwater (Eg and Tg) components. The objective of that development was to understand the system dynamics of a catchment with shallow water table, through spatio-temporal quantification of water fluxes and evaluation of their importance in water balances, focusing on the Eg and Tg components of ETss. While the Eg is computed using formulation from published study, the Tg is obtained through a novel phenomenological function, based on soil moisture availability and transpiration demand driven by climatic conditions. The MM-MF model was applied in the small La Mata catchment (~4.8 km2, Salamanca Province, Spain), characterized by semi-arid climate, granitic bedrock, shallow water table and sparse oak woodland. The main catchment characteristics were obtained using remote sensing, non-invasive hydrogeophysics and classical field data acquisition. The MM-MF model was calibrated in transient, using daily data of five hydrological years, between 1st October 2008 and 30th September 2013. The WB confirmed dependence of groundwater exfiltration on gross recharge. These two water fluxes, together with infiltration and Esoil, constituted the largest subsurface water fluxes. The Eg was higher than the Tg, which is explained by low tree coverage (~7%). Considering seasonal variability, Eg and Tg were larger in dry seasons than in wet season, when solar radiation was the largest and soil moisture the most depleted. A relevant observation with respect to tree transpiration was that during dry seasons, the decline of Tsoil, associated with the decline of soil moisture, was compensated by increase of Tg, despite continuously declining water table. However, in dry seasons, T was far below the atmospheric evaporative demand, indicating that the groundwater uptake by the tree species of this study constituted a survival strategy and not a mechanism for continued plant growth. The presented MM-MF model allowed to analyze catchment water dynamics and water balance in detail, accounting separately for impacts of evaporation and transpiration processes on groundwater resources. With its unique capability of partitioning and sourcing of ETss, the MM-MF model is particularly suitable for mapping groundwater dependent ecosystems, but also for analyzing impacts of climate and land cover changes on groundwater resources.

Stand structure of Central European forests matters more than climate for transpiration sensitivity to VPD

Abstract Temperature rise and more severe and frequent droughts will alter forest transpiration, thereby affecting the global water cycle. Yet, tree responses to increased atmospheric vapour pressure deficit (VPD) and reduced soil water content (SWC) are not fully understood due to long‐term tree adjustments to local environmental conditions that modify transpiration responses to short‐term VPD and SWC changes. We analysed sap flux density (SFD) of Fagus sylvatica, Picea abies, Pinus sylvestris and Quercus ilex from 25 sites across Europe to understand how daily variation in SWC affects the sensitivity of SFD to VPD (βVPD) and the maximum SFD (S95). Furthermore, we tested whether long‐term adjustments to site climatic conditions and stand characteristics affect βVPD and S95. The studied species showed contrasting βVPD and S95 with the largest values in F. sylvatica, followed by Q. ilex, which surpassed the two conifers that showed low βVPD and low S95. We observed that βVPD and S95 dropped during days of low SWC in F. sylvatica, P. sylvestris and Q. ilex, but not in P. abies. Both βVPD and S95 were driven by tree height, and the temperature and precipitation at the sites. However, stand basal area was the most important driver of βVPD and S95, explaining 30% of their total variance. Synthesis and applications: A future warmer and drier climate will restrict tree transpiration and thereby heavily affect the soil–plant‐atmosphere coupling. However, the effect of basal area, being the largest driver of tree transpiration sensitivity to vapour pressure deficit across a broad range of conditions, provides the opportunity to pre‐adapt European forests to future climate conditions. While stand thinning can increase the soil water availability for remaining trees, it also increases transpiration sensitivity to high air temperatures and may thereby amplify tree vulnerability to heat and drought.

Effects of Topography and Social Position on the Solar Radiation of Individual Trees on a Hillslope in Northwest China

Solar radiation is a key factor influencing the photosynthesis and transpiration of trees. In mountainous regions, solar radiation income exhibits strong spatial heterogeneity due to topographical variations and the structural complexity of the forest. However, how the solar radiation income of individual trees in different social positions varies with slope position remains unclear. In this study, the daily solar radiation of the horizontal ground (Rh), different slope positions (i.e., at different locations on a hillslope, Rs) and individual trees with different social positions in the forest (Ri) were monitored from May to October in 2020 and 2021. The daily solar radiation income of a single hillslope (Rf) was applied to quantify the Rs response to the slope and aspect (i.e., slope effect) and the shade from the opposite mountain (i.e., shaded terrain effect). Our results showed that the Rf was 27.8% lower than Rh due to the slope effect of the sample slope. In the different slope positions, 2.7%–46.9% of solar radiation was lost due to the shaded terrain effect. A stronger limitation of Rs by the shaded terrain effect was detected on the bottom slope compared to that of the upper slope. The better the social position of an individual tree (i.e., tree dominance (Dom) and the distance between trees (D)), the more solar radiation it received, ranging from 22.4 to 95.3%. The dominant factor contributing to changes in Ri was slope position followed by D and Dom and, finally, Rh. These results provide an important basis for understanding the role of topography and tree social positions in solar radiation income in mountainous regions. Forest management measures should be varied with slope positions in mountainous regions, and forest density (i.e., distance between trees) should be considered as a key factor to optimize the forest functions.

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.

The impact of tree transpiration on the safety and serviceability of pavement under seasonal variations

ABSTRACT Roadway trees planted as barriers reduce traffic noise and particle pollution in cities. However, tree roots may alter the soil's moisture content, resulting in uneven soil subsidence and maintenance issues for neighbouring structures. This study determined the safe distance between trees and pavement by investigating the effects of transpiration on pore water pressure (PWP) and pavement subsidence under seasonal variations. The root water uptake was simplified in a finite element model using multiple hydraulic head boundaries and validated using field observations. A hypoplastic model was used to model non-linear behaviour and plastic strain accumulation in unsaturated soil. Evergreen trees can reduce PWP by 72% during the dry season and 84% during the wet season, compared to bare soil. The subsidence did not decrease linearly with the distance away from trees under a rigid and impermeable pavement structure. The maximum bending moment was influenced by the distance between trees and pavement with the highest value occurring when trees were located near the pavement (up to five times the bending moments on pavement without trees). Our findings suggest that the pavement is at risk of experiencing structural failure if trees are located within 0.4 times their height away from the pavement.

Species difference of transpiration in three urban coniferous forests in a semiarid region of China

Tree transpiration and associated evaporative cooling play a critical role in water cycles and combating ‘Urban Heat Islands’ effects. However, species differences in transpiration magnitude, seasonality, and biophysical regulation in dryland urban regions are not well understood. We measured the sap flow of three widely planted coniferous species, Pinus tabulaeformis, Pinus sylvestris, and Picea meyeri, for urban greening, and concurrent soil moisture and meteorological variables in three pure urban forest plantations during three successive growing seasons (May to September during 2017–2019) under a semiarid urban environment in northern China. Stand-level transpiration (Ec) differed significantly among the three species (p < 0.001), where the mean daily Ec of P. tabulaeformis, P. sylvestris, and P. meyeri plantations were 0.78 ± 0.02 mm d–1, 0.68 ± 0.01 mm d–1, and 0.20 ± 0.01 mm d–1, respectively. Additionally, distinct differences in the seasonality of Ec were observed among tree species. Except for 2017, the Ec of P. tabulaeformis and P. sylvestris maintained at low levels in May and June and increased significantly with precipitation in July and August (p < 0.05) when soil volumetric water content (VWC) increased, but the Ec of P. meyeri remained relatively stable throughout the growing season. Responses of Ec in P. tabulaeformis and P. sylvestris plantations to meteorological factors can be changed by soil water availability. Under relatively higher water availability, the Ec of P. tabulaeformis and P. sylvestris were primarily controlled by vapor pressure deficit (VPD) and solar radiation (Rs). Under soil water stress, the Ec of P. tabulaeformis and P. sylvestris were more affected by VWC than P. meyeri. Biologically, P. tabulaeformis had a higher stomatal sensitivity to VPD than P. sylvestris and P. meyeri. At the beginning of growing seasons, the high water use in P. tabulaeformis and P. sylvestris plantations could lead to soil water depletion. Therefore, the species-specific biophysical regulations of tree water use should be carefully considered for urban forest management to achieve ecosystem services by balancing the tradeoff between water consumption and its cooling effects (through evapotranspiration) in semiarid cities.

Apple tree transpiration estimated using the Penman-Monteith model integrated with optimized jarvis model

Accurate estimates of plant transpiration (Tsf) are essential to maximizing the efficient use of water. When only considering the surface canopy resistance (rc), the Penman-Monteith (PM) model is commonly used in Tsf modeling. However, the rc is difficult to measure but it can be accurately estimated using the Jarvis canopy resistance model (JA). Our objectives were to evaluate the rc of apple trees calculated with an inverted PM model integrated with different versions of the JA model for different growth stages and to compare their accuracy using four optimization algorithms, the least squares method (LSM), genetic algorithm (GA), quantum particle swarm optimization (QPSO), and the mind evolutionary algorithm (MEA). We explored the effect of environmental constraint functions and parameter optimization of three environmental variables, vapor pressure deficit (VPD), net irradiance (Rn), and air temperature (Ta), and of soil water content (θa) on the accuracy of JA sub-models to calculate apple tree rc and Tsf. In our analysis, we used rainfed data from experiments on an apple orchard conducted during 2008–2010 at Wuwei City on the Loess Plateau of China. We compared 81 segmented JA sub-models (by canopy growth stage), comprising of the combination of the environmental constraint functions that were used to calculate rc. Moreover, the JA sub-models were optimized and results were compared to improve the accuracy of Tsf estimates with five empirical rc models, combined with the PM model. The results showed that sub-model JA3322, i.e., the third constraint for VPD, the third constraint for Rn, the second constraint for Ta, and the second constraint for θa attained the best estimate of rc with a coefficient of determination (R2 = 0.71), a Nash-Sutcliffe efficiency coefficient (NSE = 0.65), root mean squared error (RMSE = 1257.4 s m-1), and global performance indicator (GPI = 1.0) for the whole growth stage. The equivalent values for the partial canopy stage were 0.78, 0.72, 1203.3 s m-1 and 0.99, and the values for the dense canopy stage were 0.78, 0.77, 445.6 s m-1 and 0.97, respectively. Segmented JA models based on the leaf area index threshold significantly improved the accuracy of rc estimation, where the median R2 and NSE were improved by 7.1 % and 12.4 % in the partial canopy stage and by 12.2 % and 13.4 % in the dense canopy stage. Despite pointing out the best environmental constraint functions of the JA model in the different growth stages, results indicated that the MEA yielded the most accurate estimates of rc, followed by QPSO, GA, and LSM. Moreover, the JA model with environmental constraints was the most accurate method to estimate the apple tree Tsf, and MEA was the most suitable parameter optimization algorithm. Overall, the findings of this study provide accurate actual water consumption information of apple trees using easily accessible meteorological data for the effective day-to-day water management decision-making of rain-fed apple tree orchards on the Loess Plateau of China previously.

Linking evapotranspiration seasonal cycles to the water balance of headwater catchments with contrasting land uses

AbstractLand use affects evapotranspiration rates and is a primary driver of the catchment water balance. The water balance of two catchments in southeastern Australia dominated by either grazed pasture or blue gum (Eucalyptus globulus) plantation was studied, focusing on the patterns of evapotranspiration (ET) throughout the year. Rainfall, streamflow, and groundwater levels measured between 2015 and 2019 were combined to estimate annual ET using a water balance equation. In the pasture, eddy covariance was used to measure ET from the catchment. Sap flow measurements were used to estimate tree transpiration in May 2017–May 2018 and Feb 2019–Feb 2021 in two different plots within the plantation. The tree transpiration rates were added to interception, estimated as a percentage of annual rainfall, to calculate ET from the plantation catchment. ET in the pasture showed strong seasonal cycles with very low ET rates in summer and ET rates in spring that were larger than the transpiration rates in the plantation, where trees transpired consistently throughout the year. The estimated annual ET from the water balance equation was comparable to ET estimated from other measurements. In the pasture, ET on average accounted for 88% of annual rainfall, while ET in the plantation was on average 93% of rainfall, exceeding it in the years with annual rainfall lower than about 500 mm. The difference between the ET rates in the plantation and the pasture was approximately 30–50 mm y−1. The larger ET rates in the plantation were reflected in a gradual decrease in the groundwater storage. The larger ET rates were enough to cause a decrease in groundwater storage in the plantation but not in the pasture, where groundwater levels remained stable.

Tracing plant source water dynamics during drought by continuous transpiration measurements: An in-situ stable isotope approach.

The isotopic composition of xylem water (δX ) is of considerable interest for plant source water studies. In-situ monitored isotopic composition of transpired water (δT ) could provide a nondestructive proxy for δX -values. Using flow-through leaf chambers, we monitored 2-hourly δT -dynamics in two tropical plant species, one canopy-forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δT -dynamics in response to an experimental severe drought, followed by a 2 H deep-water pulse applied belowground before starting regular rain. We also sampled branches to obtain δX -values from cryogenic vacuum extraction (CVE). Daily flux-weighted δ18 OT -values were a good proxy for δ18 OX -values under well-watered and drought conditions that matched the rainforest's water source. Transpiration-derived δ18 OX -values were mostly lower than CVE-derived values. Transpiration-derived δ2 HX -values were relatively high compared to source water and consistently higher than CVE-derived values during drought. Tracing the 2 H deep-water pulse in real-time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in-situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species.

Impact of aerial humidity on seasonal malaria: an ecological study in Zambia

BackgroundSeasonal patterns of malaria cases in many parts of Africa are generally associated with rainfall, yet in the dry seasons, malaria transmission declines but does not always cease. It is important to understand what conditions support these periodic cases. Aerial moisture is thought to be important for mosquito survival and ability to forage, but its role during the dry seasons has not been well studied. During the dry season aerial moisture is minimal, but intermittent periods may arise from the transpiration of peri-domestic trees or from some other sources in the environment. These periods may provide conditions to sustain pockets of mosquitoes that become active and forage, thereby transmitting malaria. In this work, humidity along with other ecological variables that may impact malaria transmission have been examined.MethodsNegative binomial regression models were used to explore the association between peri-domestic tree humidity and local malaria incidence. This was done using sensitive temperature and humidity loggers in the rural Southern Province of Zambia over three consecutive years. Additional variables including rainfall, temperature and elevation were also explored.ResultsA negative binomial model with no lag was found to best fit the malaria cases for the full year in the evaluated sites of the Southern Province of Zambia. Local tree and granary night-time humidity and temperature were found to be associated with local health centre-reported incidence of malaria, while rainfall and elevation did not significantly contribute to this model. A no lag and one week lag model for the dry season alone also showed a significant effect of humidity, but not temperature, elevation, or rainfall.ConclusionThe study has shown that throughout the dry season, periodic conditions of sustained humidity occur that may permit foraging by resting mosquitoes, and these periods are associated with increased incidence of malaria cases. These results shed a light on conditions that impact the survival of the common malaria vector species, Anopheles arabiensis, in arid seasons and suggests how they emerge to forage when conditions permit.

Water consumption during a leafless period in a dry season in three deciduous tropical timber species

Besides mechanical protection and sugar transport through phloem, barks also transpire some amount of water. Bark transpiration may be a driving force to keep water flux during the leafless period among tropical deciduous trees, but has been hardly investigated. In the current investigation we used the sap flow data to assess the water consumption during leaf loss initiation, total leaf loss period, and leaf flush period in three economically important tropical deciduous timber species i.e, Anogeissus acuminata, Dipterocarpus tuberculatus and Tectona grandis. Eight trees of each species were measured using Granier’s heat dissipation sensors, with the circumference ranging from 59 cm to 172 cm (mean 59.84 cm) across the three species. The phenological observation for canopy leaves was also carried out on weekly basis. During the completely leafless period, a significant amount of water flux was detected in all three species, which was correlated with vapor pressure deficit (VPD), and light intensity. The whole tree daily mean water consumption during this leaf-less, flower-less and fruit-less period, mainly due to bark transpiration was around 8–14 kg across the three species. It was about 7 to 18 % of the mean whole-tree daily transpiration in the fully foliated canopies during a typical wet-season period. This water flux was supported by sufficient soil moisture as indicated by the pre-dawn leaf water potential higher than –0.4 MPa in the three species. Further, a significant and positive relationship (R2 = 0.32, P < 0.005) was found between tree circumference and whole-tree water consumption during the leaf-less period. The total stand-level water consumption during the leaf-less period ranged from 13 mm to 26.8 mm on a 30 days basis across the three species. Such water flux mainly driven by bark transpiration could have a significant role in maintaining the hydraulic integrity of trees in the dry season, although it could reduce xylem water potential to a risky level of hydraulic dysfunction during a prolonged drought or on dry sites. The study is the first report on bark transpiration in intact trees, scaled to the stand-level, which should be included in the modeling of hydraulic function of plant and ecosystem hydrology.

Quantifying water saving benefits of fixed white protective netting in irrigated apple orchards under Mediterranean-type climate conditions in South Africa

• Transpiration based crop water productivity (WP) is higher under nets compared to open orchards. • The water saving benefits of fixed nets are smaller than expected when considering ET rates possibly because of wetter soils under the nets. • There is merit in adjusting crop coefficients for irrigation scheduling under nets given the significant differences in tree transpiration rates. Apple ( Malus domestica Borkh.) producers are increasingly using nets to address climate and pest-related challenges. In the water-scarce Western Cape region of South Africa, all commercial apple orchards are irrigated. Based on limited information on tree water relations under protective nets, we hypothesised that significant water savings and improvements in crop water productivity could be achieved under nets. The lack of accurate quantitative information on water use under nets may perpetuate sub-optimal irrigation practices partly due to inaccurate crop coefficients. This study quantified tree transpiration (T) using the heat ratio sap flow method and orchard evapotranspiration (ET) using the soil water balance approach in mature irrigated ‘Rosy Glow’ apple orchards under fixed white net compared to an open (control) over two seasons. Differences in water use between treatments were analysed by comparing microclimates, plant water status, root zone soil moisture, and irrigation levels. Averaged over the two seasons, the nets reduced daily total solar radiation by ∼ 12%, wind speed by more than 36%, and reference evapotranspiration by ∼ 12%. Seasonal T was 11% lower under nets, while ET was only 4% lower. The nets reduced irrigation water requirement by 4%. Peak mid-season basal crop coefficients (K cb ) were 0.54±0.02 and 0.59±0.02 while there were no differences in the single crop coefficients (K c = 1.07±0.04 and 1.13±0.06) for the nets and control treatments, respectively. Transpiration based crop water productivity (WP) was higher under nets (20.7 kg m − 3 ) compared to 18.2 kg m − 3 in the open. There were no differences in the ET-based WP values (13.4 and 13.2 kg m − 3 ) for the nets and control treatments, respectively. This study suggests that water saving benefits of fixed nets are smaller than expected when considering ET rates because of more active ground cover under nets. But there is merit in adjusting crop coefficients for irrigation scheduling under nets given the much lower transpiration.

Janus mesoporous wood-based membrane for simultaneous oil/water separation, aromatic dyes removal, and seawater desalination

Directional transport of liquid is crucial for many applications including seawater desalination , oil/water separation, fog collection, membrane distillation and so forth. Particularly, Janus wood membranes, with asymmetric special wetting property on two sides, and mechanically robust bio-based substrate with ample pores and open nano-/micro-channels, present enticing opportunities to address this challenge. In this study, we report a Janus mesoporous wood-based membrane demonstrating the capabilities for simultaneous oil/water separation, aromatic dyes removal, and seawater desalination. Thanks to the inner driving force arising from the asymmetric special wettability, unique wood microchannels for the transpiration of trees, and the synergistic effect of PPy and Ag/AgCl NPs anchored to wood matrix, Janus mesoporous wood-based membrane would promote the desired transport without any external energy input for high-efficiency seawater desalination and switchable oil/water separation. Meanwhile, the aromatic dyes and other organic compounds in industrial wastewater or polluted seawater would get more opportunities to contact with the anchored Ag/AgCl NPs during flowing through the wood microchannels. Above advantages endow the Janus mesoporous wood-based membrane with excellent pollutant degradation efficiency (MB: 86.4 %, RhB: 91.9 %), high solar conversion efficiency (95.33 %), outstanding separation efficiency towards oil-in-water emulsion (∼99.51 %), remarkable oil adsorption capacity (∼8 times of its own weight), and great antibacterial efficiencies (E. coli: 99.87 %, S. aureus: 99.78 %, B. subtilis: 99.96 %). Results also show that this Janus mesoporous wood-based membrane has the satisfactory resistance to acid and base, robust mechanical structure, good recycling stability, high salinity tolerance and remarkable self-desalting capability. This study represents a new avenue for fabrication and design of Janus wood-based photothermal materials, and certainly will contribute to alleviating the global water and energy shortage issue. Asymmetric special wettability, unique wood microchannels, and the synergistic effect of PPy and Ag/AgCl NPs anchored to wood matrix endow the as-presented Janus mesoporous wood-based membrane with excellent aromatic dyes degradation efficiency (MB-86.4 %, RhB-91.9 %), high solar conversion efficiency (95.33 %), outstanding separation efficiency towards O/W emulsion (∼99.51 %), remarkable oil adsorption capacity (∼8 times of its own weight), great antibacterial efficiencies (E. coli-99.87 %, S. aureus-99.78 %, B. subtilis-99.96 %), and remarkable salinity tolerance. It certainly will contribute to alleviating the global water and energy shortage issue. • Mechanically robust wood membrane with asymmetric special wettability, ample pores and open microchannels was fabricated. • Janus wood membrane can simultaneously complete oil/water separation, aromatic dyes removal, and seawater desalination. • PPy/Ag/AgCl NPs endow the wood with excellent MB/RhB degradation efficiency, and high solar conversion efficiency (95.33 %). • It has oil/water separation efficiency (99.51 %), oil adsorption capacity (∼8 times), and antibacterial efficiency (99.96 %). • Such Janus mesoporous wood-based membrane will contribute to alleviating the global water and energy shortage issue.

Water use dynamics of trees in a Pinus tabuliformis plantation in semiarid sandy regions, Northeast China

Quantifying tree water use and its environmental controlling mechanisms are significantly important for understanding of adaptation strategy of trees and plantation management in semiarid and arid regions. Here, tree water consumption and tree conductance of Chinese pine (Pinus tabuliformis) plantation were quantified using sap flow measurements. Environmental variables were measured concurrently during the growing seasons of 2019 and 2020. Results showed that daily, daytime and nighttime tree water consumption averaged 27.4, 24.7, and 2.7 kg d-1 across two years, respectively. Nighttime tree water consumption accounted for 10% of the daily’s, which was mainly used for recharging stem water storage deficit caused by daytime transpiration to avoid hydraulic failure. Daytime tree water consumption was more driven by solar radiation than by vapor pressure deficit. Moreover, daytime tree conductance was 0.9 mm s-1, and significantly decreased with increasing vapor pressure deficit and decreasing soil moisture, indicating that trees had strict stomatal regulation on tree transpiration. Scaling-up from tree to stand transpiration, daytime stand transpiration averaged 1.1 mm d-1 across two years, while accumulated stand transpiration was 112.8 mm and 126.8 mm in 2019 and 2020, respectively, accounting for 33% and 37% of precipitation. However, soil evaporation occupied 35% and 51% of precipitation in 2019 and 2020, respectively. This indicated that most of water in the plantation was consumed by soil evaporation. Difference between precipitation and sum of daytime stand transpiration, canopy interception, soil evaporation, and change in soil water storage was positive in two years, indicating that water budget was balanced. These findings indicated that Chinese pine trees had strict stomatal regulation on tree transpiration, and could recharge stem water storage deficit by nighttime sap flow. Moreover, stand transpiration occupied a small part of the water balance, and thus water supply was surplus, resulting in maintaining the stability of Chinese pine plantation.

Environmental Factors Driving the Transpiration of a Betula platyphylla Sukaczev Forest in a Semi-arid Region in North China during Different Hydrological Years

More and more droughts happened during the last decades, threatening natural forests in the semi-arid regions of North China. The increase in drought pressure may have an impact on stand transpiration (T) in semi-arid regions due to rising temperature and changes in precipitation. It is unclear how the transpiration of natural forest in semi-arid regions respond to drought, which is regulated by environmental factors. In this study, a relatively simple but mechanism-based forest stand T model that couples the effects of the reference T, solar radiation (Rn), vapor pressure deficit (VPD), and relative extractable water (REW) in the 0–80 cm soil layer was developed to quantify the independent impacts of Rn, VPD, and REW on T. The model was established based on the observed sap flow of four sample trees, and environmental factors were observed from May to September in different hydrological years (2015, 2017, 2018, and 2021) in a pure white birch (Betula platyphylla Sukaczev) forest stand in the southern section of the Greater Khingan Mountains, northeastern China. The sap flow data were used to calculate tree transpiration (Tt) and T to calibrate the T model. The results indicated that (1) The Tt sharply declined in the ‘dry’ year compared with that in the ‘wetter’ year. The daily Tt for small trees in the ‘dry’ year was only one-fifth of that in the ‘wetter’ year, and the daily Tt of large trees was 48% lower than that in the ‘normal’ year; (2) Large trees transpired more water than small trees, e.g., the daily Tt of small trees was 89% lower than that of the large trees in the ‘normal’ year; (3) Daily T increased with the increase in Rn, and the response conformed to a binomial function. Daily T responded to the rise of VPD and REW in an exponential function, first increasing rapidly, gradually reaching the threshold or peak value, and then stabilizing; (4) The driving factors for the T shift in different hydrological years were the REW in the ‘dry’ year, but the Rn and REW in the ‘wet’, ‘normal’, and ‘wetter’ years. The REW in the ‘wet’ and ‘wetter’ years exerted positive effects on T, but in the ‘normal’ and ‘dry’ year, exerted negative effects on T. Thus, the environmental factors affecting T were not the same in different hydrological years.