Transpiration Characteristics Research Articles

Experimental Evaluation of Effect of Leaves on Railroad Tracks in Loss of Braking

This study aims to comprehensively assess the lubrication effect of leaves on wheel–rail contact dynamics using the Virginia Tech-Federal Railroad Administration (VT-FRA) Roller Rig, which closely simulates field conditions with precision and repeatability. Railway operators grapple with the seasonally recurring challenge of leaf contamination, which can cause partial loss of braking and lead to undesired events such as station overruns. Better understanding the adhesion-reducing impact of leaf contamination significantly improves railway engineering practices to counter their effects on train braking and traction. This experimental study evaluates the reduction in traction and braking forces (collectively called “adhesion”) as a function of leaf volume, using two leaf species that commonly grow along U.S. railroad tracks. The test methods rely on the chosen leaves’ transpiration characteristics while ensuring the result’s reproducibility. Leaves were symmetrically positioned on the wheel surface, centered around the mid-rib area within the wear band, and taped on the edges far from the wear band. The critical test parameters (i.e., wheel load, wheel velocity, and percentage creepage) are kept constant among the tests. At the same time, leaf volume is reduced from a maximum amount that covers the entire wheel surface (100% coverage) to no leaves (0%). The latter is used as the baseline. The percentage creepage is kept constant at an exaggerated amount of 2% to accelerate the test time. The results indicate a nonlinear relationship between leaf volume and the loss of braking. Even a small amount of leaf contamination causes a significant reduction in adhesion by as much as 50% compared with no contamination (i.e., baseline). Increasing leaf volume results in contact saturation, beyond which adhesion is not reduced. The minimum adhesion observed in this study is 20% of the maximum adhesion that occurs when no leaf contamination is present.

Modelling of sap flux density of oak in a humid region in China

AbstractTranspiration plays a vital role in determining the watershed water cycle. However, we still have little knowledge of the characteristics of tree transpiration in the Hanjiang River Basin, which is the water source for the middle route of South‐to‐North water diversion project. Here, we measured sap flux density of oak trees (Quercus, the dominant species here) at the 10‐min resolution for 2 years and explored its response to the environmental conditions. The incoming short‐wave radiation and vapour pressure deficit well explained the variation of daytime sap flux density, and a statistical model was then proposed to calculate the daytime sap flux density correspondingly; then a nighttime sap flux density module was proposed based on the daytime sap flux density calculation. Sap flux density showed clear counter‐clockwise hysteresis response to incoming short‐wave radiation, and clockwise hysteresis response to vapour pressure deficit. Our sap flux density model can well reproduce the corresponding hysteresis response to incoming short‐wave radiation and vapour pressure deficit. This study unravelled the environmental controls of sap flux density of the oak trees in the Hanjiang River Basin, proposed an efficient model for the sap flux density simulation, provided important knowledge for understanding the corresponding forests' water use, which is of critical significance in determining the water availability for the middle route of the South‐to‐North water diversion project.

Multi-objective optimization of transpiration cooling for high pressure turbine vane

Transpiration cooling is a promising cooling measure for high-temperature components in gas turbines. Although the flow and heat transfer characteristics of transpiration cooling have been investigated in several experimental and numerical researches, the design optimization of transpiration cooling is less studied until now. In this paper, a multi-objective optimization framework based on the ANN surrogate model and the NSGA-Ⅱ is built to optimize the transpiration cooling of C3X turbine vane. The transpiration cooled C3X vane consists of multiple porous zones whose porosities could be different and are treated as the design parameters. The optimization objectives are minimum area-averaged vane surface temperature and minimum aerodynamic loss. A total of 960 CFD simulations are conducted to prepare the sample dataset. After the validation of the surrogate-based optimization framework, the Pareto front is obtained as an output. Three representative Pareto-optimal points are chosen from the Pareto front, considering the best cooling performance, the lowest aerodynamic loss, and a balance between them. The porosity distributions and coolant allocations, the cooling performances, and the aerodynamic losses for the three Pareto points are discussed, and the involved flow and heat transfer characteristics are analyzed. The results reveal that to reach a balance between the cooling efficiency and the aerodynamic loss for an optimal transpiration cooled C3X vane, coolant should be appropriately distributed to the leading edge, as well as zones immediately downstream of the leading edge.

Potassium deficiency enhances imbalances in rice water relations under water deficit by decreasing leaf hydraulic conductance.

Potassium (K+) is an essential macronutrient for appropriate plant development and physiology. However, little is known about the mechanisms involved in the regulation of leaf water relations by K under water deficit. A pot experiment with two K supplies of 0.45 and 0 g K2O per pot (3 kg soil per pot) and two watering conditions (well-watered and water-deficit) was conducted to explore the effects of K deficiency on canopy transpiration characteristics, leaf water status, photosynthesis, and hydraulic traits in two rice genotypes with contrasting resistance to drought. The results showed that K deficiency reduced canopy transpiration rate by decreasing stomatal conductance, which led to higher canopy temperatures, resulting in limited water deficit tolerance in rice. In addition, K deficiency led to further substantial reductions in leaf relative water content and water potential under water deficit, which increased the imbalance in leaf water relations under water deficit. Notably, K deficiency limited leaf gas exchange by reducing leaf hydraulic conductance, but decreased the intrinsic water use efficiency under water deficit, especially for the drought-resistant cultivar. Further analysis of the underlying process of leaf hydraulic resistance revealed that the key limiting factor of leaf hydraulic conductance under K deficiency was the outside-xylem hydraulic conductance rather than the xylem hydraulic conductance. Overall, our results provide a comprehensive perspective for assessing leaf water relations under K deficiency, water deficit, and their combined stresses, which will be useful for optimal rice fertilization strategies.

Integrating machine learning and empirical evapotranspiration modeling with DSSAT: Implications for agricultural water management

The availability of accurate reference evapotranspiration (ETo) data is crucial for developing decision support systems for optimal water resource management. This study aimed to evaluate the accuracy of three empirical models (Hargreaves-Samani (HS), Priestly-Taylor (PT), and Turc (TU)) and three machine learning models (Multiple linear regression (LR), Random Forest (RF), and Artificial Neural Network (NN)) in estimating daily ETo compared to the Penman-Monteith FAO-56 (PM) model. Long-term data from 42 weather stations in Florida were used. Moreover, the effect of ETo model selection on sweet corn irrigation water use was investigated by integrating simulated ETo data from empirical and ML models using the Decision Support System for Agrotechnology Transfer (DSSAT) model at two locations (Citra and Homestead) in Florida. Furthermore, a linear bias correction calibration technique was employed to improve the performance of empirical models. Results were consistent in that the NN and RF models outperformed the empirical models. The empirical models tended to underestimate and overestimate small and high daily ETo values, respectively, with the HS model exhibiting the least accuracy. However, calibrated PT and TU models performed comparably to the ML models. Results also revealed that using an inappropriate ETo model could lead to over-irrigation by up to 54 mm during a single crop season. Overall, ML models have proven reliable alternatives to the PM model, especially in regions with access to long-term data due to their site-independent performance. In areas without long-term data for ML model training and testing, calibrating empirical models is viable, but site-specific calibration is needed. It is important to highlight that distinct plant species exhibit varying transpiration characteristics and, consequently, have different water requirements. These differences play a pivotal role in shaping the overall impact of ETo models on crop water use.

Transpiration characteristics and environmental controls of orange orchards in the dry-hot valley region of southwest China

Transpiration characteristics and environmental controls of orange orchards in the dry-hot valley region of southwest China

Nocturnal versus diurnal transpiration in rice plants: Analysis of five genotypes grown under different atmospheric CO2 and soil moisture conditions

The role of nocturnal transpiration in plant growth is debatable, presumably due to the soil moisture-dependent characteristics of transpiration. To address this issue, we analyzed the relationship between nocturnal and diurnal transpiration and plant growth across various soil moisture and atmospheric CO2 levels. We investigated the ratio of nocturnal to diurnal transpiration under eight soil moisture conditions (30 %, 35 %, 40 %, 50 %, 60 %, 80 %, 100 %, and 120 % [w/w]) and three CO2 concentrations (approximately 407, 610, and 843 ppm) in five rice genotypes (cv. Nipponbare, IRAT109, NERICA 1, Taichung 65 [T65], and DV85). The total plant biomass increased linearly with increase in diurnal transpiration but showed a saturated response to increase in nocturnal transpiration, suggesting that nocturnal transpiration did not contribute to growth when the soil water content was above 60 %. CO2 enrichment did not decrease the nocturnal transpiration rate (En) but decreased the diurnal transpiration rate (Ed), thereby causing an increase in the En /Ed value. The En /Ed value was the lowest when the soil water content was approximately 50 %. The graphical curves representing the change in En /Ed values plotted along with the soil water content showed a V-shaped pattern for all genotypes examined. Furthermore, the patterns of variation of En /Ed values with respect to changes in soil moisture contents were similar under different CO2 concentrations. Therefore, we concluded that suppression of nocturnal transpiration in the absence of drought would be an important target for reducing water consumption without compromising biomass production, and estimation of the En /Ed values will help in selecting individuals showing low nocturnal transpiration regardless of the CO2 concentration in the environment.

High temporal and spatial resolution characteristics of evaporation, transpiration, and evapotranspiration from a subalpine wetland by an advanced UAV technology

Wetland evapotranspiration (ET), which involves the land–atmosphere exchange of energy and water, is dynamic and affects the spatiotemporal distribution of water resources. However, due to the variability and complexity of wetlands, accurate estimation of patch-scale ET and its spatial variability remain insufficiently characterized. To overcome this challenge, an advanced unmanned aerial vehicle (UAV) technology was developed by combining the three-temperature (3T) model, which is robust to estimate transpiration and its spatial variability with UAV-based thermal infrared remote sensing, and Penman equation, which is commonly used to estimate open water evaporation. The combined approach was verified using the Bowen ratio system over a subalpine wetland. The results show that the proposed method is simple and applicable for estimating wetland ET and its spatial variability, with a determination coefficient (R2) of 0.93, mean absolute percentage error (MAPE) of 7.90%, root mean squared error (RMSE) of 0.05 mm h−1, and Nash-Sutcliffe efficiency (NSE) of 0.93. It depicts a large spatial variability in wetland ET with respect to surface vegetation characteristics, water regimes, meteorological factors, and larger transpiration rates than open water evaporation. With its limited inputs and no calibration requirements, the proposed method is concluded to be simple and to easily reveal the high temporal and spatial resolution characteristics of patch-scale ET and its components.

Water use pattern and transpiration of Mongolian pine plantations in relation to stand age on northern Loess Plateau of China

Understanding the water use pattern and transpiration rate of revegetation species is crucial for efficient water management and sustainable vegetation restoration. Mongolian pine (P. sylvestris) plantation has been implemented for decades in desert regions of northwest China, achieving significant improvements in combating soil erosion and desertification. However, the characteristics of water use pattern and transpiration of P. sylvestris at different ages remain poorly understood. We investigated seasonal variations in water uptake pattern and transpiration of different-aged P. sylvestris (4-, 10-, and 18-yr) based on stable isotope and sap flow measurements over two growing seasons. The canopy transpiration of P. sylvestris and its discrepancy between the dry and rainy season increased with increasing stand age. The 4-yr P. sylvestris extracted water mainly from shallow soil layer (0–120 cm) across the growing season (63.21–69.40%). In contrast, the older P. sylvestris (10- and 18-yr) displayed a greater degree of ecological plasticity as they shifted water uptake to deeper soils and groundwater in times of drought. In the rainy season, 10-, and 18-yr P. sylvestris extracted water predominantly from shallow soil layer (68.62%) and shallow plus middle soil layers (0–200 cm) (66.04%), respectively; in the dry season, however, they both shifted main water sources to the middle plus deep soil layers (120–300 cm) (61.29% and 63.45%). The older P. sylvestris also increased absorption of groundwater in the dry season, leading to more severe soil desiccation and faster groundwater decline. The changes in water use patterns of P. sylvestris were related to differences in root distribution and biomass, seasonal variations of plant transpiration, and changes in soil water condition. As a consequence, the variations and controls of age-dependent plant water uptake and transpiration should be considered to implement the corresponding management measures in future reforestation activities to achieve sustainable development goals.

Differences in Transpiration Characteristics among Eucalyptus Plantations of Three Species on the Leizhou Peninsula, Southern China

How much transpiration water consumption varies between eucalyptus species is unknown, making the suitability of a particular eucalyptus species for large-scale planting in a given area, or whether interspecific differences need to be taken into account for eucalyptus water consumption estimates, uncertain. Here, Eucalyptus camaldulensis Dehnh. (Ec), Eucalyptus pellita F. v. Muell. (Ep), the most resistant species, and Eucalyptus urophylla S.T. Blake × Eucalyptus grandis Hill ex Maiden (Eug), the most widely planted species, were monitored for sap flow. Their stand transpiration was also estimated and its relationship to various influencing factors analyzed for the same stand age and site, and predictive models for daily transpiration (T) developed. The results showed that the T of all eucalyptus species was jointly influenced by meteorological factors, soil water content (SWC), and leaf area index (LAI), with great variation in the T response to each influencing factor among species. Accordingly, we developed species-specific transpiration prediction models that could adequately explain the changed T of each species (R2-values: 0.863–0.911). There were significant differences in the stand daily mean sap flow density (JC) and transpiration among the three species. Although Ec had a significantly lower JC than Ep, it was significantly higher than Eug on all timescales, where the mean annual JC of Ep (0.11 cm min−1) was 1.4 and 2.6 times that of Ec (0.08 cm min−1) and Eug (0.042 cm min−1), respectively. Transpiration of Eug was significantly less than Ep, but significantly greater than Ec on all timescales, where the annual transpiration of Ep (743.41 mm) was 2.4 and 1.5 times that of Ec (311.52 mm) and Eug (493.58 mm), respectively. These results suggest that interspecific differences cannot be ignored when estimating transpiration rates in Chinese eucalyptus plantations, whose amount of water use should be considered when choosing the most optimal species to plant regionally.

Numerical investigation on the transient transport and heat transfer characteristics of transpiration cooling with liquid phase change during coolant adjustment

• A new transient model is proposed for phase-change transpiration cooling. • Dynamic fluid–solid coupling heat transfer during coolant adjustment is studied. • Dramatic variation of injection pressure with coolant mass flux is simulated. • Influence of coolant phase change location on transport characteristic is analyzed. Aimed at learning the variation law of injection pressure with coolant mass flux and promoting the active control of phase-change transpiration cooling, this study proposes a new transient mathematical model by simplifying the two-phase mixture model (TPMM) with the introduction of modified mixture enthalpy and enthalpy ratio. The smooth change with time of mixture enthalpy and enthalpy ratio improves the convergence and stability of TPMM. Experiment is first carried out to verify the accuracy of the new model. Then the two-phase flow and corresponding fluid–structure coupling heat transfer processes of transpiration cooling are studied with the new model under both steady and transient conditions. Besides, the relationship link from coolant adjustment, corresponding pressure and mass flux to the cooled wall temperature is discussed. The steady results show that injection pressure is closely related with phase change location, and the huge change of fluid kinematic viscosity and pumping effect of capillary force induces the non-linear variation of injection pressure with coolant mass flux. The transient results reveal that injection pressure changes with different laws when the mass flux increases and decreases. The changing speed of mass flux and scale of external heat flux also have a vital influence on the fluctuation of injection pressure, the faster the mass flow rate changes and the greater the heat flux is, more tempestuously the injection pressure varies.

亚热带岩溶区典型常绿和落叶树种的蒸腾特征及其对环境因子的响应

植物蒸腾是水循环的重要组成部分，为了解亚热带岩溶区树木的蒸腾耗水情况，探究气候和水文地质条件对植物蒸腾的影响，运用Granier热耗散探针技术，对亚热带岩溶区次生林内的常绿树种女贞（L.lucidum）和落叶树种刺槐（R.pseudoacacia）的树干液流进行了连续监测，并同步监测了气象因子及土壤含水率（SMC），探讨在不同时间尺度下两种生活型树种的蒸腾特征及其对环境因子的响应。结果表明：（1）在季节尺度下，影响两树种整树蒸腾量（ET）的主要因子为太阳辐射强度（Rs）、气温（T）和水汽压亏缺（VPD）；女贞蒸腾量（ET_L）表现为夏季（1.29 kg/h） > 春季（0.57 kg/h） > 冬季（0.15 kg/h） > 秋季（0.13 kg/h），刺槐蒸腾量（ET_R）表现为夏季（0.90 kg/h） > 春季（0.31 kg/h） > 秋季（0.16 kg/h） > 冬季（0.04 kg/h）。（2）在日尺度下，晴天两树种ET呈现出明显的单峰日变化，且主要影响因子均为T、VPD和Rs；但由于常绿和落叶树种的生理特征差异，降雨时ET_L受到抑制，而ET_R则显著提升。（3）从昼夜层面来看，两树种夜间蒸腾量不足日蒸腾总量的35%。在夜雨现象和树木生理特征的影响下，秋冬季夜间蒸腾量占比明显高于春夏季，刺槐的平均夜间蒸腾量及其占比（1.56 kg，24.1%）高于女贞（1.08 kg，13.9%）。;Plant transpiration is an important part of water cycle in ecosystem, that is, the process of water transfer from soil to atmosphere through plants. It is driven by solar radiation, air temperature, air humidity and wind speed, and also affected by soil water storage capacity. Understanding and quantifying the influence mechanism of transpiration is an important basis to explore the water consumption of vegetation and the water balance of ecosystem. However, although the climate conditions in the subtropical karst area are good, there is a drought period of 4-5 months. At the same time, plants are often affected by water stress because of the thin soil layer and low water holding capacity of soil in karst area. The study of plant transpiration characteristics helps us understand the water consumption of trees in subtropical karst areas, and is also of great significance for exploring the impact of climate and hydro-geological conditions on plant transpiration. Granier's thermal dissipation probe method was used to continuously monitor the trunk sap flow of the evergreen tree species (L.lucidum) and the deciduous tree species (R.pseudoacacia) in the secondary forest in the subtropical karst area. The meteorological factors and soil moisture content (SMC) are also monitored simultaneously. Our purpose is to analyze the transpiration characteristics of two life-form tree species and their responses to environmental factors on different time scales. The results show that:(1) at the seasonal scale, the main factors affecting the whole-tree transpiration (ET) are solar radiation intensity (Rs), air temperature (T) and vapor pressure deficiency (VPD). The transpiration of L.lucidum (ET_L) is shown as summer (1.29 kg/h) > spring (0.57 kg/h) > winter (0.15 kg/h) > autumn (0.13 kg/h), transpiration of R.pseudoacacia (ET_R) is shown as summer (0.90 kg/h) > spring (0.31 kg/h) > autumn (0.16 kg/h) > winter (0.04 kg/h). (2) On the daily scale, the ET of the two tree species shown obvious single peak curve on sunny day, and the main factors affecting transpiration are T, VPD and Rs. However, due to the differences in the physiological characteristics of evergreen and deciduous tree species, ET_L is weakened during rainfall, while ET_R is significantly increased. (3) in terms of day and night, the nocturnal transpiration of the two tree species accounts for less than 35% of the total daily transpiration on average. Affected by the phenomenon of night precipitation and the physiological characteristics of trees, the proportion of nocturnal transpiration in the autumn and winter is significantly higher than that in the spring and summer. At the same time, the average daily nocturnal transpiration and its proportion of R.pseudoacacia (1.56 kg, 24.1%) is higher than that of L.lucidum (1.08 kg, 13.9%).

Global Ecopotential of Dendrochronology

Drought is a meteorological phenomenon that has influenced the life of human society for ages and still has a great impact on it. This phenomenon can be divided into a group of sub-phenomena, e.g. soil and air droughts, winter droughts, etc.… From a physiological point of view, drought may have different consequences in different periods of a growing season. It seems however difficult to describe quantitative characteristics of a physiologically important drought using any meteorological parameters because within an ecosystem the results of the interaction of ecological factors may change the way plants respond to a certain limited number of values of meteorological parameters. Photosynthesis and transpiration characteristics predetermine that the function modelling annual tree ring variability will always and under any conditions have a variable that will depend on drought. The objective of dendroclimatic studies can be either to separate a drought-associated climatic signal from the tree ring chronology or, on the contrary, to search a chronology with a “clear” and easily detected drought-associated signal. Having retrospective aspects, Dendroclimatic studies show considerable promise for the evaluation of droughts including those that have an impact on crop yields. Such studies can give further information for crop yield forecasts.

Distinct transpiration characteristics of black locust plantations acclimated to semiarid and subhumid sites in the Loess Plateau, China

Black locust (Robinia pseudoacacia) is widely planted throughout semiarid and subhumid regions of the Loess Plateau of China. Determining the changes in transpiration of this species in different climatic areas is important for revealing the acclimation mechanism of black locust and developing suitable forest management practices, particularly in the context of global climate change. Here, sap flow and canopy conductance of black locust plantation trees in semiarid (Yan’an) and subhumid (Yongshou) sites were quantified using Granier-type thermal dissipation probes and concurrent environmental observations from 2012 to 2017. Several physiological parameters were measured throughout the growing season. The results showed that sap flow was correlated with phenological factors across seasons within a year. However, interannual changes in sap flow were affected mainly by the reference evapotranspiration (ET0) at the Yongshou site, and jointly by precipitation (P), soil water content, and P/ET0 at the Yan’an site. Sap flow response to meteorological factors showed less discrepancy between pre- and post-rainfall periods at the Yan’an site. Moreover, canopy conductance fluctuated less with a wider range of vapor pressure deficit (VPD) and the slope of canopy resistance as a function of VPD was lower, indicating relatively lower sensitivity of stomatal conductance to environmental factors in Yan’an site. Physiological parameters, except for predawn leaf water potential, were significantly different between the two sites. The results suggested that black locust tended to reduce transpiration, modify leaf morphology, and improve water use efficiency to enhance its adaptability to the dryer site. The species changes stomatal regulation characteristics and general growth rate to acclimatize to distinct water habitats.

基于Penman-Monteith方程模拟青海云杉生长季日蒸腾过程

青海云杉（Picea crassifolia）作为我国黄土高原与青藏高原地区的主要造林树种，对其林分蒸腾耗水特征的研究，能够更合理的指导该地区植被重建与林分调控，以加强林分的稳定性，提高水分利用效率。为了揭示青海云杉在生长季内的冠层蒸腾规律以及冠层整体气孔阻力与环境因子的响应，评价Penman-Monteith方程在青海云杉冠层尺度上的适用性，采用探针式热扩散茎流计（TDP）进行测定，同步长期监测了环境数据，利用反推法建立冠层整体气孔阻力（r_sT）与环境因子之间的回归模型，结合Penman-Monteith方程模拟出青海云杉生长季的日蒸腾量，采用均方根误差、平均绝对误差和平均相对误差对蒸腾量的实测值与模拟值进行误差分析，以验证模型的准确性。得出的主要结论有：（1）生长季内青海云杉日蒸腾量随月份变化呈先增高后降低的趋势；各月蒸腾量占潜在蒸散量的比例为7月（79.68%） > 8月（72.71%） > 6月（72.53%） > 5月（67.08%） > 9月（66.48%） > 10月（64.29%）；（2）树干液流对气象因子的滞后时间为0.5 h；（3）不同月份云杉冠层整体气孔阻力（r_sT）与空气相对湿度（RH）呈正相关关系，与大气温度（T）、饱和水汽压差（VPD）呈负相关关系；（4）应用所建立的多因素回归模型结合Penman-Monteith方程对青海云杉蒸腾量进行模拟验证，累计平均相对误差为14.381%，平均绝对误差为0.160 mm，均方根误差为0.2。综上所述，Penman-Monteith方程在林分冠层尺度上有较好适用性根据所建立的多因素回归模型并结合Penman-Monteith方程，可以利用饱和水汽压差、温度和空气相对湿度三个气象因子较好地模拟日蒸腾过程。;Picea crassifolia is the main afforestation tree species in the Loess Plateau in China. Vegetation reconstruction and forest stand regulation can be guided reasonably by studying water consumption characteristics in this area. It is important for forest stand to strengthen stability and to improve water use efficiency (WUE). In order to reveal the canopy transpiration characteristics of Picea crassifolia during the growing season and the response of the overall canopy stomatal resistance to environmental factors, and to understand the equation of Penman-Monteith whether or not suitable for scale of canopy of Picea crassifolia, we used thermal dissipation probe (TDP) to measure sap flow of Picea crassifolia and monitored the factor of environment by weather station (DAVIS). Then we framed the regression model between Canopy overall stomatal resistance (r_sT) and the environmental factors by inverse method. Lastly, we used equation of Penman-Monteith to predict the transpiration of Picea crassifolia during the growing season. The Root Mean Squared Error, Mean Absolute Deviation and Mean Relative Error would be the indexes to test and verify the accuracy of model we framed when we compared the predicted value with the measured value. The results show that (1) the daily evapotranspiration of Picea crassifolia increases firstly and then decreases with the monthly changes during the growth season. The ratio of monthly evapotranspiration to potential evapotranspiration is Jul (79.68%) > Aug (72.71%)> Jun (72.53%) > May (67.08%) > Sep (66.48%) > Oct (64.29%). (2) There is an obvious Lag-time phenomenon between sap flow and meteorological factors about 0.5 h. (3) The r_sT is positively correlated with relative humidity (RH), and negatively correlated with atmospheric temperature (T) and saturated water vapor pressure difference (VPD). (4) We applied the established multi-factor regression model and Penman-Monteith equation to simulate and verify transpiration of Picea crassifolia. The results indicated that the cumulatively average relative error is 14.381%, the average absolute error is 0.160 mm, and the root mean square error is 0.2. The simulation effect is basically feasible. In summary, the equation of Penman-Monteith has good applicability on the stand canopy scale. We can accord to the established multi-factor regression model combined with the equation of Penman-Monteith, three meteorological factors of saturated water VPD, T and RH can be used to simulate the evapotranspiration on any day, avoiding the inconvenience of long-term observation.

Estimation of the transpiration of urban shrubs using the modified three-dimensional three-temperature model and infrared remote sensing

Urban shrubs are one of the main components of urban vegetation and play a significant role in mitigating the urban heat island (UHI) through transpiration. As there are no suitable methods to measure the transpiration rate of urban shrubs, many issues related with urban transpiration rate and UHI are still not properly understood. In this study, the two-dimensional (2D) three-temperature (3T) model is extended to three-dimensional (3D) scale and applied to estimate the transpiration rate of typical urban shrubs from May to December in 2018. Three common species of shrubs are observed in this study, namely Lagerstroemia indica cv “Bush”, Podocarpus macrophyllus var. angustifolius, and Ligustrum vicaryi. Estimated transpiration rates are proofed by isotope method. Results show that (1) the modified 3D-3T model is a reliable method for estimating urban shrub transpiration (R2 = 0.61, MAE = 0.10 mm·h−1); (2) urban shrubs have a quite high transpiration rate, ranging from 0.2 to 0.6 mm·h−1 in the daytime; (3) the rate of transpiration varies greatly in different directions of the shrub canopy (top, south, north, east, and west); (4) the 3D-3T model is able to better reflect transpiration characteristics under the heterogeneous condition in urban environments. To the best of our knowledge, the improved 3D-3T model is one of the very few methods that can measuring the transpiration rate of urban shrubs. The results from this study provide meaningful information and a useful tool for measuring urban evapotranspiration and urban planning.

Role of Species and Planting Configuration on Transpiration and Microclimate for Urban Trees

Research Highlights: To demonstrate the effectiveness of configuration modes and tree types in regulating local microclimate. Background and Objectives: Urban trees play an essential role in reducing the city’s heat load. However, the influence of urban trees with different configurations on the urban thermal environment has not received enough attention. Herein we show how spatial arrangement and foliage longevity, deciduous versus evergreen, affect transpiration and the urban microclimate. Materials and Methods: We analyzed the differences between physiological parameters (transpiration rate, stomatal conductance) and meteorological parameters (air temperature, relative humidity, vapor pressure deficit) of 10 different species of urban trees (five evergreen and five deciduous tree species), each of which had been planted in three configuration modes in a park and the campus green space in Xi’an. By manipulating physiological parameters, crown morphology, and plant configurations, we explored how local urban microclimate could be altered. Results: (1) Microclimate regulation capacity: group planting (GP) > linear planting (LP) > individual planting (IP). (2) Deciduous trees (DT) regulated microclimate better than evergreen trees (ET). Significant differences between all planting configurations during 8 to 16 h were noted for evergreen trees whereas for deciduous trees, all measurement times were significantly different. (3) Transpiration characteristics: GP > LP > IP. The transpiration rate (E) and stomatal conductance (Gs) of GP were the highest. Total daily transpiration was ranked as group planting of deciduous (DGP) > linear planting of deciduous (DLP) > group planting of evergreen (EGP) > linear planting of evergreen (ELP) > isolated planting of deciduous (DIP) > isolated planting of evergreen (EIP). (4) The microclimate effects of different tree species and configuration modes were positively correlated with E, Gs, and three dimensional green quantity (3DGQ), but weakly correlated with vapor pressure deficit (VpdL). (5) A microclimate regulation capability model of urban trees was developed. E, Gs, and 3DGQ could explain 93% variation of cooling effect, while E, Gs, VpdL, and 3DGQ could explain 85% variation of humidifying effect. Conclusions: This study demonstrated that the urban heat island could be mitigated by selecting deciduous broadleaf tree species and planting them in groups.

Sap flow characteristics in growing and non-growing seasons in three tree species in the semiarid Loess Plateau region of China

Year-round observation suggests that sap flow in growing season represents species-specific transpiration characteristics and sap flow of deciduous broadleaved species in non-growing season is related to daily changes in stem diameter. Year-round observation is necessary to investigate species-specific characteristics of sap flow and its possible contribution in non-growing season. The middle Loess Plateau in China is characterized by temperate climate and vegetation cover of generally drought-tolerant deciduous broadleaved and some evergreen coniferous trees. In this study, Granier-type thermal dissipation probes were applied to measure year-round xylem sap flow in three species from April 2017 to April 2018. Solar radiation, air temperature, relative air humidity, precipitation, and soil moisture were monitored continuously. The results showed that the peak time of sap flux density (Fd) in the diurnal courses of Robinia pseudoacacia was later than that of Quercus liaotungensis and Platycladus orientalis. The peak time of Fd for R. pseudoacacia was significantly postponed during the period with higher soil moisture when compared with that at lower soil moisture, whereas the peak time was reached slightly earlier in the other two species. Sap flow during the non-growing season was relatively higher in P. orientalis than in the two broadleaved species. The diurnal course of Fd in non-growing season showed high values at nighttime and low values in daytime, opposite to that in growing season. Daily variation in stem diameter was inverse of the change in sap flow in the two broadleaved species, whereas the daily change in stem diameter of P. orientalis was very small and showed no clear relation with sap flow. The results suggest that changes in transpiration characteristics with soil moisture conditions vary with tree species; sap flow in deciduous broadleaved trees in non-growing season is small and closely related to daily changes in stem diameter.

半干旱区城市环境下油松林分蒸腾特征及其影响因子

PDF HTML阅读 XML下载 导出引用 引用提醒 半干旱区城市环境下油松林分蒸腾特征及其影响因子 DOI: 10.5846/stxb201811262571 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家科技支撑计划项目（2015BAD07B06-4） The stand transpiration characteristics of Pinus tabulaeformis and its influential factors in a semi-arid urban environment Author: Affiliation: Fund Project: The research and demonstration on the cultivation and management technology system of water-saving urban landscape forests in Inner Mongolia and Beijing 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:在城市环境下，由于不透水地面面积的增加，土壤-植物-大气之间水汽循环减弱，水汽调节能力差，因而研究城市树木蒸腾对环境因子的响应对于城市进行合理的水汽调节具有重要意义。于2017年生长季，在内蒙古呼和浩特市区树木园内选择58年生油松（Pinus tabulaeformis Carr.）作为研究树种，采用热扩散法测定其树干液流，并同步监测气象因子和土壤含水量变化，利用彭曼公式计算冠层气孔导度。结果表明：（1）生长季内，油松林分蒸腾存在明显日、月变化，晴天天气下林分蒸腾日变化呈单峰曲线，月林分蒸腾量5月最大，其次是7月、8月、6月和9月，分别为20.96、19.89、18.09、17.25 mm和7.49 mm。（2）油松林分蒸腾与饱和水汽压差、太阳总辐射、土壤含水量和风速均存在极显著相关关系（P<0.01），太阳总辐射、饱和水汽压差和土壤含水量是影响林分蒸腾的主要环境因子（R2=0.47、R2=0.31和R2=0.16），风速对林分蒸腾的影响程度最小（R2=0.12）；不同降雨量对林分蒸腾的影响作用不同，10 mm以上的日降雨量对油松林分蒸腾作用明显。（3）除环境因子外，油松叶片气孔通过响应环境变化控制蒸腾作用，当饱和水汽压差<1.5 kPa时，叶片气孔对饱和水汽压差的响应更敏感；当太阳总辐射<250 W/m2时，叶片气孔对蒸腾起促进作用，超过该阈值，叶片气孔关闭从而抑制树木蒸腾。 Abstract:The water vapor cycle between soil-plant-atmosphere is weakened and its ability for regulation is poor due to the increase in impervious surface areas in urban environments. Therefore, it is of great significance to study the transpiration response of urban trees to environmental factors amidst reasonable water vapor regulation in urban areas. In the growing season of 2017, 58-year-old Pinus tabulaeformis Carr. was selected as a research tree species for sap flow measurement, by thermal diffusion, at the Hohhot arboretum, Inner Mongolia. We measured meteorological factors and soil water content simultaneously and estimated canopy stomatal conductance using the Penman Formula. The results showed that (1) stand transpiration of P. tabulaeformis had obvious daily and monthly changes in the growing season. The diurnal stand transpiration variation graph showed a single peak curve on sunny days. Monthly stand transpiration reached the maximum in May, followed by July, August, June and September, which was 20.96, 19.89, 18.09, 17.25 mm and 7.49 mm, respectively. (2) There were extreme significant correlations between stand transpiration and vapor pressure deficits, global radiation, soil water content, and wind speed (P<0.01). Global radiation, vapor pressure deficit, and soil water content were the main environmental factors affecting forest transpiration (R2=0.47, R2=0.31 and R2=0.16), and wind speed had the smallest impact on forest transpiration (R2=0.12). Different rainfall events had varied effects on stand transpiration, but a significant effect when daily rainfall exceeded 10 mm. (3) In addition to environmental factors, leaf stomata of P. tabulaeformis controlled transpiration in response to environmental factors. Leaf stomata were more sensitive to vapor pressure deficits when it was < 1.5 kPa, whereas, global radiation, when < 250 W/m2, promoted transpiration. When the threshold was exceeded, leaf stomata would close to suppress tree transpiration. 参考文献 相似文献 引证文献

Time lag of stem sap flow and its relationships with transpiration characteristics in Quercus liaotungensis and Robina pseudoacacia in the loess hilly region, China

Using Granier-type thermal dissipation probes (TDP), we measured stem xylem sap flow of the natural dominant species Quercus liaotungensis and a reforestation species Robinia pseudoacacia from July to September in 2016 in the semiarid loess hilly region. Meteorological factors and soil water content were simultaneously monitored during the study period. Using cross-correlation analysis, time lag between diurnal patterns of sap flux density and vapor pressure deficit (VPD) was quantitatively estimated. Differences in the time lag between the two species and possible influence by different diameter classes and soil water contents were analyzed. The results showed that the diurnal courses of sap flux density were similar to those of meteorological factors, with daily peaks ear-lier than VPD. The peak of VPD lagged behind the sap flux densities of Q. liaotungensis and R. pseudoacacia 118.2 min and 39.5 min, respectively. The peak of PAR lagged behind the sap flux density of Q. liaotungensis 12.4 min, but was 68.5 min ahead of that for R. pseudoacacia. Time lag between sap flux density and VPD significantly varied between tree species and was affected by soil water content. Those during higher soil water content period were about 32.2 min and 68.2 min longer than those during the period with lower soil water content for the two species, respectively. There was no correlation between time lag and tree diameter classes. The time lag between VPD and sap flux density for R. pseudoacacia was about 21.4 min longer in smaller diameter trees than in larger trees, which was significantly different under the lower soil water content. Our results suggested that the time lag effect between VPD and sap flux densities in the two species reflected their sensitivities to driving factors of transpiration, and that higher soil water content was favorable to sap flux density reaching its peak early. The lower soil water content might lead to lower sensitivity of the trees to meteorological factors. R. pseudoacacia was more sensitive to changes of soil water content.