Transpiration and early growth of tree plantations established on degraded cropland over shallow saline groundwater table in northwest Uzbekistan

This study examined the early growth and water use of tree plantations established on a marginalized irrigated cropland in northwest Uzbekistan, where salinization of agricultural soils is widespread due to shallow saline groundwater tables. During the first two growing seasons in 2003–2004, the tree stands consisting of Elaeagnus angustifolia L., Populus euphratica Oliv., and Ulmus pumila L. were irrigated with 80 mm year⁻¹, and, in 2005, were left to rely on the shallow (0.9–2.0 m deep) groundwater with a salinity of 1–5 dS m⁻¹. Soil salinity increased but remained within the range of moderate-to-strong (4–14 dS m⁻¹) during the three years. In the course of the growing season, plantations transpired 0.1–7 mm day⁻¹ in 2003 and 1–13 mm day⁻¹ in 2004–2005, as determined with the Penman–Monteith model. In the absence of irrigation, the annual stand transpiration averaged 1250, 1030, and 670 mm for E. angustifolia, P. euphratica and U. pumila, respectively. In 2005, the leaf area index of E. angustifolia ranged from 5 to 10, surpassing that of the other two species more than two-fold. Differences in canopy conductance and transpiration were significant among the tree species and the decoupling coefficient at no time exceeded 0.3, indicating strong physiological control of transpiration. The vigorous juvenile growth and high transpiration under deficit irrigation and after irrigation was terminated, suggested that afforestation with well-adapted tree species is a viable land use option for degraded cropland. The plantation responses to increasing soil salinity must be monitored to determine potential leaching demands in the long run.     

Relationships between single tree canopy and grass net radiations

It is reported a simple approach to transform daily values of grass net (all-wave) radiation (R_n, MJ m⁻² day⁻¹), as measured over standard grass surface at meteorological stations, into whole tree canopy net radiation (A, MJ tree⁻¹ day⁻¹). The revolving Whirligig device [McNaughton, K.G., Green, S.R., Black, T.A., Tynam, B.R., Edwards, W.R.N., 1992. Direct measurement of net radiation and photosynthetically active radiation absorbed by a single tree. Agric. For. Meteorol. 62, 87–107] describing a sphere about the tree measured A in five trees of different species (walnut, dwarf apple, normal apple, olives and citrus), with leaf area L_A varying from 8.65 to 40 m². For each tree, A and R_n were linearly related (A = bR_n), as previously reported elsewhere, but it was found that the slope of such regression was also a linear function of L_A or, b = 0.303 (±0.032) L_A. Consequently, the hypothesis that total daily tree canopy net radiation per unit leaf area is linearly related to grass net radiation could not be rejected after 86 days of measurements in five locations, and the empirical relationship is A = 0.303 (±0.032) R_nL_A (R² = 0.9306).     

Evaporation of intercepted rainfall from isolated evergreen oak trees: Do the crowns behave as wet bulbs?

A new approach is suggested for estimating evaporation of intercepted rainfall from single trees in sparse forests. It is shown that, theoretically, the surface temperature of a wet tree crown will depend on the available energy and windspeed. But for a fully saturated canopy under rainy conditions, surface temperature will approach the wet bulb temperature when available energy tends to zero. This was confirmed experimentally from measurements of the radiation balance, aerodynamic conductance for water vapour and surface temperature on an isolated tree crown. Net radiation over a virtual cylindrical surface, enclosing the tree crown, was monitored by a set of radiometers positioned around that surface. Aerodynamic conductance for the tree crown was derived by scaling up measurements of leaf boundary layer conductance using the heated leaf replica method. Thermocouples were used to measure the average leaf surface temperature. Results showed that a fully wet single tree crown behaves like a wet bulb, allowing evaporation of intercepted rainfall to be estimated by a simple diffusion equation for water vapour, which is not restricted by the assumptions of one-dimensional transfer models usually used at the stand scale. Using this approach, mean evaporation rate from wet, saturated tree crowns was 0.27 or 0.30 mm h^?1, when surface temperature was taken equal to the air wet bulb temperature or estimated accounting for the available energy, respectively.     

Rhizosphere soil microbial index of tree species in a coal mining ecosystem

Microbial characterization of the tree rhizosphere provides important information relating to the screening of tree species for re-vegetation of degraded land. Rhizosphere soil samples collected from a few predominant tree species growing in the coal mining ecosystem of Dhanbad, India, were analyzed for soil organic carbon (SOC), mineralizable N, microbial biomass carbon (MBC), active microbial biomass carbon (AMBC), basal soil respiration (BSR), and soil enzyme activities (dehydrogenase, urease, catalase, phenol oxidase, and peroxidase). Among the tree species studied, Aegle marmelos recorded the highest value for MBC (590 mg kg⁻¹), urease (190.5 μg NH₄⁺-N g⁻¹ h⁻¹), catalase (513 μg H₂O₂ g⁻¹ h⁻¹), dehydrogenase (92.3 μg TPF g⁻¹ h⁻¹), phenol oxidase (0.057 μM g⁻¹ h⁻¹) and BSR/AMBC (0.498 mg CO₂-C mg biomass⁻¹ day⁻¹); Tamarindus indica for mineralizable N (69.5 mg kg⁻¹); Morus alba for catalase (513 μg H₂O₂ g⁻¹ h⁻¹) and phenol oxidase (0.058 μM g⁻¹ h⁻¹); Tectona grandis for peroxidase (0.276 μM g⁻¹ h⁻¹), AMBC/MBC (99.4%), and BSR/MBC (0.108 mg CO₂-C mg biomass⁻¹ day⁻¹); Ficus religiosa for AMBC (128.4 mg kg⁻¹) and BSR (12.85 mg CO₂-C kg⁻¹ day⁻¹); Eugenia jambolana for MBC/SOC (8.03%); Butea monosoperma for AMBC/SOC (1.32%) and Azadirachta indica for BSR/AMBC (0.1134 mg CO₂-C mg biomass⁻¹ day⁻¹). Principal component analysis was employed to derive a rhizosphere soil microbial index (RSMI) and accordingly, dehydrogenase, BSR/MBC, MBC/SOC, EC, phenol oxidase and AMBC were found to be the most critical properties. The observed values for the above properties were converted into a unitless score (0–1.00) and the scores were integrated into RSMI. The tree species could be arranged in decreasing order of the RSMI as: A. marmelos (0.718), A. indica (0.715), Bauhinia bauhinia (0.693), B. monosperma (0.611), E. jambolana (0.601), Moringa oleifera (0.565), Dalbergia sissoo (0.498), T. indica (0.488), Morus alba (0.415), F. religiosa (0.291), Eucalyptus sp. (0.232) and T. grandis (0.181). It was concluded that tree species in coal mining areas had diverse effects on their respective rhizosphere microbial processes, which could directly or indirectly determine the survival and performance of the planted tree species in degraded coal mining areas. Tree species with higher RSMI values could be recommended for re-vegetation of degraded coal mining area.     

Drought constraints on transpiration and canopy conductance in mature aspen and jack pine stands

Half-hourly mean values of transpiration measured by eddy covariance over the course of six growing seasons in two boreal forest sites were used to develop stand-level relationships between transpiration and soil water content. The two sites were an aspen site on fine-textured soil and over five growing seasons for a jack pine site on coarse-textured soil in Saskatchewan, Canada. About half of the data record covered a multi-year drought that was more severe at the aspen site than the jack pine site. Measurements of transpiration and environmental variables were used to adjust a transpiration model to each site, with environmental variables retained in the model based on their capacity to improve the model adjustment. The model was also used to produce estimates of maximum canopy conductance (g_cMAX). The fit of the model to the aspen half-hourly transpiration is better than to the jack pine data (r² of 0.86 versus 0.60). Relative soil water content explains more of the variability in half-hourly transpiration at the aspen site (46%) than at the jack pine site (10%). The relationships between transpiration and environmental variables are stable throughout the drought suggesting an absence of acclimation. Published soil water modifier curves for loamy clay soils compare well with the modifier function we obtained for a similar soil at the aspen site, but the agreement between the published curve and our curve is poor for the sandy soil of the jack pine site. Values of g_cMAX computed at the half-hourly scale are greater at the aspen site (14.3 mm s⁻¹) than at the jack pine site (10.2 mm s⁻¹), but we hypothesize that the coarse soil and perennially lower water content of the jack pine site may cause this difference. Finally, we also present values of g_cMAX computed at the daily and monthly scales for use in models that operate at these time steps.     

Evapotranspiration of a young irrigated olive orchard in southern Spain

This paper assesses the relationship between evapotranspiration (ET) of olive orchards and canopy size, under both dry and wet soil conditions. Measurements of ET of a 4 ha young, uniformly drip irrigated olive cv. ‘Arbequina’ orchard were performed using the eddy covariance and the water balance techniques, for 3 years, while leaf area index (LAI) varied from 0.01 to 1. The two techniques showed excellent agreement. Daily ET varied significantly depending on soil surface wetness. Data with dry soil were used to assess the energy partition as a function of LAI. An analysis for days with and without the irrigation drippers wet spots was used to establish the olive crop coefficient (K_c) under typical “summer” conditions and its relation with LAI and ground cover. The linear model proposed here predicts olive K_c of about 0.15 for 5% ground cover, increasing to 0.3 at 25%. The presence of wet spots from drip irrigation increased K_c by 37 and 8% for ground cover fractions of 5 and 25%, respectively. Further research is needed to parameterise more sophisticated models of canopy conductance in order to estimate tree transpiration and evaporation from the soil surface independently.     

Transpiration from coppiced poplar and willow measured using sap-flow methods

Transpiration rates from poplar (Beaupré, Populus trichocarpa×deltoides) and willow (Germany, Salix burjatica) clones, grown as short-rotation coppice (three-year-old stems on four-year-old stools) at a site in south-west England, were measured through the summer of 1995. Area-averaged transpiration was estimated by scaling sap-flow rates measured in individual stems to a stand area basis using measurements of leaf area and stem diameter distribution. Sap flow in poplar was measured using the stem heat balance, heat pulse velocity and deuterium tracing techniques; in willow only the stem heat balance method was used. In June and early July the mean daily transpiration from the poplar was 6±0.5 mm day⁻¹, stomatal conductances averaged 0.33 mol m⁻² s⁻¹ for leaves in the upper layer of the canopy and daily latent heat flux often exceeded the daily net radiation flux. Similarly high transpiration was estimated for the willow. The transpiration rates were higher than any reported rates from agricultural or tree crops grown in the UK and arose because of high aerodynamic and stomatal conductances. The high stomatal conductances were maintained even when atmospheric humidity deficits and soil water deficits were large. Much lower rates (1±1 mm day⁻¹) from both clones were recorded in August at the end of a drought period.These results suggest that extensive plantation of poplar or willow short-rotation coppice will result in reduced drainage to stream flow and aquifer recharge.     

The distribution of leaf area,radiation, photosynthesis and transpiration in a Shamouti orange hedgerow orchard. Part II. Photosynthesis,transpiration, and the effect of row shape and direction

The influence of the distribution of radiation in an orange canopy on transpiration and photosynthesis was examined by developing a model of these processes.The leaf energy balance, microclimate relationships and climatic data are combined with radiation, leaf conductance, and leaf carbon uptake models to simulate orchard photosynthesis and transpiration over 2 days. Calculated hourly values of transpiration showed good agreement with measured values of sap flow in the orange orchard.Calculated carbon uptake during the six summer months was 22 kg CO₂ per tree; however, experimental estimates of annual dry matter production yield 55 kg CO₂ per tree. The calculated figure is therefore considerably in error and indicates that present information used in carbon balance modeling of Citrus is inadequate. Even so, it is shown that radiation levels deep in the canopy, where a significant amount of leaf area and transpiration is located, are too low for significant carbon uptake to occur.As an example of the usefulness of the model, the distributions of photosynthesis, transpiration and photosynthetic radiation were simulated in hedgerow canopies of three different shapes following current pruning practices in Israel. The distribution of foliage inside the given hedgerow cross-section was calculated based on the relationship of average measured foliage density to calculated diffuse photosynthetic irradiance in the canopy. The simulation was run for rows oriented north-south and east-west and for climatic conditions of midsummer. The results of the simulation indicated that: (a) The highest photosynthesis in citrus orchards is obtained by covering the largest ground areas possible with a thick canopy, i.e., maximum leaf area index (LAI). Under such conditions most photosynthesis occurs in the upper 1 m of the canopy. (b) Although rows with slanted walls do not have the highest photosynthesis, they allow more light penetration into the canopy and have productive regions on the periphery of the canopy at all heights within the orchard. (c) Whereas row orientation has little influence on total photosynthesis of the orchard, a N-S orientation allows more light penetration into rows with slanted walls and/or wide inter-row alleys, thus reducing spatial variation in the computed photosynthesis. (d) Water use of vertically pruned citrus orchards can be decreased significantly without seriously affecting photosynthesis by reducing canopy height to as low as 3 m.     

Estimating the three-dimensional structure of canopy foliage based on the light measurements in a Betula ermanii stand

In order to more accurately evaluate the functional activity of forest stands by canopy production and evapotranspiration, we improved the methods for field measurements and statistical modeling to estimate foliage configuration (spatial distribution of leaves) while simultaneously reconstructing the three-dimensional photosynthetically active photon flux density (PPFD) distribution (PPFD pattern) in a forest canopy. By using a sensor (photodiode) array, a PPFD pattern was observed in summer 2002 under the canopy in an even-aged, pure stand of Japanese mountain birch Betula ermanii Cham. (17-years old) in Hokkaido, northern Japan. A Markov chain Monte Carlo (MCMC) sampling technique is developed such that a set of foliage configurations generated by the model referred to as the Gibbs foliage canopy (GFC) approximates the field-measured PPFD pattern. The posterior distribution of the foliage configurations is generated by the parallel tempering MCMC of eight independent series of foliage configurations. The GFC model generated the posterior distribution of the LAI estimates (mean 4.56) that appeared to be appropriate in comparison to other LAI estimates of the B. ermanii stand based on the indirect and nondestructive methods by LAI-2000 (LAI = 3.43) and litterfall traps (LAI = 5.56) because they could be under- and overestimated, respectively. Our evaluations of the canopy production and evapotranspiration rates suggest that the relationship between LAI and canopy functions was not very simple because it depended on the nonlinear functional forms of the leaf responses of photosynthesis and transpiration to PPFD. The current study demonstrates an application of MCMC techniques that can generate a set of possible structures of unobserved/unobservable objects based on the high-resolution dataset obtained by some indirect (or remote-sensing) methods.     

Assessing the effects of the clumping phenomenon on BRDF of a maize crop based on 3D numerical scenes using DART model

Inverting radiative transfer (R-T) models against remote sensing observations to retrieve key biogeophysical parameters such as leaf area index (LAI) is a common approach. Even if new inversion techniques allow the use of three-dimensional (3D) models for that purpose, one-dimensional (1D) models are still widely used because of their ease of implementation and computational efficiency. Nevertheless, they assume a random distribution of foliage elements whereas most canopies show a clumped organization. Due to that crude simplification in the representation of the canopy structure, sizeable discrepancies can occur between 1D simulations and real canopy reflectance, which may further lead to false LAI values. The present investigation aims to appraise to which extent the incorporation of a clumping index (noted λ) into 1D R-T model could improve the simulations of Bidirectional Reflectance Distribution Function (BRDF). Canopy BRDF is simulated here for three growth stages of a maize crop with the Discrete Anisotropic Radiative Transfer (DART) model in the visible and near infrared spectral bands, for two contrasted soil types (dark and bright) and different levels of heterogeneity to represent the canopy structure. 3D numerical scenes are based on in-situ structural measurements and associated BRDF simulations are thus considered as references. 1D scenarios assume either that leaves are randomly distributed (λ = 1) or clumped (λ < 1). If BRDF simulations seem globally reliable under the assumption of a random distribution in near infrared, it can also lead to relative errors on the total BRDF up to 30% in the red spectral band. It comes out that the use of a clumping index in a 1D reflectance model generally improves BRDF simulations in the red considering a bright soil, which seems relatively independent of LAI. In the near infrared, best results are usually obtained with homogeneous canopies, except with the dark soil. Clearly, influent factors are mainly the LAI and the spectral contrast between soil and leaves.     

华北平原灌溉麦田水分利用效率的SEM多因素影响研究

水分利用效率(WUE)常被嵌入到多种生态系统模型中,用于评估生态系统对气候变化的响应。然而,自然条件下多种因素不仅直接影响WUE,还通过影响冠层结构等间接影响WUE,其中的影响机制仍不明晰。为了明确多种因素对冬小麦WUE的协同影响,本研究基于2015年(温暖湿润年)和2016年(温暖干旱年)涡度相关系统观测的小麦关键生育期(返青、拔节、抽穗、灌浆)的数据,分析了WUE的变化,并借助结构方程模型(SEM),以叶面积指数(LAI)为中间变量,分析了多种因素[净辐射(R_n)、空气温度(T_a)、饱和水汽压差(VPD)、风速(WS)、土壤含水量(SWC)]对WUE的影响机制。结果表明,2015年平均WUE为1.52g(C)·kg~(-1)(H_2O),2016年平均WUE为1.22g(C)·kg~(-1)(H_2O)。不管在温暖湿润年还是温暖干旱年, T_a、LAI和VPD均是影响WUE的主要因素。WUE随LAI增加而增加, Ta增加也有助于提高WUE,而当温度相近时, VPD增加会降低WUE。T_a、LAI和VPD对WUE的影响在温暖湿润年和温暖干旱年重要性程度不同,温暖湿润年最重要的影响因素为LAI,温暖干旱年为T_a; VPD在温暖湿润年既直接影响WUE,同时又通过影响LAI的变化间接作用于WUE,但在温暖干旱年仅具有直接影响。R_n在温暖干旱年和温暖湿润年表现也不相同:在温暖湿润年对WUE具有显著的影响,在温暖干旱年影响不显著,这与温暖湿润年降雨量大及降雨频次高有关。显然,模拟WUE时考虑不同年份气象条件会使结果更为准确。WS未对WUE产生显著的影响,潜在原因可能是其对冠层上部接收辐射充足的叶片影响较大,而对冠层内部叶片无显著影响。农田生态系统不同生育阶段对辐射、温度等的耐受性及响应方式不同,SEM可以将LAI设置为中间变量以综合这种阶段性的变化,因此,对于冠层结构季节变幅大的生态系统, SEM是研究其环境控制机制的有力工具。这些研究结果可为今后精确模拟生态系统WUE以及预测WUE对气候变化的响应提供科学依据。     

Responses of transpiration and canopy conductance to partial defoliation of Eucalyptus globulus trees

Partial defoliation has been shown to affect the water relations and transpiration (gas exchange) of plants. Over one growing season, the water relations in response to partial (∼45%) defoliation were examined in four-year-old Eucalyptus globulus trees in southern Australia. Daily maximum transpiration rates (E_max), maximum canopy conductance (GCmax), and diurnal patterns of tree water-use were measured over a period of 215 days using the heat-pulse technique in adjacent control (non-defoliated) and defoliated trees. Sap-flux measurements were used to estimate canopy conductance and soil-to-leaf hydraulic conductance (K_P); leaf water potential (Ψ) and climate data were also collected. Following the removal of the upper canopy layer, defoliated trees exhibited compensatory responses in transpiration rate and canopy conductance of the remaining foliage. Defoliated E. globulus had similar predawn but higher midday Ψ_l, transpiration rates (E), canopy conductance (G_C) and K_P compared to the non-defoliated controls, possibly in response to increased water supply per unit leaf area demonstrated by higher midday Ψ_l. Higher E in defoliated E. globulus trees was the result of higher G_C in the morning and early afternoon. This paper also incorporates the cumulative effect of defoliation, in a phenomenological model of maximum canopy conductance of E. globulus. These results contribute to a mechanistic understanding of plant responses to defoliation, in particular the often observed up-regulation of photosynthesis that also occurs in response to defoliation.     

Automated measurement of canopy stomatal conductance based on infrared temperature

Decreased water uptake closes stomates, which reduces transpiration and increases leaf temperature. The leaf or canopy temperature has long been used to make an empirical estimate of plant water stress. However, with a few supplemental measurements and application of biophysical principles, infrared measurement of canopy temperature can be used to calculate canopy stomatal conductance (g_C), a physiological variable derived from the energy balance for a plant canopy. Calculation of g_C requires an accurate measurement of canopy temperature and an estimate of plant height, but all of the other measurements are available on automated weather stations. Canopy stomatal conductance provides a field-scale measurement of daily and seasonal stomatal response to prevailing soil water and atmospheric conditions, and facilitates a comparison of models that scale conductance from single leaves (measured with porometers) to canopies. A sensitivity analysis of the input measurements/estimates showed g_C is highly sensitive to small changes in canopy and air temperature, and less sensitive to the other required measurements (relative humidity, net radiation, wind speed, and plant canopy height). The measurement of g_C becomes increasingly sensitive to all of the component factors as the conditions become cloudier, cooler, and more humid. We determined g_C for alfalfa and turfgrass by making the necessary environmental measurements and coupling them with a two-source (plant canopy layer and soil layer) energy balance model. We then compared these g_C values to maximum single leaf values scaled-up to the canopy level (g_CP, defined as potential canopy stomatal conductance herein) for the two crops. For both crops, g_C matched g_CP within approximately 10% after irrigation. The turfgrass g_C measurements were also compared to mean single leaf values measured with a porometer. At mid-day, g_C values were typically about double the single leaf values. Because this approach for determining g_C allows continuous, non-contact measurement, it has considerable potential for coupling with measurements of soil moisture to better understand plant–soil water relations. It also has potential for use in precision drought stress and irrigation scheduling.     

A model to simulate response of plant stomata to environmental conditions

In order to simulate plant transpiration under different field climatic conditions we have developed and verified a semi-empirical model for predicting the stomatal response to solar global radiation, leaf temperature, vapour pressure difference between the leaf and ambient air, ambient air CO₂ concentration and soil water potential. The transpiration and the stomatal relative conductance of a Nicotania Tabaccum var “samsun” plant leaves were measured in a laboratory apparatus and compared to those predicted by the model: good agreement was obtained between them for the different investigated cases. The model was incorporated in a numerical greenhouse microclimate model and its effects on the canopy microclimate are discussed here.     

Responses of net ecosystem CO2 exchange to nitrogen fertilization in experimentally manipulated grassland ecosystems

Nitrogen (N) addition enhances primary productivity of terrestrial ecosystems. However, the effects of N fertilization and/or deposition on net ecosystem CO₂ exchange (NEE) are not fully understood. The effects of N on NEE were investigated in two experimental cheatgrass ecosystems in Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs), Reno, Nevada. In this experiment, no N fertilization was added to the two EcoCELLs in the first year and two different N fertilization regimes were applied in the second year. N fertilizer was applied once to one EcoCELL (pulse fertilization, PF), and the same total amount of N in biweekly increments to the other EcoCell (gradual fertilization, GF). NEE, photosynthetically active radiation (PAR) and canopy green leaf area index (LAI) were continuously measured in the two EcoCELLs during the pretreatment and N-fertilized years. Plant N content and biomass were measured at the end of the growing season in each year. Radiation-use efficiency (RUE_CO2) was calculated as the ratio of gross ecosystem photosynthesis (GEP) to the intercepted photosynthetically active radiation (IPAR). The responses of NEE to IPAR were used to estimate the maximum ecosystem photosynthetic capacity (F_max). N fertilization stimulated canopy LAI, plant N content, F_max, RUE_CO2, NEE and biomass in both methods of N supply applications. PF led to higher LAI, F_max and NEE than GF, but both had a similar RUE_CO2 during the early growing season. GF maintained higher LAI, F_max, RUE_CO2 and NEE than PF during the late growing season. At the ecosystem level, N fertilization stimulated daily NEE directly by increasing canopy LAI, plant N content, shoot/root ratio and the maximum ecosystem photosynthetic capacity, and increased the seasonally accumulated NEE indirectly by extending the growing season. PF differed significantly from GF in its effects on NEE and RUE_CO2, possibly due to differential rates and timing of N availability. Our study suggested that these changes in the canopy RUE_CO2 and growing season under N fertilization or N deposition regimes should be considered in modeling studies of ecosystem C sequestration.     

Quantifying the uncertainties of transpiration calculations with the Penman–Monteith equation under different climate and optimum water supply conditions

The uncertainties of transpiration calculations with the Penman–Monteith equation were quantified under different climate conditions of Brazil, Germany and Israel using maize as a common crop type. All experiments were carried out under non-limiting growing conditions. Canopy resistance was determined by scaling to canopy level specific relations between in situ measurements of incident radiation and stomatal conductance using a light penetration model. The model was tested against heat-pulse measured sap flow in plant stems. The root mean square error (RMSE) of daily calculated transpiration minus measured sap flow was 0.4 mm/day. It was dominated by its variance component (variance = 0.2 {mm/day}²; bias = 0.0 mm/day). Calculated transpiration closely matched the measured trends at the three locations. No significant differences were found between seasons and locations. Uncertainties of canopy conductance parameterizations led to errors of up to 2.1 mm/day. The model responded most sensitively to a 30% change of net radiation (absolute bias error = 1.6 mm/day), followed by corresponding alterations of canopy resistances (0.8 mm/day), vapour pressure deficits (0.5 mm/day) and aerodynamic resistances (0.34 mm/day). Measured and calculated 30-min or hourly averaged transpiration rates are highly correlated (r² = 0.95; n = 10634), and the slope of the regression line is close to unity. The overall RMSE of calculated transpiration minus measured sap flow was 0.08 mm/h and was dominated by its variance component (0.005 {mm/h}²). Measured sap flow consistently lagged behind calculated transpiration, because plant hydraulic capacitance delays the change of leaf water potential that drives water uptake. Calculated transpiration significantly overestimated sap flow during morning hours (mean = 0.068 mm/h, n = 321) and underestimated it during afternoon hours (mean = ?0.065 mm/h; n = 316). The Penman–Monteith approach as implemented in the present study is sufficiently sensitive to detect small differences between transpiration and water uptake and provides a robust tool to manage plant water supply under unstressed conditions.     

Energy and water balance measurements for water productivity analysis in irrigated mango trees, Northeast Brazil

Crop water parameters, including actual evapotranspiration, transpiration, soil evaporation, crop coefficients, evaporative fractions, aerodynamic resistances, surface resistances and percolation fluxes were estimated in a commercial mango orchard during two growing seasons in Northeast Brazil. The actual evapotranspiration (E_a) was obtained by the eddy covariance (EC) technique, while for the reference evapotranspiration (E₀); the FAO Penman–Monteith equation was applied. The energy balance closure showed a gap of 12%. For water productivity analysis the E_a was then computed with the Bowen ratio determined from the eddy covariance fluxes. The mean accumulated E_a for the two seasons was 1419 mm year⁻¹, which corresponded to a daily average rate of 3.7 mm day⁻¹. The mean values of the crop coefficients based on evapotranspiration (K_c) and based on transpiration (K_cb) were 0.91 and 0.73, respectively. The single layer K_c was fitted with a degree days function. Twenty percent of evapotranspiration originated from direct soil evaporation. The evaporative fraction was 0.83 on average. The average relative water supply was 1.1, revealing that, in general, irrigation water supply was in good harmony with the crop water requirements. The resulting evapotranspiration deficit was 73–95 mm per season only. The mean aerodynamic resistance (r_a) was 37 s m⁻¹ and the bulk surface resistance (r_s) was 135 s m⁻¹. The mean unit yield was 45 tonne ha⁻¹ being equivalent to a crop water productivity of 3.2 kg m⁻³ when based on E_a with an economic counterpart of US$ 3.27 m⁻³. The drawback of this highly productive use of water resources is an unavoidable percolation flux of approximately 300 mm per growing season that is detrimental to the downstream environment and water users.     

Use of a greenhouse as an open chamber for canopy gas exchange measurements: Methodology and validation

Measurements of whole-canopy gas exchange - of CO₂ and H₂O - are important for agricultural and ecological reasons. The objective of this study was to investigate the use of a full-size greenhouse as an open-chamber system for measuring canopy-scale gas exchange. Measurements were validated by comparison with gas exchange scaled up from leaf- and plant-level measurements. Leaf-level measurements used a conventional hand-held cuvette gas exchange system at many points in the greenhouse. The experiments were done in a greenhouse with an area of (15 × 24) m² in which a pepper crop was grown. Within the canopy photosynthetic activity and transpiration changed with height, as expected. In addition, it was shown both theoretically and experimentally that in the absence of air mixing within the chamber, gradients of CO₂ and H₂O developed along the airflow direction. Theoretical estimates of the gradients were in good agreement with measured values. In spite of the gradients, canopy photosynthesis and transpiration could be estimated relatively accurately. For instance, the values of canopy photosynthesis and transpiration, during the course of the day, as measured with the open-chamber approach, were in good agreement with mean values obtained from measurements on individual leaves. However, transpiration values obtained both from open-chamber measurements and from individual leaves were generally a little lower than those obtained with lysimeters.     

Plant water relations in lychee: Effects of solar radiation interception on leaf conductance and leaf water potential

The effect of radiation interception on leaf conductance and leaf water potential in six-years old lychee trees (Litchi chinensis Sonn. cv. Bengal) was investigated during the dry season in subtropical Queensland, Australia. A high degree of exposure of leaves to direct radiation raised leaf-air water vapour concentration gradient (Δw) and resulted in lower leaf conductance and leaf water potential. Interior leaves of the south side of trees were less sensitive to atmospheric and radiation effects and are the best indicator of drought stress in lychee. Completely random or stratified sampling is necessary to estimate a true mean value for calculation of canopy transpiration or photosynthesis.     

Improved legume tree fallows and tillage effects on structural stability and infiltration rates of a kaolinitic sandy soil from central Zimbabwe

Improved legume tree fallows have great potential to increase soil organic carbon (SOC), aggregate stability and soil infiltration rates during the fallowing phase. However, persistence of the residual effects of improved fallowing on SOC, aggregate stability and infiltration rates, under different tillage systems in Zimbabwe is not well documented. The relationships between SOC, aggregate stability and infiltration in fallow-maize rotation systems are also not well documented. We therefore evaluated effects of tillage on SOC, aggregate stability and infiltration rates of a kaolinitic sandy soil during the cropping phase of an improved fallow-maize rotation system. Plots that were under legume tree fallows (Sesbania sesban; Acacia angustissima), natural fallow (NF) and under continuous maize during the previous 2 years were divided into conventional tillage (CT) and no-till (NT) subplots soon after fallow termination, and maize was cropped in all plots during the following two seasons. Aggregate stability was investigated using water stable macroaggregation index (I_ma), water dispersible clay (WDC) and using the mean weight diameter (MWD) after different wetting procedures. Infiltration rates were determined using simulated rainfall at intensity of 35 mm h⁻¹ on 1 m² plots. Soil organic carbon was significantly higher (P < 0.05) under fallows than continuous maize. For the 0–5 cm depth SOC was 11.0, 10.0, 9.4 and 6.6 g kg⁻¹ for A. angustissima, S. sesban, NF and continuous maize, respectively, at fallow termination. After 2 years of cropping SOC was 8.0, 7.0, 6.1 and 5.9 g kg⁻¹ under CT and 9.1, 9.0, 8.0 and 6.0 g kg⁻¹ under NT for A. angustissima, S. sesban, NF and continuous maize, respectively. Aggregate stability was significantly greater (P < 0.05) under fallows than under continuous maize and also higher under NT than under CT. The macroaggregation index (I_ma) for the 0–5 cm depth was 466, 416, 515 and 301 for A. angustissima, S. sesban, NF and continuous maize, respectively at fallow termination, decreasing to 385, 274, 286 and 255 under CT and 438, 300, 325 and 270 under NT, for A. angustissima, S. sesban, NF and continuous maize, respectively, after 2 years of cropping. Percent WDC was also significantly lower (P < 0.05) in fallows than in continuous maize, and for the 0–5 cm it was 11, 10, 8 and 17 for A. angustissima, S. sesban, NF and continuous maize, respectively at fallow termination. After 2 years of cropping WDC (%) was 12, 14, 15 and 17 under CT and 10, 12, 12 and 16 under NT for A. angustissima, S. sesban, NF and continuous maize, respectively. MWD also showed significantly higher (P < 0.05) aggregate stability in fallows than in continuous maize. Water infiltration rates were significantly greater under fallows than continuous maize but these declined significantly during the cropping phase in plots that had been fallowed. In October 2000, infiltration rates in the A. angustissima and NF plots were above 35 mm h⁻¹ as no runoff was observed. Steady-state infiltration rates were 24 mm h⁻¹ in S. sesban and 5 mm h⁻¹ for continuous maize after 30 min of rainfall simulations. After 2 years of cropping infiltration rates remained above 35 mm h⁻¹ in A. angustissima plots, but declined to 18 and 8 mm h⁻¹ for NF, CT and NT respectively and 12 mm h⁻¹ for S. sesban, CT and NT. It is concluded that legume tree fallows improved SOC, aggregate stability and infiltration rates, but these benefits accrued during fallowing decreased significantly after 2 years of cropping following the termination of fallows. The decrease in SOC and aggregate stability was higher under CT than NT. Coppicing fallows of A. angustissima were the best long-term fallow species when integrated with NT as improved soil physical properties were maintained beyond 2 years of post-fallow cropping.