Function Of Vapor Pressure Deficit Research Articles

Less pronounced drought responses in ring-porous than in diffuse-porous temperate tree species

Tree species differ in their physiological responses to drought, but the underlying causes are often unclear. Here we explored responses of radial growth to centennial drought events and sap flow (Fs) to seasonal drought in four mixed forests on either moist or drier sites in northwestern Switzerland. While the diffuse-porous species (Fagus sylvatica, Prunus avium, Tilia platyphyllos) showed marked growth reductions in 1976 and 2003, two known marker years for severe drought, growth of the two ring-porous species (Quercus petraea and Fraxinus excelsior) was less severely affected. During a dry early to midsummer, diffuse-porous species strongly reduced Fs at the two drier sites but not (or less so) at the two moister sites. Regardless of soil moisture availability, the deep-rooting, ring-porous trees invariably down-regulated Fs to 60–70% of their maxima in response to vapour pressure deficit (VPD) and maintained similar fluxes across sites, irrespective of upper soil moisture conditions. A generalised additive model of normalised Fs as a function of VPD and soil matric potential yielded a drought-sensitivity ranking of Fs led by the two insensitive ring-porous species followed by the diffuse-porous trees (ordered by increasing sensitivity: Fraxinus excelsior < Quercus petraea < Prunus avium < Acer pseudoplatanus < Fagus sylvatica < Tilia platyphyllos). In conclusion, ring-porous tree species exhibited stronger VPD-driven stomatal control over Fs, and tree-ring formation was less sensitive to severe drought than in their neighbouring diffuse-porous species. The Fs regulation explained the greater drought tolerance of the ring-porous trees.

Optimal irrigation management for avocado (cv. 'Hass') trees by monitoring soil water content and plant water status

Irrigation scheduling based on soil water content (Ɵw) sensors requires that Ɵw be maintained within a range (management lines) that is optimal for plant growth. The lower limit or “breaking point” is determined following the soil water content dynamics on the transition of a rapid rate of depletion to a slower, under similar reference evapotranspiration. Although this criterion is practical, its implementation should be validated with plant water status measurement that contemplate weather condition, such as stem water potential “non-stressed” baseline (Ψₓ as a function of vapor-pressure deficit (VPD) in Ɵw conditions that do not limit yield). A study was conducted on a mature cv. ‘Hass’ avocado orchard in Central Chile during two seasons. There were 5 irrigation treatments: T1, Control; T2 and T3 with 29% less and 25% more of what was applied in T1, respectively; T4 and T5 same as Control until first and second fruit drop abscission, respectively, and then with 29% less. T1 trees were irrigated using a continuous frequency domain reflectometry (FDR) probe to maintain the root zone between field capacity and the breaking point. There was biweekly monitoring of the Ɵw prior to irrigation, Ψₓ and VPD. The Ψₓ decline proportional to the intensity and the timing of water restriction; however, no treatment affected the crop load in either season. T2 did not show significant detrimental in fruit size, production and maturation, despite that frequently reached water content levels at the limit of the breaking point, and showed lower levels of stem water potential than Control, being the treatment with the highest water productivity. The results confirm that breaking point is an effective criterion to establish irrigation management. Additionally, when comparing the baseline for our non-stressed trees with a baseline from full irrigation treatments obtained from the literature, 30% water savings were achieved.

Optimization of canopy resistance models for alpine meadow in the northeastern Tibetan Plateau

• Twelve combination models of the Jarvis-type model were compared for canopy resistance estimation of alpine meadows. • Six-year eddy covariance data were used to evaluate the model performance. • The model 10 (M10) performed best for the alpine meadow. • Selection of appropriate stress functions is essential for canopy resistance modeling. Canopy resistance ( r c ) is a critical parameter for estimating vegetation transpiration. The site-specific r c can be calculated using the inversed Penman-Monteith (PM) equation with the effective leaf area index (LAI), which requires meteorological and turbulent flux data. The spatial distribution of r c is difficult to characterize due to the harsh environment of the Tibetan Plateau. The Jarvis-type model for modeling r c , described as a multiplicative function of environmental variables, has been widely used. However, the differences and optimization of different Jarvis-type models for alpine meadows have not been fully addressed. Consequently, our overall objective was to determine the appropriate functions for r c estimation and improve its accuracy for the alpine meadow ecosystem. Twelve Jarvis-type models composed of different stress functions were examined and compared with the observed r c calculated using PM equation at the Arou site in the northeastern Tibetan Plateau. The results suggest that the proper air temperature function and vapor pressure deficit function could improve model performance obviously. There was no obvious difference between the two different stress functions of downward shortwave radiation. The best model (M10), which was composed of an asymptotical function of downward shortwave radiation, a linear function of air temperature, an exponential function of vapor pressure deficit and a piecewise function of soil water content, had best performance with coefficient of determination of 0.93, root mean square error of 60.2 s m −1 and Nash-Sutcliffe efficiency coefficient of 0.92. The selection of proper stress functions is important for r c modeling. Models that considered the air temperature for r c calculations produced better results than those without temperature. The sensitivity analysis of r c to environmental variables indicated that r c was most sensitive to vapor pressure deficit, followed by LAI and downward shortwave radiation, whereas r c was less sensitive to soil water content. For all optimized parameters, r c was the most sensitive to k T (a fitting parameter for temperature), followed by k D (a fitting parameter for vapor pressure deficit) and r cmin (minimum r c under the optimal physiological condition). This study addresses the selection of proper stress functions in modeling r c for the alpine meadow site, which can also provide a reference for other ecosystems.

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.

Assessment of soybean yield with altered water-related genetic improvement traits under climate change in Southern Brazil

Abstract Water deficit is a major factor responsible for soybean yield gap in Southern Brazil and tends to increase under climate change. An alternative to reduce such gap is to identify soybean cultivars with traits associated to drought tolerance. Thus, the aim of this study was to assess soybean adaptive traits to water deficit that can improve yield under current and future climates, providing guidelines for soybean cultivar breeding in Southern Brazil. The following soybean traits were manipulated in the CSM-CROPGRO-Soybean crop model: deeper root depth in the soil profile; maximum fraction of shoot dry matter diverted to root growth under water stress; early reduction of transpiration under mild stress; transpiration limited as a function of vapor pressure deficit; N2 fixation drought tolerance; and sensitivity of grain filling period to water deficit. The yields were predicted for standard and altered traits using climate data for the current (1961–2014) and future (middle-century) scenarios. The traits with greater improvement in soybean yield were deeper rooting profile, with yield gains of ≈300 kg ha−1, followed by transpiration limited as a function of vapor pressure deficit and less drought-induced shortening of the grain filling period. The maximum fraction of shoot dry matter diverted to root and N2 fixation drought tolerance increased yield by less than 75 kg ha−1, while early reduction of transpiration resulted in a small area of country showing gains. When these traits were combined, the simulations resulted in higher yield gains than using any single trait. These results show that traits associated with deeper and greater root profile in the soil, reducing transpiration under water deficit more than photosynthesis, creating tolerance of nitrogen fixation to drought, and reducing sensitivity of grain filling period to water deficit should be included in new soybean cultivars to improve soybean drought tolerance in Southern Brazil.

Influence of different cultivars–locations on maximum daily shrinkage indicators: Limits to the reference baseline approach

The use of biosensors for irrigation scheduling is an interesting way to improve water use efficiency. Trunk diameter variations (TDV) have been studied in several species as a biosensor capable of detecting water stress. The most frequently used parameter for this purpose from TDV is maximum daily shrinkage (MDS). In the present study, MDS was studied in olive trees in two locations in Spain (Badajoz and Ciudad Real), with different cultivars in each one (‘Morisca’ in Badajoz and ‘Cornicabra’ in Ciudad Real), for three years in one location and four years in the other. In order to establish a reference baseline for both locations the relationship between vapour pressure deficit (VPD) and MDS was calculated. Four irrigation treatments were used: one control, irrigated by the FAO method, two in relation to stem water potential threshold and one based on both the trunk growth and MDS from linear variable differential transducers (LVDT). Heteroscedasticity was found in MDS data, so the transformation of the square root (SQR) of the data was necessary be done. The reference baseline with SQR of MDS vs VPD was significantly different between varieties/locations. A local calculation is thus needed to obtain an accurate reference baseline. The relationship between SQR of MDS vs stem water potential (SWP) showed a lineal fit with values higher than −1.8MPa. In order to ensure that the relationship between SWP and MDS is not due to their relation to VPD, SWP and MDS were divided by their theoretical values in function of VPD. It was observed that the response of MDS to water stress is much weaker than SWP and in this range of SWP (higher of −1.8MPa) variations of MDS are only due to VPD. The number of sensors necessary for precise measurement of MDS was, on average, more than thirty.

Modelling transpiration of greenhouse gerbera (Gerbera jamesonii H. Bolus) grown in substrate with saline water in a Mediterranean climate

Gerbera plants were grown in semi-closed rockwool culture under greenhouse conditions in different seasons in a Mediterranean climate. The plants were irrigated using either fresh (FW; 1.0molm−3 NaCl) or moderately saline (SW; 9.0molm−3 NaCl) water. In autumn, NaCl concentration did not influence significantly plant growth, flower production and transpiration (E), which instead were reduced in spring in the plants irrigated with SW. In both seasons, water salinity did not affect leaf stomatal resistance (rl), which was determined by the inversion of the Penman–Monteith (PM) equation or measured with a diffusion porometer. The PM formula and two regression equations were calibrated and validated for estimating the hourly rate of daytime transpiration (Ed); a regression model was also fit to nocturnal transpiration (En). Regression models predicted Ed as a function of vapour pressure deficit (VPD) and/or the radiation intercepted by the canopy. Leaf area index (LAI), which is required by all the equations, was modelled as function of crop thermal time (i.e. growing degree days). The PM model predicted Ed using a constant value of rl. Model calibration and validation were performed using independent datasets. The irrigation with FW or SW did not require a different calibration of transpiration models. Both PM formula and regression equations provided accurate estimates of Ed; fitted equations explained between 80% and 96% of the variance in measured Ed. A linear regression of En against (LAI·VPD) accounted for 92% of measured En.

Structural Uncertainty in Model-Simulated Trends of Global Gross Primary Production

Projected changes in the frequency and severity of droughts as a result of increase in greenhouse gases have a significant impact on the role of vegetation in regulating the global carbon cycle. Drought effect on vegetation Gross Primary Production (GPP) is usually modeled as a function of Vapor Pressure Deficit (VPD) and/or soil moisture. Climate projections suggest a strong likelihood of increasing trend in VPD, while regional changes in precipitation are less certain. This difference in projections between VPD and precipitation can cause considerable discrepancies in the predictions of vegetation behavior depending on how ecosystem models represent the drought effect. In this study, we scrutinized the model responses to drought using the 30-year record of Global Inventory Modeling and Mapping Studies (GIMMS) 3g Normalized Difference Vegetation Index (NDVI) dataset. A diagnostic ecosystem model, Terrestrial Observation and Prediction System (TOPS), was used to estimate global GPP from 1982 to 2009 under nine different experimental simulations. The control run of global GPP increased until 2000, but stayed constant after 2000. Among the simulations with single climate constraint (temperature, VPD, rainfall and solar radiation), only the VPD-driven simulation showed a decrease in 2000s, while the other scenarios simulated an increase in GPP. The diverging responses in 2000s can be attributed to the difference in the representation of the impact of water stress on vegetation in models, i.e., using VPD and/or precipitation. Spatial map of trend in simulated GPP using GIMMS 3g data is consistent with the GPP driven by soil moisture than the GPP driven by VPD, confirming the need for a soil moisture constraint in modeling global GPP.

Evaluation of wheat and maize evapotranspiration determination by direct use of the Penman–Monteith equation in a semi-arid region

The Penman–Monteith (PM) equation was introduced as one of the most reliable equations to determine crop ETc, without using crop coefficient or ETo values. In this study, the PM equation was evaluated using lysimeters in a semi-arid region for wheat and maize. Different equations for aerodynamic resistance (r a) and canopy resistance (r c) were tested in the PM equation and they were ranked using statistical analysis. It was shown that the combined method of r a and r c in FAO-56 does not lead to a good prediction of ETc for wheat and maize in comparison with the lysimeter-measured data. The results indicated that a modified equation for r c was the most accurate method for both wheat and maize. Using this equation, the suggested model of FAO-56 and another investigation for r a led to the best results for wheat and maize, respectively. Furthermore, it was shown that the previously modified equation for r c was newly modified as a function of vapor pressure deficit (VPD) and the results were as accurate as before. Therefore, an equation as a function of VPD can be used when solar radiation (R n) is not available easily.

Single- and Dual-Surface Iterative Energy Balance Solutions for Reference ET

The concept of a reference evapotranspiration (ETr) calculated from daily or hourly weather data, and multiplied by a crop coefficient (Kc) in order to estimate crop water use (ETc), is widely established in agricultural science and engineering. To find region- and variety-specific values of Kc from field-measured ETc values, the equation is inverted to: Kc = ETc/ETr. Forms of the Penman-Monteith (PM) formula for calculation of reference alfalfa or grass evapotranspiration (ETr and ETo, respectively) were promulgated by ASCE in 1990, FAO in 1998, and ASCE in 2005. The PM formulations are sensitive to climatic conditions, producing estimates of ETr and ETo that are more or less close to measured values depending on regional climate, and yielding values of Kc that vary from region to region and so are not transferrable. Theoretical shortcomings may be the basis of some of these problems, including the explicit nature of the calculation, which relies on the implied assumption that canopy and air temperatures are equal. We examined the ETr estimation of two surface energy balance formulations that stipulated different air and canopy temperatures: a two-layer (soil and canopy) approach, and a one-layer (big leaf) approach that included soil heat flux. Since canopy temperature is implicit in these formulations, they must be solved iteratively. Iterative solutions of ETr were compared with the ASCE PM formulation and against lysimeter-measured ETr. All three methods of ETr estimation produced ET values that compared very well with field-measured ET for alfalfa grown under reference ET conditions. Errors may occur with any of the three approaches to ETr estimation when stomatal resistance changes due to weather conditions; thus, assumptions of constant daytime and nighttime surface resistances cause mis-estimation of surface energy fluxes. It appears that a surface resistance value of 200 s m-1 at night for alfalfa grown under reference ET conditions is too large. It also appears that assuming constant daytime surface resistance of 30 s m-1 is probably not ideal, and that presenting daytime surface resistance as a function of vapor pressure deficit might improve ETr calculation.

Up-scaling of water use efficiency from leaf to canopy as based on leaf gas exchange relationships and the modeled in-canopy light distribution

The aim of this study was to evaluate the extent to which water use efficiency (WUE) at leaf scale can be used to assess WUE at canopy scale, leaf WUE being assumed to be a constant function of vapor pressure deficit and to thus not be dependent upon other environmental factors or varying leaf properties. Leaf WUE and its variability and dependencies were assessed using leaf gas-exchange measurements obtained during two growing seasons, 1999 and 2000, at the Soroe beech forest study site on Zealand in Denmark. It was found that the VPD-normalized leaf WUE, WUE normleaf, although dependent on incoming PAR below 500 μmol m −2 s −1 is independent, both of the canopy levels and of variations in the environmental parameters. The average WUE normleaf for PAR above 500 μmol m −2 s −1 was found to be 5.5 μmol CO 2 (mmol H 2O) −1 hPa and, for the full range, 2.3 μmol CO 2 (mmol H 2O) −1 hPa. These results showed that WUE can be up-scaled from leaf to canopy on the basis of WUE normleaf and the PAR distribution within the canopy. The up-scaling conducted was based on this WUE normleaf – PAR relationship, the light distribution being assessed using the MAESTRA model, parameterized in accordance with measurements obtained for the Soroe forest. The up-scaled WUE was then compared with WUE as estimated from turbulent flux data measured above the forest with the eddy-covariance technique. The modeled daily canopy WUE obtained for daytime fluxes (6:00 AM–6:00 PM) was found to be in agreement with corresponding canopy WUE estimates based on the turbulent fluxes observed and to be dependent on VDP and light intensity alone, its thus being independent of other environmental factors. Accordingly, canopy WUE can be estimated on the basis of the up-scaled WUE relationships, provided incident PAR and VPD within the canopy are known.

Solar energy conversions in the greenhouses

Abstract For the greenhouse thermal analysis, it is essential to know the amount of solar radiation that was utilized by the greenhouse and consequently converted into sensible and latent heat via convection and evapo-transpiration. Parameters are frequently used for this purpose are: utilization efficiency factor (π), and sensible and latent heat factors (η and δ). Previous studies considered these factors as constants, although they depend on the time of the day, the environmental conditions inside and outside the greenhouse, plants and soil characteristics and the greenhouse structure, orientation and its location. These factors in addition to a proper correlation to estimate the evapo-transpiration rate in a greenhouse have not yet been investigated under the arid climatic conditions of the Arabian Peninsula. This study was to investigate π, η and δ factors; conversion of solar radiation into sensible and latent heat; and the evapo-transpiration rate (ET) in the greenhouse under arid conditions. A polyethylene-covered, evaporatively cooled, planted greenhouse with a floor area of 34 m2 was used. The parameters required for the analysis were measured in a hot summer and sunny day. The results showed that value of π was almost constant (≅0.75); whereas values of η and δ strongly depended on the net solar radiation over the plants (Sn1). ET depended on the vapor pressure deficit (VPD) much more than Sn1. Correlations to estimate ET, as a function of VPD and Sn1 and to estimate η and δ factors as functions of Sn1, were investigated. At a plant density corresponding to a leaf area index (LAI) of 3, about 54% of the integrated solar radiation that was utilized by the greenhouse was converted to sensible heat and about 46% converted to latent heat via evapo-transpiration.

Evapotranspiration components determined by sap flow and microlysimetry techniques of a vineyard in northwest China: Dynamics and influential factors

► Sap flow and evaporation were measured separately by heat balance sap flow system and micro-lysimeters in our vineyard . Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. ► Sap flow rate fluctuated diurnally and seasonally along with solar radiation and vapor pressure deficit. Volumetric soil water content to a depth of 1.0 m (VWC) influenced sap flow rate only when it fell below 0.188 cm 3 cm −3 . However, soil evaporation increased exponentially with VWC. ► When scaling vineyard transpiration from sap flow, leaf area, rather than stem diameter or vine cross-sectional area, should be selected as an indicator of individual sap flow variability. ► The sum of transpiration and evaporation was satisfactorily consistent to evapotranspiration measured by the Bowen ratio energy balance system, demonstrating the applicability of sap flow for measuring grape water use in this region. ► Soil evaporation accounted for 47.3% of total evapotranspiration. Keeping the VWC in such vineyards slightly above ∼60% of the field capacity could reduce the ratio of soil evaporation to evapotranspiration, according to the different response to VWC for sap flow rate and soil evaporation. Large areas of vineyards have been established in recent years in arid region of northwest China, despite limited water resources. Water to support these vineyards is mainly supplied by irrigation. Accurate estimation of vineyard evapotranspiration ( ET ) can provide a scientific basis for developing irrigation management. Transpiration and soil evaporation, as two main components of ET , were measured separately in a vineyard in this region by heat balance sap flow system and micro-lysimeters during the growing season of 2009. Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. Daily sap flow rate ( SR l ) increased linearly with solar radiation ( R s ), but showed an exponential increase to its maximum curve as a function of vapor pressure deficit (VPD). Residuals of the two regressions both depended on volumetric soil water content to a depth of 1.0 m (VWC). VWC also significantly influenced SR l . The relationship of them could be expressed by a piecewise regression with the turnover point of VWC = 0.188 cm 3 cm −3 , which was ∼60% of the field capacity. Conversely, soil evaporation ( E s ) increased exponentially with VWC. Thus, we recommended keeping VWC in such vineyards slightly above ∼60% of the field capacity to maintain transpiration while reducing soil evaporation. Vineyard transpiration ( T s ) was scaled from sap flow by using leaf area ( A l ) as it explained 60% of the spatial variability of sap flow. Vine transpiration was 202.0 mm during the period from April 28 to October 5; while that of E s was 181.0 mm. The sum of these two components was very close to ET estimated by the Bowen ratio energy balance method (386.9 mm), demonstrating the applicability of sap flow for measuring grape water use in this region.

A Model to Predict Diurnal Pollen Shed in Maize

ABSTRACTWe have developed a mathematical model to predict the diurnal pattern of maize (Zea mays L.) pollen shed on the basis of local meteorological conditions. Our goal is to improve simulations of maize pollen dispersion that have typically released pollen at a constant rate in contrast with measurements of pollen shed that show diurnal variation in the rate of shed. Measurements coupling pollen shed and local meteorological variables were made during controlled experiments and a 2004 field experiment to examine the influence of meteorological conditions on pollen shed. From these data, a model was developed to predict the diurnal pattern of pollen shed as a function of vapor pressure deficit, solar radiation, temperature, and the amount of pollen remaining to be shed. The model was validated by predicting the rate of pollen shed, normalized by the daily total of pollen shed, that occurred hourly for days during a 2003 field study () and results from van Hout et al. (2008; ). The model captured the general trend of pollen shed and predicted the time of peak shed within an hour of the measured peak on most days. The model, however, tended to underpredict the magnitude of the normalized peak rate of shed and did not account for secondary peaks in pollen shed that were occasionally observed. Thus, future model refinements will depend on identifying additional biological or environmental factors that impact the instantaneous rate of pollen shed.

Energy balance measurements and a simple model for estimating pecan water use efficiency

Plant water use efficiency (WUE) is defined as the net dry matter production (DM) per unit of consumptive water use. It is a vital variable for plant growth, yield, and irrigation-management models. Pecan [Carya illinoinensis (Wangenh.) C. Koch] WUE has been estimated for above ground biomass growth using evapotranspiration (ET) and DM data measured separately in different experiments. The WUE for whole pecan trees (including above and below ground parts), a direct measurement (ET and DM measured in one experiment), and a simple model for WUE based on weather conditions, are currently lacking. A 16.5 m walk-up tower in a flood irrigated pecan orchard, located in the Mesilla Valley of NM, was instrumented with energy budget and eddy flux sensors. Continuous, above canopy measurements of vertical fluxes of sensible heat, H 2O vapor, and CO 2 were made by the eddy covariance technique in growing seasons from 2002 to 2005. ET was calculated from vapor flux. DM production was calculated from CO 2 flux assuming that dry matter of pecan trees was 46.4% carbon. During the growing seasons (May through November), the mean ET was 122.7 cm (48 inch) per season; the mean dry matter production for the whole trees was 22082.3 kg ha −1 (19684.4 lb acre −1) per season. The average seasonal water use efficiency for the whole trees was 179.7 kg ha −1 cm −1 (406.5 lbs acre −1 inch −1). In ‘on’ years (high-yield years) 13.8% of the dry matter produced was allocated to the harvested nut crop, while in ‘off’ years only 8.0% was so allocated. Similarly, the nut WUE as nut yield per unit water used (as cm depth) was higher in ‘on’ years, at 26.2 kg nuts ha −1 cm −1 versus 14.9 kg nuts ha −1 cm −1. A simple model for monthly WUE (kg ha −1 cm −1) as a function of vapor pressure deficit (VPD) and relative humidity (RH) was obtained. It is anticipated that the measured WUE, the simple WUE model, and other data obtained in this study will be useful in developing and validating pecan growth, yield, and irrigation-management models.

Relationships among Ambient Temperature and Vapor Pressure Deficit and Leaf and Stem Water Potentials of Fully Irrigated, Field-Grown Grapevines

Four <i>Vitis vinifera</i> L. cultivars grown at five locations throughout California were studied to determine the relationships among temperature and vapor pressure deficit (VPD) and leaf water potential (Ψ_l) measured under clear skies at midday (solar noon) or in some instances midmorning to midafternoon. Stem water potential (Ψ_stem) was also measured on several occasions. Vines were irrigated at 100% or greater of measured or estimated vineyard evapotranspiration, and deficit or nonirrigated vines were included for comparison. Temperature and VPD were determined at the time of measurement. The highest and lowest values of Ψ_l measured on well-watered grapevines were −0.51 and −1.15 MPa, respectively. Leaf and stem water potentials were linearly related to VPD and ambient temperature. The coefficient of determination was greater for the relationship between Ψ_l and VPD (<i>R</i>² = 0.74) than ambient temperature (<i>R</i>² = 0.58). Based on the regressions, estimates of Ψ_l at a VPD of 2 and 5 kPa for fully irrigated grapevines would be −0.65 and −0.89, respectively, while those of Ψ_stem at the same VPDs would be −0.37 and −0.57 MPa, respectively. Leaf water potential of water-stressed vines was less responsive to VPD or temperature when Ψ_l values ranged from −1.2 to −1.45 MPa. The values of Ψ_l and Ψ_stem as a function of VPD or temperature could serve as baselines indicating whether grapevines are fully irrigated or not water stressed under the environmental conditions found in semiarid grapegrowing regions.

Seasonal responses of xylem sap velocity to VPD and solar radiation during drought in a stand of native trees in temperate Australia.

Xylem sap velocity of two dominant tree species, Eucalyptus crebra F. Muell. and Callitris glaucophylla J. Thompson & L.A.S. Johnson, in a native remnant forest of eastern Australia was measured in winter and summer during a prolonged (> 12 months) and extensive drought. The influence of vapour pressure deficit (VPD) and solar radiation levels on the velocity of sap was determined. Pronounced hysteresis in sap velocity was observed in both species as a function of VPD and solar radiation. However, the rotation of the hysteresis curve was clockwise for the response of sap velocity to VPD but anti-clockwise in the response of sap velocity to radiation levels. A possible reason for this difference is discussed.The degree of hysteresis (area bounded by the curve) was larger for the VPD response than the response to solar radiation and also varied with season. A simple linear model was able to predict sap velocity from knowledge of VPD and solar radiation in winter and summer. The consistent presence of hysteresis in the response to sap velocity to VPD and solar radiation suggests that large temporal and spatial models of vegetation water use may require some provision for the different responses of sap velocity, and hence water use, to VPD and solar radiation, between morning and afternoon and between seasons.

Modelling Site Productivity of Eucalyptus globulus in Response to Climatic and Site Factors

A simple model, PROMOD, predicts the growth of a forest following canopy closure, i.e. under conditions in which the foliage biomass has attained a steady state. The principal output from PROMOD is peak mean annual increment. However, additional output available includes the closed-canopy leaf area index, evapotranspiration and water use efficiency. In addition, an indication of biomass partitioning around the time of peak MAI and the relative effects different environmental factors play in limiting production can be obtained. PROMOD is based on a generalisation of a simple forest growth model which predicts biomass production and partitioning at the stand level with a daily or annual time step. The minimum level of inputs required by PROMOD are of a quality and quantity that forest managers can readily and cheaply obtain for screening prospective plantation sites: the latitude, longitude, altitude, slope and aspect of the site and a classification of the soil depth, texture, stoniness, drainage and a rating of site fertility. However, PROMOD can be run using daily inputs of weather data and hence predict the seasonal variation of production. The closed-canopy leaf area index is calculated from the mean annual rainfall and temperature at the site, and a simple rating of site fertility. Annual production is calculated as the sum of daily production and takes diurnal temperature variation and possible seasonal photosynthetic acclimation into account. A simple soil water balance model is included in which water use is based on a crop factor which is a function of soil water content and a water use efficiency which is a function of vapour pressure deficit. The model was developed on the basis of data from nine plots of Eucalyptus globulus in south-eastern Tasmania and in Western Australia, and was validated using data from 19 plots in northern Tasmania.

The effects of water availability on transpiration, water potential and growth of Picea abies during a growing season

Transpiration, pre-dawn shoot water potential and growth of Norway spruce trees were measured for one growing season, under three water regimes, giving a seasonal water input of 696 mm (irrigated), 579 mm (control) and 270 mm (drought). Potential transpiration was calculated from the Penman-Monteith equation, using maximal canopy conductance as a function of vapour pressure deficit. Sap flow was measured by a tree-trunk heat balance method and the data were expressed per unit ground surface. This allowed calculation of canopy conductance ( g c) for the three treatments. The median of the daytime g c was about 1.1 cm s −1 for non-limiting water conditions, and about 0.5 cm s −1 for the conditions of a moderate water deficit. Pre-dawn water potential did not decrease until daily transpiration rates were substantially reduced, with corresponding g c of 0.2 cm s −1. The type of control which limits transpiration is discussed. The basal-area growth was related closely to the amount of water infiltrated into the soil. The estimated long-term tree water-use efficiency (the total dry matter produced per unit of water transpired) was on average 4.8 g kg −1, with insignificant differences between the treatments. Seasonal transpiration reached 380 mm, 325 mm and 209 mm for the irrigated, control and drought treatment, respectively.

Evapotranspiration Measurement and Modeling for a Potted Chrysanthemum Crop

A crop of chrysanthemums was grown on a lysimeter in a variable shade greenhouse. Measurements were made of evapotranspiration (ET), air velocity, air temperature, air dew point temperature, carbon dioxide concentration, leaf temperature, leaf area, and photosynthetically active and global solar radiation. Comparisons were made between the evapotranspiration of the crop and the predicted water use from a computer model that used temperature, relative humidity, and solar irradiance levels as inputs (the combination model). The ET was studied as a function of vapor pressure deficit (VPD) and solar irradiance levels both separately and together. A relationship between stomatal resistance and solar irradiance levels for chrysanthemums was established.