Foliage Temperature Research Articles

Evaporative cooling of a ventilated greenhouse rose crop

Greenhouse crops may suffer from excessive heat load in warm weather. A procedure is developed to evaluate latent heat cooling by means of crop transpiration and free water evaporation from a wet pad and fan system. The procedure uses concurrent external climatic factors as input data. It treats construction characteristics (dimensions and radiometric properties of the roof cover), plant foliage (leaf area, stomatal conductance) and ventilation rate as parameters to calculate heat transport coefficients. Measurements in a greenhouse rose crop ( Rosa indica L cv ‘Mercedes Long’) show that the numerical solution of the energy balance equation predicts accurately crop transpiration, foliage temperature, air temperature and humidity inside the greenhouse. With ventilation rates of 30 volume changes per hour and external air humidity below 50%, transpiration of a plant well supplied with water, cools the foliage and the air in the greenhouse below external temperature even when solar radiation is at its maximum value. Cooling obtained with an evaporative wet pad at the air inlet lowers vapour pressure deficit in the greenhouse and decreases transpiration rate. Still, total latent heat dissipation added to pad evaporation and crop transpiration is higher than that obtained by crop transpiration without the wet pad. The combined solution of the energy balance of the air passing through the evaporative pad and of the crop predicts accurately transpiration of the rose crop and internal temperature and humidity. The evaporative pad cools the air considerably; but the lowering of transpiring leaf temperature is only minor. Evaporation from the pad decreases when external humidity increases. Crop transpiration rate when the wet pad operates is nearly independent of external humidity and ventilation rate.

Canopy directional emissivity: Comparison between models

Land surface temperature plays an important role in many environmental studies, as for example the estimation of heat fluxes and evapotranspiration. In order to obtain accurate values of land surface temperature, atmospheric, emissivity and angular effects should be corrected. This paper focuses on the analysis of the angular variation of canopy emissivity, which is an important variable that has to be known to correct surface radiances and obtain surface temperatures. Emissivity is also involved in the atmospheric corrections since it appears in the reflected downwelling atmospheric term. For this purpose, five different methods for simulating directional canopy emissivity have been analyzed and compared. The five methods are composed of two geometrical models, developed by Sobrino et al. [J. A. Sobrino, V. Caselles, & F. Becker (1990). Significance of the remotely sensed thermal infrared measurements obtained over a citrus orchard. ISPRS Photogrammetric Engineering and Remote Sensing 44, 343–354] and Snyder and Wan [W. C. Snyder & Z. Wan, (1998). BRDF models to predict spectral reflectance and emissivity in the thermal infrared. IEEE Transactions on Geoscience and Remote Sensing 36, 214–225], in which the vegetation is considered as an opaque medium, and three are based on radiative transfer models, developed by François et al. [C. François, C. Ottlé, & L. Prévot (1997). Analytical parametrisation of canopy emissivity and directional radiance in the thermal infrared: Application on the retrieval of soil and foliage temperatures using two directional measurements. International Journal of Remote Sensing 12, 2587–2621], Snyder and Wan [W. C. Snyder & Z. Wan (1998). BRDF models to predict spectral reflectance and emissivity in the thermal infrared. IEEE Transactions on Geoscience and Remote Sensing 36, 214–225.] and Verhoef et al. [W. Verhoef, Q. Xiao, L. Jia, & Z. Su (submitted for publication). Extension of SAIL to a 4-component optical–thermal radiative transfer model simulating thermodynamically heterogenous canopies. IEEE Transactions on Geoscience and Remote Sensing], in which the vegetation is considered as a turbid medium. Over surfaces with sparse and low vegetation cover, high angular variations of canopy emissivity are obtained, with differences between at-nadir view and 80° of 0.03. Over fully vegetated surfaces angular effects on emissivity are negligible when radiative transfer models are applied, so in these situations the angular variations on emissivity are not critical on the retrieved land surface temperature from remote sensing data. Angular variations on emissivity are lower when the emissivity of the soil and the emissivity of the vegetation are closer. All the models considered assume Lambertian behaviour for the soil and the leaves. This assumption is also discussed, showing a different behaviour of directional canopy emissivity when a non-Lambertian soil is considered.

Heat flux parameterization for sparse and dense grasslands with the analytical land-atmosphere radiometer model (alarm)

The dependence of radiometric surface temperature (\(\theta\)s) on view angle and the unclear definition of the aerodynamic temperature, which is the temperature that gives the correct sensible heat flux estimate at a specified roughness length, bring about a challenge in estimating sensible heat flux from \(\theta\)s. An analytical-land-atmosphere-radiometer model (ALARM) has been developed to convert \(\theta\)s taken at any zenith view angle to a clearly defined equivalent isothermal surface temperature, \(\theta\)i, at a defined scalar roughness length. ALARM is an analytical model based on K-theory that links the foliage temperature profile to the radiometric surface temperature and the temperature ‘felt’ by the turbulent lower atmosphere. ALARM has previously been applied with slightly different values of its parameters to several grassland sites of varying canopy density. Our objective in this study was to apply ALARM to these and to one additional dataset with a single parameterization. When compared to the reference (measured) values of sensible heat flux H, ALARM estimates of H had root mean square errors of about 35 W m-2. These results were comparable to those from two other simple canopy models also tested with these datasets.

RED MANGROVES EMIT HYDROCARBONS

Abstract The objective of this study is to investigate hydrocarbon species and amounts released by red mangrove foliage and determine if these quantities warrant future research on atmospheric chemical processing of these compounds. The field investigation took place during July 2001 at Key Largo, Florida Bay, Florida. Foliage still attached to plants was enclosed in cuvettes while air of known flow rates circulated around leaves to study hydrocarbon emissions. Cuvette air samples underwent gas chromatographic analyses to determine species and amounts of hydrocarbons released by mangrove foliage. Red mangrove foliage emits isoprene and trace amounts of the monoterpenes of α-pinene, β-pinene, camphene, and d-limonene. The mangrove flowers released these latter compounds in amounts ranging from 0.5 to 10 mg (monoterpene) per gram of dry biomass per hour. These fluxes are normalized to the foliage temperature of 30 °C. When normalized to the foliage temperature of 30 °C and light levels of 1000 µmol m−2 s−1,...

Carbon Dioxide and Temperature Effects on Evapotranspiration and Water Use Efficiency of Soybean

Rising CO2 and potential global warming will cause changes in evapotranspiration (ET). Our research objective was to determine the impact of CO2 and air temperature on canopy ET, water use efficiency (WUE), foliage temperature, and canopy resistance (Rc) of soybean [Glycine max (L.) Merr.]. Plants were grown in sunlit, controlled‐environment chambers at cyclic maximum/minimum air temperatures from 28/18°C to 44/34°C and CO2 of 350 or 700 μmol mol−1 Maximum ET rate in the early afternoon at 35 d after planting ranged from 7.5 mol m−2 s−1 at 28/18°C to 19.0 mol m−2 s−1 at 44/34°C. Daily ET during the middle of the season ranged from 260 mol H2O m−2 d−1 (4.7 mm d−1) at 28/18°C to 660 mol H2O m−2 d−1 (11.9 mm d−1) at 44/34°C. Mean daily ET was linearly related to mean air temperature (Tair) as: [Mean daily ET = 21.4 × Tair − 306, r2 = 0.99 (mol H2O m−2 d−1), or mean daily ET = 0.385 × Tair − 5.5 (mm d−1)]. Doubled CO2 caused a 9% decrease in ET at 28/18°C, but CO2 had little effect at 40/30°C or 44/34°C. Whole‐day WUE declined linearly with air temperature, with a slope of −0.150 [(μmol CO2 mmol−1 H2O) °C−1]. Changes in ET and WUE were governed by changes in foliage temperature and Rc. In conclusion, increases in temperature anticipated by climate change could more than offset decreases of ET that would be caused by increases in CO2

Remote Sensing Research in Hydrometeorology

An overview of remote sensing research in hydrometeorology, with an emphasis on the major contributions that have been made by United States Department of Agriculture-Agricultural Research Service (USDA-ARS) scientists, is provided. The major contributions are separated into deriving from remote sensing (1) hydrometeorological state variables and (2) energy fluxes, particularly evapotranspiration which includes plant water stress. For the state variables, remote sensing algorithms have been developed for estimating land surface temperatures from brightness temperature observations correcting for atmospheric and emissivity effects, estimating near-surface soil moisture from passive microwave remote sensing, determining snow cover from visible and snow water equivalent from microwave data, and estimating landscape roughness, topography, vegetation height, and fractional cover from lidar distancing technology. For the hydrometeorological fluxes, including plant water stress, models estimating evapotranspiration have been developed using land surface temperature as a key boundary condition with recent schemes designed to more reliably handle partial vegetation cover conditions. These research efforts in estimating evapotranspiration with remotely sensed surface temperatures have been utilized by ARS researchers in the development of the Crop Water Stress Index and Water Deficit Index for assessing plant water stress. In addition, the development of the Thermal Kinetic Window and Crop Specific Temperatures have revealed the dynamic interactions among foliage temperature, plant species, and the physical environment. ARS researchers continue to develop new and improved remote sensing algorithms for evaluating state variables and fluxes. Moreover, they are involved in new research directions to address science questions impeding hydrometeorological research. These include investigating tie utility of combining multifrequency remote sensing data for improved estimation of land surface properties, and incorporating remote sensing for evaluating the effects of landscape heterogeneity on atmospheric dynamics and mean air properties and resulting feedbacks on the land surface fluxes.

Relating satellite multiangular thermal infrared observations to soil and foliage temperature

The anisotropy of thermal infrared (TIR) radiance emitted by terrestrial targets comprising a mixture of soil and foliage is well documented by both ground and space — based radiometric data. Since TIR radiance depends on surface temperature and surface emissivity, observed anisotropy can be interpreted in terms of either anisotropy in emissivity or, for heterogeneous terrestrial targets, of differences in component temperatures. This paper focuses on the latter. A simple linear mixing model is proposed to relate the component temperatures of soil and foliage to the radiance emitted by a soil — foliage mixture. The underlying assumptions are briefly reviewed. It is finally shown that the simple linear model can be used to retrieve soil and foliage component temperatures from radiometric observations at different view angles. This is illustrated with a case — study based on TIR radiometric data collected by the Along Track Scanning Radiometer (ATSR-2) on board the ERS- 2 during the SGP'97 and HEIFE experiments.

Conversion of radiometric to aerodynamic surface temperature with an anisothermal canopy model

A land surface‐atmosphere‐radiometer model was applied to field data at the First International Satellite‐Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). The model converts from radiometric surface temperature taken at any zenith view angle to an equivalent isothermal (or aerodynamic) surface temperature. With data from one day at a grassy site with a leaf area index of 1.0, values of the three unknowns in the model, which describe the shape of the vertical foliage temperature profile, were found which best agree with reference radiometric and aerodynamic surface temperatures at the site. Having shown that the model can be optimized to be consistent with field data, the model was applied to data taken at the same site on the next day. In this case the model was not optimized but was used in a predictive mode to convert from a radiometric to an aerodynamic surface temperature. The aerodynamic temperatures predicted by the model were closer to the reference values (that is, estimates of aerodynamic temperature obtained by inverting Monin‐Obukhov Similarity Theory) than were the radiometric surface temperatures in all cases, for an average improvement of 3.6 K.

Analytical Land–Atmosphere Radiometer Model

Abstract Conversion of radiometric land surface temperature (θr) to an equivalent isothermal (aerodynamic) surface temperature (θi) is important in balancing the land surface energy budget with satellite-based θr measurements. An analytical land–atmosphere radiometer model (ALARM) has been developed to convert θr taken at any zenith view angle to θi at a specified scalar roughness length z0h,i. Field data from 1987 and 1989 at the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) were used to evaluate the performance of ALARM. It was possible to find an optimal foliage temperature profile such that ALARM is consistent with these radiometric and atmospheric field data. The errors were significantly less when radiometer zenith angles were less than 40°. The foliage temperature at the canopy top θfh and the foliage temperature profile curvature parameter b were parameterized as functions of air temperature and leaf area index, respectively. Using these parameteri...

Study on the law of radiant directionality of row crops

The style of crops planting is frequently in row-structure, the row-structure style may result in big difference among the sunlit, shaded soil surface and foliage temperatures and cause pixel component to vary in azimuth orientation, these further lead to the change of radiant directionlity of row crops in the zenith and azimuth orientations. Since the row crops are often tackled as isotropic in the azimuth orientation based on continuous vegetation assumption, big errors will be brought about. In order to eliminate the errors, it is necessary to study the law of radiant directionality of the row crops. In this paper, Monte Carlo method has been employed to simulate the angular effects on radiation caused by row architecture parameters. The simulated results show that the parameters, for example, row height, row width, row interval between the neighbor rows and the leaf area index have significant influences on the radiant directionality, but the azimuth orientation ranks the first among the parameters.

Canopy Microenvironments and Applied Bacteria Population Dynamics in Shaded Tall Fescue

Shading of the turfgrass canopy is assumed to support increased microbial activity and therefore more potential disease problems. The goal of this research was to determine how shading affects microenvironmental conditions, and how these conditions affect phylloplane colonization by applied bacterial strains. Shade cloth was suspended over tall fescue (Festuca arundinacea Schreb.) turf in field plots and environmental parameters were measured in shaded and full sun areas. Shading increased leaf wetness duration and relative humidity (RH), but decreased the average and range of canopy air and foliage temperatures. Three species of bacteria (Bacillus megaterium, Stenotrophomonas maltophilia, and Enterobacter cloacae) were applied to all plots. Populations of B. megaterium were the lowest of the three species in both study years. Data showed that measurable differences in microclimate occur as a result of shading and that bacterial colonization was generally favored by shading. The lack of consistency between the two assay methods and among bacterial species regarding the effects of shading suggests that differences in response to microclimate conditions may exist among bacterial species or strains. Results indicate that shading should be considered when screening biological control agents for disease management.

Theoretical relationship between stomatal resistance and surface temperatures in sparse vegetation

The relationship between stomatal resistance and foliage temperature in sparse vegetation has been the subject of previous papers [Smith, R.C.G., Barrs, H.D., Fischer, R.A., 1988. Agric. Forest. Meteorol. 42, 183–198; Shuttleworth, W.J., Gurney, R.J., 1990. Q. J. R. Meteorol. Soc. 116, 497–519], in which the modeling is based upon the one-dimensional two-layer approach of Shuttleworth and Wallace [Shuttleworth, W.J., Wallace, J.S., 1985. Q. J. R. Meteorol. Soc. 111, 839–855]. In both studies, however, a major assumption exists concerning the contribution of the substrate to the evaporation process. Using the same approach as these previous studies, an extended and upgraded model is presented in the sense that it relates stomatal resistance to foliage and substrate temperatures ( T f and T s) without any assumption on substrate contribution. A comparison of stomatal resistances estimated from component temperatures ( T f and T s) with values measured on fallow savannah during the HAPEX–Sahel experiment confirms the good performance of the model. Numerical simulations show the general behavior of the relationship between stomatal resistance and foliage temperature in several scenarios involving various weather conditions and canopy characteristics. The sensitivity of the calculated stomatal resistance to input variables and model parameters is investigated. It is shown that the calculation of stomatal resistance exhibits a significant sensitivity to foliage temperature and a much lesser one to substrate temperature. Uncertainties in leaf area index have a relatively weak impact on the calculated stomatal resistance. The sensitivity of stomatal resistance to the two main coefficients involved in the partitioning of available energy has also been investigated.

429 Detecting Incipient Water Stress in Buffalograss and Kentucky Bluegrass through Foliage Temperature using Lowcost Infrared Sensors

Infrared sensors were used to quantify canopy temperature and thus detect differences in incipient water stress between a cool-season grass [Kentucky bluegrass (KBG) (Poa pratensis)] and a warm-season grass [buffalograss (BG) (Buchloe dactyloides)]. The infrared sensors, connected to a datalogger, measured average hourly leaf–air temperatures (TL–TA) 1 m above eight replicate plots of Kentucky bluegrass and eight replicate plots of buffalograss. Air temperature and relative humidity from a nearby weather station were used to calculate the average hourly vapor pressure deficit (VPD). In late July, we ceased irrigating and measured TL–TA and soil water content while allowing the turf to dry down for 5 weeks. Soil water content was measured with a neutron probe. Both species exhibited a significant relationship between TL–TA and VPD. As the VPD increased, TL–TA decreased in both species (KBG r2 = 0.73, BG r2 = 0.71) on the 2nd day after an irrigation during well-watered conditions. An artifact was created on the first day after an irrigation as a result of excessive surface evaporation. KBG and BG were similar under well-watered conditions. KBG had a higher TL–TA after 4 to 5 days without irrigation. By contrast, BG did not have a higher TL–TA until 25 to 30 days without irrigation. Part of BG's drought avoidance was extraction of soil water down to 0.9 m vs. 0.45 m for KBG.

Estimation of Volatile Organic Compound Fluxes Using the Forest–Land–Atmosphere Model (FLAME)

Abstract The prognostic one-dimensional Forest–Land–Atmosphere Model (FLAME) has been further extended to simulate diurnal cycles of volatile organic compound (VOC) fluxes inside and above an idealized mixed forest mainly composed of oaks and pines. The tree height is 12 m and the leaf area index is 4. The canopy crown is divided into five layers of equal leaf area increment. The algorithms developed by Guenther et al. are applied to predict the monoterpene emission from leaves in each canopy layer. The value of photosynthetic active radiation (PAR) and foliage temperature (Tleaf) required by these algorithms are provided by FLAME. The modeled PAR and Tleaf are used for different leaf angle classes i from sun to shaded leaves within each layer j in the canopy. In the present study the authors examine the VOC fluxes modeled at a reference level above the forest for two cases, A and B, for which the modeled canopy temperature Tc reaches a maximum of approximately 21° and 31°C, respectively. It is supposed t...

Uncertainties in Isoprene Emissions from a Mixed Deciduous Forest Estimated Using a Canopy Microclimate Model

Abstract Utilizing the concepts of localized near-field diffusion, a modeling system was developed to estimate isoprene emissions from foliage of a mixed deciduous forest. The model determined radiation disposition and foliage temperature inside the canopy using as input forcing variables measured above the forest. Such model outputs were then combined with an isoprene emission algorithm to estimate emissions from the forest ecosystem and thus assess uncertainties in inventory emissions. Under conditions of atmospheric stability, modeled temperature in the bottom half of the forest stand was overestimated by as much as 4°C. For unstable conditions modeled and measured temperatures agreed more closely, the model overestimating temperature by less than 2°C. Based on an uncertainty analysis, parameters exerting the greatest influence on calculated leaf temperature and light levels inside the forest included radiation extinction coefficients and the partitioning between sunlit and shaded leaves. The uncertain...

592 Use of Infrared Thermometry and Heat-pulse Technique for Detection of Water Stress in Apple Trees

The recent development of small portable infrared thermometers has made canopy temperature an easily measured characteristc in the field. Our objective was to correlate a reduction of soil water with foliage temperature and to compare it with other indicators of plant stress (Pn, E, gs, leaf expansion, sap flow). During Summer 1998, we evaluated the responses of potted apple rootstocks (cultivars Budagowski 9, M9, and Mark) to soil water deficit. Irrigation was withheld for 7 days, and the canopy temperature (Tc) was measured daily with an infrared camera. Tc was always higher than air temperature (Ta). Tc between control and stress plants began to differentiate from day 3. In Mark, this difference was maintained until the end of the experiment. However, gas exchange in Mark seemed to be less affected by the stress than in the other two cultivars. At day 7, midday stomatal conductance (gs) was 38.0, 32.3, and 72.0 mmol·m–2·s–1 in Budagowski 9, M9, and Mark, respectively (control values varied between 161.6 and 164.3 mmol·m–2·s–1 for all the cultivars). Heat-pulse sapflow sensors installed on Mark indicated that the speed of the xylem sap was affected by the stress from day 4 (19-26 cm/h for the controls vs. 15–21 cm/h for the stressed plants). Specific details on the physiological data will be presented.

Radiometric and equivalent isothermal surface temperatures

The analytical solution for the heat flux from an anisothermal canopy developed from K theory by Brutsaert and Sugita [1996] (hereinafter referred to as B&S) has been extended to provide a parameterization of the difference between the radiometric and the equivalent isothermal surface temperature. The latter is the isothermal temperature at which a canopy would give the correct sensible heat flux if the sensible heat roughness length z0h was given its theoretical, isothermal value derived by B&S. A parameterization of the canopy temperature seen by a radiometer is developed which incorporates the same foliage temperature profile as that used by B&S. From this the weighting fraction (i.e., the fraction of canopy top versus canopy bottom temperature “seen” by the radiometer) can be derived, as can an equation for the view zenith angle at which the radiometric and equivalent isothermal surface temperatures are the same. Field data indicate that multiangle radiometric surface temperatures alone cannot determine the three unknowns describing the foliage temperature profile. Several strategies to overcome this problem are outlined. The parameterization may eventually help to interpret remotely sensed infrared surface temperatures and to improve estimates of sensible heat flux and evapotranspiration, but comprehensive field testing is still needed.

Analytical parameterization of canopy directional emissivity and directional radiance in the thermal infrared. Application on the retrieval of soil and foliage temperatures using two directional measurements

This work is aimed at deriving canopy component (soil and foliage) temperatures from remote sensing measurements. A simulation study above sparse, partial and dense vegetation canopies has been performed to improve the knowledge of the behaviour of the composite radiative temperature and emissivity. Canopy structural parameters have been introduced in the analytical parameterization of the directional canopy emissivity and directional canopy radiance:namely, the leaf area index (LAI), directional gap fraction and angular cavity effect coefficient. The parameterization has been physically defined allowing its extension to a wide range of Leaf Inclination Distribution Functions (LIDF). When single values are used as leaves and soil temperatures, they prove to be retrieved with insignificant errors from two directional measurements of the canopy radiance (namely at 0 and 55 from nadir), provided that the canopy structure parameters are known. A sensitivity study to the different parameters shows the great importance of the accuracy on LAI estimation (an accuracy of 10 per cent is required to retrieve the leaves temperature with an accuracy better than 0.5 degK, the same requirement being 5 per cent for the retrieval of soil temperature). The radiometric noise is important too, but its effects may be limited by using very different angles for the measurements: for 0 and 55, the effect of a Gaussian noise (NEDeltaT 0.05deg K) is lower than 0.5degK on the retrieved soil and foliage temperatures). Uncertainties on the leaf and soil emissivities (Delta epsilon 0.01) cause little errors in the retrieval (lower than 0.5degK). If the inclination dependence of the leaves temperature is considered, a 1 degK error is observed in the retrieved soil and foliage temperatures. This error is due to the fact that the effective foliage temperature varies with the view angle (a few 10 -1 deg K at 55 ), which implies errors in the inversion scheme. This effect may be corrected for by using an angular corrective term delta depending only on the off-nadir angle used.

An intercomparison study on models of sensible heat flux over partial canopy surfaces with remotely sensed surface temperature

Using remotely sensed surface temperature to estimate the sensible heat flux over partial canopy covered surfaces, one faces the problem of how the bare soil and plant foliage temperatures contributing to the radiometric surface temperature are related to the turbulent transport of sensible heat across the surface-atmosphere interface. To solve this problem, several sensible heat models, using radiometric surface temperature, have appeared in the literature. In this study, using the observational data from three interdisciplinary field experiments [the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE), Monsoon '90 and Washita '92, the performance of four models using average values of their parameters in predicting land surface sensible heat flux was evaluated. By analyzing the sensitivity of the models to common parameters and input variables, reasons for differences in the performances of the models and the potential to improve the agreement with observations have been ascertained. From the comparisons of modeled versus measured sensible heat flux for the different surfaces, the dual-source model in Norman et al. (1995) had the best agreement with its mean absolute percent difference (MAPD) values being similar to the observational accuracy (i.e., ∼ 20%). From the sensitivity analysis the model appears to have the greatest potential for operational applications since it requires relatively few parameters and is not very sensitive to the uncertainty in most of the model parameters. The single-source models in Kustas et al. (1989) and Troufleau et al. (1996) require accurate estimates of the surface roughness z 0m and empirical relationships to account for differences between aerodynamic and radiometric temperature. Therefore, it may be difficult to improve their performance without some independent means of estimating the empirical coefficients and a reliable method for determining z 0m. Estimates from the dual-source model in Lhomme et al. (1994) tend to produce the largest scatter with the observations. This may be related to the fact that the model is more sensitive to variations in most of the parameters common in dual-source models. In addition, its response to high surface-air temperature differences in the sensitivity analysis differs from all other models. Therefore, it may be more difficult to obtain reliable estimates from this model on an operational basis.

Simulation of climate change with infrared heaters reduces the productivity of Lolium perenne L. in summer

Field-grown perennial ryegrass was subjected to climate warming and elevated CO 2 concentration during summer in free air conditions (no enclosure of the vegetation). Increased foliage temperature (2.5°C above fluctuating ambient) was induced by heating the stand with infrared radiation sources, modulated by an electronic control device (FATI, Free Air Temperature Increase). Enhanced CO 2 was produced by a FACE system (Free Air CO 2 Enrichment). Exposure to simulated climate warming drastically reduced above-ground harvestable dry matter (52% loss). The nitrogen allocated to the leaf fraction was thus concentrated into less dry matter, which enhanced the nitrogen concentration on a mass basis (+17%) but also per unit leaf area (+47%). As a consequence, CO 2 assimilation rates were not affected in these slower growing plants in the +2.5°C treatment, and the photochemical efficiency of non-cyclic electron transport of photosystem II was also unaffected. Although the plants were grown in the field without root restrictions, long-term exposure to elevated CO 2 concentration induced noticeable acclimation of the photosynthetic apparatus (40% loss of fixation potential), which largely outweighed the direct stimulation in this summer period. Part of the reduced rates could be attributed to lower N concentration on a leaf area basis. The results are compared with responses of this species in sunlit conditioned greenhouses, which indicates that experiments in enclosures may underestimate effects in the field. This also emphasizes the need to validate other plant responses to climate warming and CO 2 enrichment in free air conditions.