Leaf Boundary Layer Conductance Research Articles

Adsorption of particulate matter and uptake of metal and non-metal elements from PM in leaves of Pinus densiflora and Quercus acutissima: a comparative study

Trees can serve as effective biofilters of Particulate matter (PM) pollution, making them valuable for managing air pollution and promoting public health. Leaves of trees can reduce PM through absorption, adsorption, and fallout mechanisms, which are influenced by species-specific characteristics and environmental factors. Although several studies have explored the impact of various leaf characteristics on their ability to adsorb PM from field conditions, few have been conducted in controlled chambers to analyze the adsorption of PM on leaf surfaces and the uptake of metal and non-metal elements from PM on leaves. To fill these knowledge gaps, this study investigated the PM adsorption and leaf characteristics of two different tree species, Pinus densiflora (an evergreen coniferous species) and Quercus acutissima (a deciduous broad-leaved species) under controlled conditions in a PM exposure chamber with a target concentration of 300 μg m−3. The main aim of this study was to measure and compare the rate of PM component (metal and non-metal elements) uptake in two species and investigate the leaf characteristics that contribute to PM adsorption. We investigated the relationship between PM adsorption and physiolog, micro-morphology, and chemical properties of the leaf surface in two species. This study used a Pearson’s correlation analysis and a principal component analysis (PCA) to evaluate correlation between PM adsorption and leaf characteristics and uptake of metal/non-metal elements in PM on leaves. This result showed that leaf characteristics such as stomatal size, leaf roughness, and wax content played a crucial role in PM10 adsorption, while physiological factors like transpiration and leaf boundary layer conductance were identified as important determinants of PM2.5 adsorption on plant leaves. It also observed significant variations in the uptake of aluminum, iron, magnesium, phosphorus, and sulfur. This study not only advances our understanding of the mechanisms behind PM adsorption by tree leaves but also underscores the importance of selecting appropriate tree species based on their leaf characteristics for urban forestry and green infrastructure projects. The ability to strategically use tree species for PM pollution mitigation highlights a practical approach to enhancing environmental sustainability and public health.

Alternative method for determining leaf CO2 assimilation without gas exchange measurements: Performance, comparison and sensitivity analysis.

We present an alternative method to determine leaf CO2 assimilation rate (An ), eliminating the need for gas exchange measurements in proximal and remote sensing. This method combines the Farquhar-von Caemmerer-Berry photosynthesis model with mechanistic light reaction (MLR) theory and leaf energy balance (EB) analysis. The MLR theory estimates the actual electron transport rate (J) by leveraging chlorophyll fluorescence via pulse amplitude-modulated fluorometry for proximal sensing or sun-induced chlorophyll fluorescence measurements for remote sensing, along with spectral reflectance. The EB equation is used to directly estimate stomatal conductance from leaf temperature. In wheat and soybean, the MLR-EB model successfully estimated An variations, including midday depression, under various environmental and phenological conditions. Sensitivity analysis revealed that the leaf boundary layer conductance (gb ) played an equal, if not more, crucial role compared to the variables for J. This was primarily caused by the indirect influence of gb through the EB equation rather than its direct impact on convective CO2 exchange on the leaf. Although the MLR-EB model requires an accurate estimation of gb , it can potentially reduce uncertainties and enhance applicability in photosynthesis assessment when gas exchange measurements are unavailable.

Spatiotemporal dynamics of leaf transpiration quantified with time-series thermal imaging

Accurately capturing the spatiotemporal dynamics of transpiration from sub-leaf to ecosystem scales remains a key challenge in eco-physiology and hydrology as typical methods face a trade-off between spatial coverage and temporal resolution. Here, we developed a new scalable, semi-automated method to produce highly precise estimates of water and energy fluxes and applied it to single leaves. High-resolution thermal infrared (TIR) images and paired colour photographs of excised soybean leaves were captured at 15 s intervals until wilting, automatically registered and segmented, and used as input for transient energy balance models to estimate latent heat flux (transpiration) at a temporal resolution of one second. Three approaches to estimating leaf boundary layer conductance to heat (gHa) and sensible heat flux were compared, two of which did not require the use of any dry or wet reference surface. The accuracy of water loss modeled using average leaf temperature was also compared to models retaining pixel-scale temperature heterogeneity at a spatial resolution of 0.326 mm2. Cumulative leaf water-losses modeled using average leaf temperature closely matched gravimetric measurements (r2 = 0.95) and pixel-scale models identified striking spatiotemporal patterns of water loss at the sub-leaf scale. Different methods of estimating gHa did not significantly alter model results. Use of leaf energy balance models with time series thermal images to quantify transient transpiration fluxes was able to accurately resolve 1-s time-varying leaf water loss in outdoor conditions, did not require any reference surfaces, and also produced data on the characteristic length scales of heterogeneous sub-leaf response. Given the ability to omit reference surfaces and retain accuracy, this approach also has the potential to be scaled-up to quantify energy fluxes in more complex plant canopies.

Dynamic distribution of thermal effects of an oscillating frost protective fan in a tea field

Wind machines are one of the most effective and prevalent methods of frost protection for crops. However, their physical and physiological effectiveness should be re-examined because climate change from global warming can reduce the cold tolerance of crops, leading to higher risk of frost damage. Here, we experimentally elucidate thermal effects of a wind machine by spatiotemporal analysis of leaf heat balance, with the aid of a new method facilitating continuous and multipoint evaluation of leaf boundary layer conductance ( G A ). G A regulates convective heat exchange between leaves and ambient air. This analysis was performed in a tea field with a frost protective fan (oscillating wind machine), thereby visualising the spatiotemporal distribution of thermal effects from the oscillating fan. Those effects showed a dynamically varying distribution in space and time, as shown in the supplementary animation, under strong influences of both changes in G A and air-to-leaf temperature difference. The change in G A was synchronised with airflow from the oscillating fan onto leaves. However, the change in air-to-leaf temperature difference was complicated, with delay in transient thermal responses of leaves and ambient air. These spatiotemporal characteristics of thermal effects varied substantially with field location, owing to different intervals and durations of airflow from the oscillating fan. Consequently, remarkable non-uniformities in thermal effects appeared across the field, increasing potential risk of frost damage. These results indicate that the effectiveness of the fan still has room for improvement by considering the spatiotemporal characteristics of thermal effects reported herein, toward more reliable frost protection under global warming.

Seasonal Variation in Canopy Aerodynamics and the Sensitivity of Transpiration Estimates to Wind Velocity in Broadleaved Deciduous Species

Abstract Characterization of seasonal dynamics in wind speed attenuation within a plant canopy α is necessary for modeling leaf boundary layer conductance , canopy–atmosphere coupling Ω, and transpiration at multiple scales. The goals of this study were to characterize seasonal variation in α in four tree species with canopy wind profiles and a canopy-structure model, to quantify the impact of α on estimates of and Ω, and to determine the influence of variable wind speed on transpiration estimates from a biophysical model [Multi-Array Evaporation Stand Tree Radiation Assemblage (MAESTRA)]. Among species, α varied significantly with above-canopy wind speed and seasonal canopy development. At the mean above-canopy wind speed (1.5 m s−1), α could be predicted using a linear model with leaf area index as the input variable (coefficient of determination R2 = 0.78). However, the canopy-structure model yielded improved predictions (R2 = 0.92) by including canopy height and leaf width. By midseason, increasing canopy leaf area and α resulted in lower within-canopy wind speeds, a decrease in by 20%–50%, and a peak in Ω. Testing a discrete increase in wind speed (0.6–2.4 m s−1; seasonal mean plus/minus one standard deviation) had variable influence on transpiration estimates (from −30% to +20%), which correlated strongly with vapor pressure deficit (R2 = 0.83). Given the importance of α in accurate representation of , Ω, and transpiration, it is concluded that α needs to be given special attention in plant canopies that undergo substantial seasonal changes, especially densely foliated canopies (i.e., leaf area index >1) and in areas with lower native wind speeds (i.e., <2 m s−1).

Leaf Boundary Layer Conductance in a Tomato Canopy under the Convective Effect of Circulating Fans in a Greenhouse Heated by an Air Duct Heater

Leaf Boundary Layer Conductance in a Tomato Canopy under the Convective Effect of Circulating Fans in a Greenhouse Heated by an Air Duct Heater

Cross-scale modelling of transpiration from stomata via the leaf boundary layer.

Leaf transpiration is a key parameter for understanding land surface-climate interactions, plant stress and plant structure–function relationships. Transpiration takes place at the microscale level, namely via stomata that are distributed discretely over the leaf surface with a very low surface coverage (approx. 0·2-5%). The present study aims to shed more light on the dependency of the leaf boundary-layer conductance (BLC) on stomatal surface coverage and air speed. An innovative three-dimensional cross-scale modelling approach was applied to investigate convective mass transport from leaves, using computational fluid dynamics. The gap between stomatal and leaf scale was bridged by including all these scales in the same computational model (10⁻⁵-10⁻¹ m), which implies explicitly modelling individual stomata. BLC was strongly dependent on stomatal surface coverage and air speed. Leaf BLC at low surface coverage ratios (CR), typical for stomata, was still relatively high, compared with BLC of a fully wet leaf (hypothetical CR of 100%). Nevertheless, these conventional BLCs (CR of 100%), as obtained from experiments or simulations on leaf models, were found to overpredict the convective exchange. In addition, small variations in stomatal CR were found to result in large variations in BLCs. Furthermore, stomata of a certain size exhibited a higher mass transfer rate at lower CRs. The proposed cross-scale modelling approach allows us to increase our understanding of transpiration at the sub-leaf level as well as the boundary-layer microclimate in a way currently not feasible experimentally. The influence of stomatal size, aperture and surface density, and also flow-field parameters can be studied using the model, and prospects for further improvement of the model are presented. An important conclusion of the study is that existing measures of conductances (e.g. from artificial leaves) can be significantly erroneous because they do not account for microscopic stomata, but instead assume a uniform distribution of evaporation such as found for a fully-wet leaf. The model output can be used to correct or upgrade existing BLCs or to feed into higher-scale models, for example within a multiscale framework.

Comparison of leaf water use efficiency of oak and sycamore in the canopy over two growing seasons

The seasonal trends in water use efficiency of sun and shade leaves of mature oak (Quercus robur) and sycamore (Acer pseudoplatanus) trees were assessed in the upper canopy of an English woodland. Intrinsic water use efficiency (net CO2 assimilation rate/leaf conductance, A/g) was measured by gas exchange and inferred from C isotope discrimination (δ13C) methods. Shade leaves had consistently lower δ13C than sun leaves (by 1–2‰), the difference being larger in sycamore. Buds had distinct sun and shade isotopic signatures before bud break and received an influx of 13C-rich C before becoming net autotrophs. After leaf full expansion, δ13C declined by 1–2‰ gradually through the season, emphasising the importance of imported carbon in the interpretation of leaf δ13C values in perennial species. There was no significant difference between the two species in the value of intrinsic water use efficiency for either sun or shade leaves. For sun leaves, season-long A/g calculated from δ13C (72–78 μmol CO2 [mol H2O]−1) was 10–16% higher than that obtained from gas exchange and in situ estimates of leaf boundary layer conductance. For shade leaves, the gas exchange–derived values were low, only 10–18% of the δ13C-derived values. This is ascribed to difficulties in obtaining a comprehensive sample of gas exchange measurements in the rapidly changing light environment.

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.

Leaf boundary layer conductance in ventilated greenhouses: An experimental approach

Improving the design, control and management of greenhouse ventilation systems requires a better insight into the coupling and feedback mechanisms between the crop, the internal atmosphere and the outside atmosphere. The objective of this work was to characterize the leaf boundary-layer conductance ( g b) in highly ventilated greenhouse conditions and to study its dependence on the ventilation conductance ( g v) and outside conditions. For this purpose, measurements of g b were carried out in a greenhouse cultivated with rose plants, using rose leaf replicas consisting of a pair of artificial leaves, one being heated and the other unheated. The relation between g b and g v was established and quantified, and further generalised to the case of large greenhouses. By implementing the relation into the expression of the canopy aerodynamic conductance, the influence of the leaf area index (LAI) on the total greenhouse conductance was addressed. It was shown that LAI plays a major role in determining the degree of coupling between the greenhouse canopy and the outside atmosphere. The results underline the importance of integrating estimates of the canopy aerodynamic conductance in greenhouse climate control algorithms.

Boundary layer conductance for contrasting leaf shapes in a deciduous broadleaved forest canopy

A new method of constructing light, flexible and more realistic replica leaves for continuous determination of leaf boundary layer conductance to heat transfer ( g b h ) was developed and tested in a mature oak ( Quercus robur L.) and sycamore ( Acer pseudoplatanus L.) tree canopy. The replicas were used to determine the difference between oak and sycamore leaf g b h in exposed sites in the upper canopy, the relationship of g b h with wind speed, the seasonal changes in g b h in the canopy as leaf cover developed, and values for Ω, the decoupling coefficient. The replicas showed similar gradients in temperature at their margins to those in real leaves. When exposed, the g b h of the larger sycamore leaves was 66% of that of oak leaves under the same conditions. Linear relationships were found with g b h and wind speed across the measured range of 0.3–3.5 m s −1, and flow in the replica boundary layers was laminar in all conditions. The leafless canopy produced a substantial sheltering effect, reducing g b h by 12–28% in light winds. Sycamore replicas in the leafed canopy showed a 19–29% lower g b h at a given external wind speed than when outside, but there was little difference between ‘sun’ and ‘shade’ position shoots, because of the density of the shoots, and the close branching pattern. In contrast, in oak g b h at a given wind speed was 15–21% lower for ‘sun’ leaves than that for replicas outside the canopy, with a larger reduction (approximately 28%) in denser ‘shade’ sites. Although wind speed in the canopy was often low, leaves of both species were usually well coupled to the canopy airstream ( Ω < 0.3). Sun leaves were substantially less well coupled than shade leaves, despite the lower shelter effect, because of their higher stomatal conductance values. In the lightest winds (<0.5 m s −1) and with high stomatal conductance, coupling may on many occasions be poor for sun leaves, particularly for the larger sycamore leaves.

Control of transpiration in three coffee cultivars: the role of hydraulic and crown architecture

Water use and hydraulic architecture were studied in the coffee (Coffea arabica) cultivars San Ramon, Yellow Caturra and Typica growing in the field under similar environmental conditions. The cultivars differed in growth habit, crown architecture, basal sapwood area and total leaf surface area. Transpiration per unit leaf area (E), stomatal conductance (g s), crown conductance (g c), total hydraulic conductance of the soil/leaf pathway (G t) and the stomatal decoupling coefficient, omega (Ω) (Jarvis and McNaughton 1986) were assessed over a range of soil moisture and during partial defoliation treatments. The relationship between sap flow and sapwood area was linear and appeared to be similar for the three cultivars. Variation in g c, E, and G t of intact plants and leaf area-specific hydraulic conductivity (k l) of excised lateral branches was negatively correlated with variation in the ratio of leaf area to sapwood area. Transpiration, g c, and g s were positively correlated with G t. Transpiration and G t varied with total leaf area and were greatest at intermediate values (10 m2) of leaf area. Omega was greatest in Yellow Caturra, the cultivar with the greatest leaf area and a dense crown, and was smallest in Typica, the cultivar with an open crown. Differences in omega were attributable primarily to differences in leaf boundary layer conductance among the cultivars. Plants of each cultivar that were 40% defoliated maintained sap flows comparable to pretreatment plants, but expected compensatory increases in g s were not consistently observed. Despite their contrasting crown morphologies and hydraulic architecture, the three cultivars shared common relationships between water use and hydraulic architectural traits.

Wind speed and leaf boundary layer conductance variation within tree crown: Consequences on leaf-to-atmosphere coupling and tree functions

In situ measurements and model simulations were used to analyse the spatial variations of wind speed ( U) and leaf boundary layer conductance ( g b H), and their effects on leaf transpiration and photosynthesis within the crown of a 20-year-old walnut tree ( Juglans regia L.). Wind speed attenuation within the tree crown was strongly correlated to the cumulative leaf area along the wind path into the crown, as deduced from the 3D distribution of the tree foliage, obtained by the combined use of digitizing and allometric relationships. Simulations by the RATP model, i.e., a 3D numerical model of radiation transfer, leaf transpiration and photosynthesis ( Sinoquet et al., 1999), were used (i) to study the spatial variations of transpiration, photosynthesis and leaf-to-atmosphere coupling within the tree crown; and (ii) to analyse coupling at tree level, in order to evaluate the sensitivity of total transpiration to climate variations. The main results of the simulations were: (i) spatial variations of stomatal conductance were mainly responsible for the important spatial variation of the leaf-to-air decoupling factor (Ω) within the tree crown; (ii) high coupling was observed at tree level, so that whole tree transpiration was primarily controlled by air VPD and secondarily by radiation; (iii) wind velocity and direction influenced only weakly the local transpiration, and had no effect on photosynthetic rates.

Vertical profiles of boundary layer conductance and wind speed in a cotton canopy measured with heated brass surrogate leaves

Knowledge of single leaf boundary layer conductance ( g b) within extensive canopies is required to scale stomatally regulated gas exchange from leaf to canopy. Fluxes of terpenes and heat, formation of dew, and other processes occurring at the leaf surface may be controlled by g b. We develop, test, and deploy at five canopy heights, a set of paired, heated and unheated brass surrogate leaves, with realistic 3-dimensional shape and characteristic dimension ( d) for cotton ( Gossypium hirsutum L.) in California. An equation describing heat transfer from a flat plate, enhanced by a factor of 1.15 for complex leaf shape, adequately described g b of the surrogate leaves. In the field, values of g b measured with the surrogate leaves near the top of the canopy were closely related to wind speed, as expected from theory. The dependence of g b on leaf size was apparent at all heights in the canopy using surrogate leaves differing in d. The profile of within-canopy wind speed ( u) was determined using the profile of g b and d of the surrogate leaves. This profile of effective u in the leaf environment was well described by a conventional exponential decay model with an empirically derived wind extinction coefficient ( K = 1.5). This profile of u was combined with the directly measured profile of d of biological leaves to determine the actual profile of g b in this canopy. The increasing leaf size toward the middle of the canopy caused actual g b to decline more steeply than predicted from constant d. With depth in the canopy sunflecks increasingly contributed to unequal radiation interception between paired surrogate leaves and therefore to anomalies in calculated g b. A simple data filtering protocol, rejecting data outside the range observed at the top of the canopy, is introduced. Data rejection increased appropriately with increasing sunfleck frequency and depth in the canopy. These results extend the applicability of the heated, paired surrogate leaf technique to sheltered positions in dense canopies, and to large-leafed species, allowing direct measurement of an important canopy parameter ( g b) that has typically been estimated or calculated as a residual.

Soil moisture dynamics of calcareous grassland under elevated CO2.

Water relations of nutrient-poor calcareous grassland under long-term CO2 enrichment were investigated. Understanding CO2 effects on soil moisture is critical because productivity in these grasslands is water limited. In general, leaf conductance was reduced at elevated CO2, but responses strongly depended on date and species. Evapotranspiration (measured as H2O gas exchange) revealed only small, non-significant reductions at elevated CO2, indicating that leaf conductance effects were strongly buffered by leaf boundary layer and canopy conductance (leaf area index was not or only marginally increased under elevated CO2). However, these minute and non-significant responses of water vapour loss accumulated over time and resulted in significantly higher soil moisture in CO2-enriched plots (gravimetric spot measurements and continuous readings using a network of time-domain reflectometry probes). Differences strongly depended on date, with the smallest effects when soil moisture was very high (after heavy precipitation) and effects were largest at intermediate soil moisture. Elevated CO2 also affected diurnal soil moisture courses and rewetting of soils after precipitation. We conclude that ecosystem-level controls of the water balance (including soil feedbacks) overshadow by far the physiological effects observed at the leaf level. Indirect effects of CO2 enrichment mediated by trends in soil moisture will have far-ranging consequences on plant species composition, soil bacterial and faunal activity as well as on soil physical structure and may indirectly also affect hydrology and trace gas emissions and atmospheric chemistry.

Physiological and environmental control of transpiration by trees in windbreaks

Knowledge of the environmental and physiological variables controlling water use by trees in windbreaks should assist efforts to manage competition for water between trees and crops in agroforestry. Coupling between windbreak trees and the atmosphere was therefore assessed by determining values of the decoupling coefficient ( Ω) for windbreaks during four periods of the year, using experimentally-derived aerodynamic and surface conductances for trees of Azadirachta inidica A. Juss in windbreaks at Sadoré, Niger. Aerodynamic conductances were determined by scaling-up measurements of leaf boundary layer conductances made using leaf replicas. Surface conductances for the trees were derived from measurements of whole-tree transpiration made using the heat-pulse technique. Values of Ω for the windbreaks were ≈0.3 at moderate wind speeds in the more humid seasons of the year, but fell to 0.1 during the driest period of the year. Thus, windbreaks are generally well-coupled to the atmosphere, so that transpiration is predominantly driven by the ambient vapour pressure deficit at leaf surfaces and the trees are able to exert close physiological control over water use. Management strategies to reduce the surface conductances of windbreak canopies should, consequently, effectively reduce demand for water by windbreaks and help to control competition for water between trees and crops.

Leaf boundary layer conductance in a vineyard in Portugal

Construction and calibration of a new type of heated leaf replica for measuring leaf boundary layer conductance g b H is described, offering two novel features: (i) the heated and unheated surfaces are situated on the same replica, and (ii) the temperature difference is measured inside the replicas instead of at the surface. Wind tunnel experiments were used for calibration against completely heated standard replicas. Definition of a normalised characteristic length L to be used in the Reynolds and Nusselt numbers of a flat surface is proposed. Good agreement between experimental data and engineering relationships were found for circular and vine-leaf shaped standard replicas. Six partially heated vine-leaf replicas were installed in a vineyard in central Portugal together with complementary meteorological sensors. Leaf boundary layer conductance was continuously measured, at different locations within a vine, for 40 days, and compared with a reference wind speed U ref measured above the canopy (2.84 m). The measured conductances were closely correlated with U ref but due to local shelter effects and/or directional effects on convective transfers, the correlation depended on wind direction and location of the replicas within the canopy. Although further improvements are needed in their construction and calibration, some features of these partially heated leaf replicas may be advantageous for long term analysis of micrometeorological variations in complex canopies.

Aerodynamic conductances of trees in windbreaks

A scheme for scaling leaf boundary layer conductances ( b b) to tree crowns was developed for Azadirachta indica A. Juss. trees in windbreaks in the Sahel. This scheme was derived from measurements of g b, made with heated leaf-replica pairs mounted in the crowns of windbreak trees, and values of bulk aerodynamic conductances for whole trees ( g a) in windbreaks, which were determined from the rate of evaporation from two artificially wetted, excised trees, using the hanging-tree technique. Heated pairs of leaf replicas were constructed and tested in a wind tunnel before being deployed in the field. The effects of wind speed ( u) on g b in the wind tunnel agreed with expectations based on observations by others of the aerodynamic properties of leaves. Similar responses to wind speed were found when the heated leaf-replica pairs were used in situ; aggregated leaf boundary layer conductances ( g bt), calculated by summing g b over the total leaf area of each tree as conductances in parallel, were proportional to u z , where z varied between 0.5 and 0.8, the values expected for laminar and turbulent boundary layers, respectively. In contrast, g a was proportional to u 1.1t and was much smaller than g bttb,, even if effects of differences in leaf areas among trees were accounted for. The differences between measured values of ga and g bt were used to derive an empirical model of g ac, the conductance for scalar transfer from the limits of the leaf boundary layers to the reference position outside the tree crown. This model can be used to estimate g a from wind speed, so that g a can be estimated by summing, as conductances in series, g ac and values of g bt determined from in situ measurements of g b. Thus, at sites where trees are sparsely or non-homogeneously distributed and the cutting of more than a few trees to measure g a is not practicable, heated leaf-replica pairs and the hanging-tree technique can be used together to develop a scheme for scaling g b to the whole tree. This approach should be particularly useful in studies examining energy budgets in agroforestry, horticulture or other settings where trees are isolated or do not form a closed canopy.

Leaf boundary layer conductance of two native species in southeast Spain

Leaf boundary layer conductance was measured in the field using heated leaf replicas placed in the canopies of two of the three main semi-arid species found in the Rambla Honda (Almería, South East of Spain), Retama sphaerocarpa (L.) Boiss, and Stipa tenacissima L. R. sphaerocarpa is a woody perennial bush with approximately cylindrical cladodes arranged on randomly orientated stems. In contrast S. tenacissima is a perennial tussock grass with leaves that can roll during periods of water stress. Cylindrical leaf replicas were constructed with dimensions similar to the leaves of S. tenacissima and cladodes of R. sphaerocarpa. The relationship between wind speed ( u) measured outside the plant canopy and boundary layer conductance ( g a) was analysed on the basis of an equation, derived from engineering formulae describing forced convection for a cylinder in laminar flow. The coefficients a and b were derived empirically from experimental data for replicas placed within and outside the canopies of the two species. Measured boundary layer conductance in the open tended to be higher than that estimated from engineering equations resulting in higher estimates of a. There was no significant difference in the estimate of b between replicas in the open and the engineering value. In the R. sphaerocarpa canopy the estimate of a tended to decrease with depth into the canopy but there was no significant difference in estimates of b between replicas inside and outside the canopy. Inside the S. tenacissima canopy both coefficients a and b tended to be significantly lower than those derived from engineering equations. Significant differences between the parameters of the relationship inside and outside the canopy showed the difficulty in deriving a unique equation to calculate the boundary layer conductance of a whole plant from wind speed measured outside the canopy. This work confirms that serious errors can be made if engineering equations are applied without a better knowledge of the relationship between boundary layer conductance and wind speed. The results suggest that reduction of average wind speed by shelter within the canopy and increased turbulence are important in influencing leaf boundary layer. The heated leaf replica method used in this work provides a simple and effective system for measuring leaf boundary layer conductance for plants in the field, and is preferable to the use of uncalibrated empirical equations These results can be related to the importance of leaf shape and canopy structure in determining water use of natural vegetation in semi-arid regions. R. sphaerocarpa has an open canopy allowing a better penetration of air flow. S. tenacissima has a dense canopy producing shelter decreasing wind speed rather than increasing turbulence. While both species have similar shaped transpiring plant parts, the strategy of S. tenacissima to maintain dead materials as part of the plant canopy could be important in control of water loss.

A heated leaf replica technique for determination of leaf boundary layer conductance in the field

This paper presents a robust field method for continuous estimation of boundary layer conductance by measurement of the temperature difference between a replica millet leaf which is heated with an electric current and an identical replica used as a reference. The measurements of boundary layer conductance in a wind tunnel were compared with conductances calculated using equations from an engineering text. The two sets of values are similar at low wind speeds. At wind speeds above 2.5 m s −1 replica boundary layer conductances were higher than those calculated for a laminar boundary layer. Errors and assumptions of the technique were analysed. Heating current was switched between the replicas to avoid systematic errors. Using this system, leaf boundary layer conductance was measured in the field in Niger for a millet crop behind a windbreak. The relationship between wind speed (u) and boundary layer conductance ( g a) was analysed for four distances from the windbreak and three periods at the start of the millet cropping season. The relationship g a = au b gave a series of values of parameters a and b. Values of b lay between 0.5 and 0.8 and were related linearly to the low frequency turbulence of the air flow (given by the standard deviation of the wind speed).