Leaf Boundary Layer Resistance Research Articles

Arboreal Urban Cooling Is Driven by Leaf Area Index, Leaf Boundary Layer Resistance, and Dry Leaf Mass per Leaf Area: Evidence from a System Dynamics Model

Heat waves are becoming more frequent due to climate change. Summer heat waves can be particularly deadly in cities, where temperatures are already inflated by abundant impervious, dark surfaces (i.e., the heat island effect). Urban heat waves might be ameliorated by planting and maintaining urban forests. Previous observational research has suggested that conifers may be particularly effective in cooling cities. However, the observational nature of these studies has prevented the identification of the direct and indirect mechanisms that drive this differential cooling. Here, we develop a systems dynamics representation of urban forests to model the effects of the percentage cover of either conifers or broadleaf trees on temperature. Our model includes physiological and morphological differences between conifers and broadleaf trees, and physical feedback among temperature and energy fluxes. We apply the model to a case study of Vancouver, BC, Canada. Our model suggests that in temperate rainforest cities, conifers may by 1.0 °C cooler than broadleaf trees; this differential increases to 1.2 °C when percentage tree cover increases from 17% to 22% and to 1.7 °C at 30% cover. Our model suggests that these differences are due to three key tree traits: leaf area index, leaf boundary layer resistance, and dry mass per leaf area. Creating urban forests that optimize these three variables may not only sequester CO2 to mitigate global climate change but also be most effective at locally minimizing deadly urban heat waves.

Lettuce plant growth and tipburn occurrence as affected by airflow using a multi-fan system in a plant factory with artificial light.

A multi-fan system (MFS) for single culture beds was developed to improve the airflow in a plant factory with artificial light. The MFS had seven fans which were installed on both the front and back sides of culture beds to generate airflow from two opposite horizontal directions. The fans that push the air into the culture bed were air inlets while those that pull the air out of the culture bed were air outlets. In this study, three airflow patterns were evaluated: T1, the front and back sides of the culture bed were air inlets; T2, the front side was an air inlet and the backside was an air outlet; and T3, both the front and back sides were air outlets. A culture bed with no MFS was used as a control (T4). Lettuce growth and tipburn occurrence were evaluated and leaf boundary layer resistance (1/gbv), sensible heat flux (Sh), and latent heat flux (Lh) of lettuce plants were estimated. The airflow pattern in T1 improved the air velocity (Va) by an average of 0.75ms-1 and a variation coefficient of 65%. The 1/gbv decreased significantly with the increase in Va, and the lowest value of 54.0sm-1 was observed in T1. The low resistance to heat and moisture transfer enhanced the Sh and Lh of lettuce plants. The average Sh and Lh were 40% and 46% higher in T1 compared with those in T4. The fresh and dry weights of lettuce plants in T1 were 1.13 and 1.06 higher than those in T4, respectively. No tipburn occurrence was observed in lettuce plants grown under the MFS while five leaves per plant were injured with tipburn in T4. The results indicated that improving the airflow can improve the growth of indoor cultured lettuce and alleviate the occurrence of tipburn due to the decrease in the 1/gbv and the increase in the transpiration rate.

Evaluating Land–Atmosphere Coupling Using a Resistance Pathway Framework

Abstract This paper presents a methodology for examining land–atmosphere coupling in a regional climate model by examining how the resistances to moisture transfer from the land to the atmosphere control the surface turbulent energy fluxes. Perturbations were applied individually to the aerodynamic resistance from the soil surface to the displacement height, the aerodynamic resistance from the displacement height to the reference level, the stomatal resistance, and the leaf boundary layer resistance. Only perturbations to the aerodynamic resistance from the soil surface to the displacement height systematically affected 2-m air temperature for the shrub and evergreen boreal forest plant functional types (PFTs). This was associated with this resistance systematically increasing the terrestrial and atmospheric components of the land–atmosphere coupling strength through changes in the partitioning of the surface energy balance. Perturbing the other resistances did contribute to changing the partitioning of the surface energy balance but did not lead to systematic changes in the 2-m air temperature. The results suggest that land–atmosphere coupling in the modeling system presented here acts mostly through the aerodynamic resistance from the soil surface to the displacement height, which is a function of both the friction velocity and vegetation height and cover. The results show that a resistance pathway framework can be used to examine how changes in the resistances affect the partitioning of the surface energy balance and how this subsequently influences surface climate through land–atmosphere coupling. Limitations in the present analysis include grid-scale rather than PFT-scale analysis, the exclusion of resistance dependencies, and the linearity assumption of how temperature responds to a resistance perturbation.

Ecophysiology of leaf trichomes.

This review examines how leaf trichomes influence leaf physiological responses to abiotic environmental drivers. Leaf trichomes are known to modulate leaf traits, particularly radiation absorptance, but studies in recent decades have demonstrated that trichomes have a more expansive role in the plant-environment interaction. Although best known as light reflectors, dense trichome canopies modulate leaf heat balance and photon interception, and consequently affect gas exchange traits. Analysis of published studies shows that dense pubescence generally increases reflectance of visible light and near-infrared and infrared radiation. Reflective trichomes are also protective, reducing photoinhibition and UV-B related damage to leaf photochemistry. Little support exists for a strong trichome effect on leaf boundary layer resistance and transpiration, but recent studies indicate they may play a substantive role in leaf water relations affecting leaf wettability, droplet retention and leaf water uptake. Different lines of evidence indicate that adaxial and abaxial trichomes may function quite differently, even within the same leaf. Overall, this review synthesises and re-examines the diverse array of relevant studies from the past 40 years, illustrating our current understanding of how trichomes influence the energy, carbon and water balance of plants, and highlighting promising areas for future research.

Tree surface temperature in an urban environment

Trees are essential in a dense urban environment not only because of their aesthetic value, but also for their cooling effect during hot periods, which impacts directly on the local microclimate. However, certain trees cope better with high urban temperatures than others. Here, we report tree crown temperatures of 10 common tree species frequently planted in Central European cities (in part, supplemented with stomatal conductance data, g s ). Parts of the city of Basel, Switzerland (7 ° 4 1 ′ E /47 ° 3 4 ′ N ) were scanned from a helicopter using a high-resolution thermal camera. A histogram of the composite image shows peaks at 18 ° C (water), 26 ° C (vegetation), 37 ° C (streets) and a less obvious one at 45 ° C (roofs). At an ambient temperature of c. 25 ° C, tree crown temperatures ranged from c. 24 ° C ( Aesculus hippocastanum trees located in a park) to 29 ° C in Acer platanoides trees, located in a street. Trees in parks were significantly cooler (c. 26 ° C) than trees surrounded by sealed ground (c. 27 ° C). The only coniferous species, Pinus sylvestris did not vary in temperature with location (park or street) and exhibited foliage temperature close to air temperature. Generally, small-leaved trees remained cooler than large-leaved trees. Stomatal conductance data collected during similar weather conditions suggest that there was no bias in crown temperatures due to locally different water supply between trees. Although the highest leaf temperatures of individuals of A. platanoides reached over 5 K leaf-to-air temperature difference ( Δ T L − A ), we do not expect temperature stress to occur in these conditions. In order to estimate possible effects of future temperature extremes on Δ T L − A , we evaluated the leaf energy balance for a range of stomatal responses and air temperatures, using leaf size, wind speed and the measured species-specific leaf boundary layer resistance. At an ambient temperature of 40 ° C, Δ T L − A ranged from 2 to 5 K when g s was assumed to drop linearly to 50% of its maximum value. When g s was compromised further (20% of species-specific maxima), the difference in Δ T L − A between species became larger with rising ambient temperature (range 4–10 K). Those species with the lowest leaf temperatures at 25 ° C were not necessarily coolest at 40 ° C. Species-specific differences in Δ T L − A under extreme temperatures as shown here may be useful for urban tree planning in order to optimise management cost and human comfort.

A coupled dispersion and exchange model for short-range dry deposition of atmospheric ammonia

The MODDAS-2D model (MOdel of Dispersion and Deposition of Ammonia over the Short-range in two dimensions) is presented. This stationary model couples a two-dimensional Lagrangian stochastic model for short-range dispersion, with a leaf-scale bi-directional exchange model for ammonia (NH3), which includes cuticular uptake and a stomatal compensation point. The coupling is obtained by splitting the upward and downward components of the flux, which can be generalized for any trace gas, and hence provides a way of simply incorporating bi-directional exchanges in existing deposition velocity models. The leaf boundary-layer resistance is parametrized to account for mixed convection in the canopy, and the model incorporates a stability correction for the Lagrangian time-scale for vertical velocity, which tends to increase the Lagrangian time-scale in very stable conditions compared with usual parametrizations. The model is validated against three datasets, where concentrations of atmospheric NH3 were measured at several distances from a line source. Two datasets are over grassland and one is over maize, giving a range of canopy structure. The model correctly simulates the concentration in one situation, but consistently overestimates it at further distances or underestimates it at small distances in the two other situations. It is argued that these discrepancies are mainly due to the lack of length of one of the line sources and non-aligned winds. Analysis shows that the surface exchange parameters and the turbulent mixing at the source level are the predominant factors controlling short-range deposition of NH3. Copyright © 2006 Royal Meteorological Society

A multilayer biochemical dry deposition model. 1. Model formulation

A multilayer biochemical dry deposition model has been developed based on the NOAA Multilayer Model (MLM; Meyers et al. [1998]) to study gaseous exchanges between the soil, plants, and the atmosphere. Most of the parameterizations and submodels have been updated or replaced. The numerical integration was improved, and an aerodynamic resistance based on Monin‐Obukhov theory was added. An appropriate parameterization for the leaf boundary layer resistance was chosen. A biochemical stomatal resistance model was chosen based on comparisons of four different existing stomatal resistance schemes. It describes photosynthesis and respiration and their coupling with stomatal resistance for sunlit and shaded leaves separately. Various aspects of the photosynthetic process in both C3 and C4 plants are considered in the model. To drive the photosynthesis model, the canopy radiation scheme has been updated. Leaf area index measurements are adjusted to account for stem area index. A normalized soil water stress factor was applied to potential photosynthesis to account for plant response to both drought and water‐logging stresses. A new cuticle resistance model was derived based on membrane passive transport theory and Fick's first law. It accounts for the effects of diffusivity and solubility of specific gases in the cuticle membrane, as well as the thickness of the cuticle membrane. The model is designed for use in the nationwide dry deposition networks, for example, the Clean Air Status And Trends Network (CASTNet), and mesoscale models, for example, the Community Multiscale Air Quality model (CMAQ) and even the Weather Research and Forecasting model (WRF).

EVALUATI NG SURFACE RESISTANCE FOR ESTIMATING CORN AND POTATO EVAPOTRANSPIRATION WITH THE PENMAN–MONTEITH MODEL

The PenmanMonteith (PM) evapotranspiration (ET) model has been shown to be adequate for estimating dailyreference crop ET (ETo). However, the proper evaluation of surface resistance to vapor exchange (rs) has been a limiting factorfor using the model to directly estimate daily ET for other crops. A popular approach to quantify rs, referred to as canopyresistance (rc), is based on singleleaf resistance (rL) and leaf area index (LAI) but does not account for nonstomatalcontributions (excess resistance) to rs such as leaf boundary layer resistance and aerodynamic resistance within the canopy.Field measurements of ET, rL, and LAI for fullcanopy conditions were used to develop an empirical crop height andLAIdependent relation to estimate excess resistance (ro). An alternative rs term was defined as ro + rc and used in conjunctionwith an aerodynamic resistance term (ra) calculated from the top of the canopy. Using climatic data collected over corn andpotato canopies, three variations of daily PM ET were compared:PM1, which used the rs definition including excess resistance (rs = ro + rc), ra calculated from the top of the canopy, andvapor pressure deficit (VPD) and solar irradiance (Rs) functions to adjust rL.PM2, which was essentially identical to PM1 with no VPD or Rs adjustment to rL.PM3, which used standard rs (rS = rc) and ra (based on a zeroplane displacement height approximately 2/3 of cropheight) calculations with VPD and Rs adjustments to rL.All three methods of PM ET were compared with daily crop ET measurements made with a Bowen ratio energy balance(BREB) system. For corn, the results of this study suggested significant resistance to vapor transfer in excess of the rc usedin standard calculations of daily ET with the PM equation. This behavior was not seen with the calculation of daily ET forpotatoes. The direct application of the standard PM equation to estimate daily corn ET (i.e., PM3) is not recommendedbecause excess resistance not accounted for by this model will often lead to substantial ET overestimation. However,estimation of excess resistance based on canopy features was not found satisfactory (i.e., PM1 and PM2).

Molecular diffusivities of Hg vapor in air, O 2 and N 2 near STP and the kinematic viscosity and thermal diffusivity of air near STP

Accurate knowledge of the coefficients of molecular diffusivity ( D) of trace gases, the kinematic viscosity of air ( ν) and the thermal diffusivity of air ( k) has important application to modeling and observational studies of atmosphere–biosphere interactions. For example, leaf boundary-layer resistances are often parameterized in term of the Schmidt number for mass exchange and the Prandtl number for heat exchange. This study reviews and re-analyzes the historical data and some modeling results for D for Hg vapor in air, N 2 and O 2 and recasts results of an independent review of dynamic viscosity and thermal conductivity of air in a form that are consistent with one another and with this author’s previous study of D for simple gases and air, N 2 and O 2 near STP. Because D, ν and k are functions of temperature and pressure, all modeling results and data are corrected to 1 atmosphere pressure and then used with a one- and two-parameter regression model to determine optimal values for the temperature exponent and the value of D, ν and k at 0°C. Statistical/regression methods for treating data and modeling results follow this author’s previous study of D, except that data for D for Hg vapor is not restricted to values below 100°C.

On the Concept of Leaf Boundary Layer Resistance for Forced Convection

The definition of leaf boundary layer resistance is reconsidered in respect of the three-dimensional diffusion-controlled mass transport region just above the leaf surface. Due to the existence of this superstomatal air layer, the conventional convective boundary layer is not in direct contact with the surface. Thus, in terms of plant physiology, the diffusive “end correction” to the stomatal resistance should be included in the boundary layer resistance. This is true for laminar as well as turbulent flows. When the surface mole fraction of an exchanged gas is estimated using the boundary layer resistance ignoring the diffusive term may lead to a noticeable error. The self-consistent approach is used to clarify the problems of the boundary layer formation and stomatal interference. If the correction is taken into account, the boundary layer resistance becomes dependent also on stomatal shape and distribution on the leaf. The traditional semiempirical formula corrected by the superstomatal diffusion is applied in numerical calculations. In estimates of the water vapour mole fraction on the surface of a transpiring leaf the relative error ranges from insignificant (quiescent air, large leaf and large stomatal pores) to 20 % (low humidity, strong wind, small leaf and small elliptic pores). The boundary layer resistance can decrease by a factor of 3 when the semiaxis lengths of the stomata increase from 1 and 0.5 μm to 10 and 5 μm. The effective thickness of the superstomatal air layer is maximally several millimetres (small stomatal surface concentration and small pores).

Estimating sensible and latent heat fluxes from a temperate broad-leaved forest using the Simple Biosphere (SiB) model

Sensible ( H ) and latent heat ( λE ) flux densities from a well-watered, broad-leaved forest of Nothofagus trees were estimated using the simple biosphere (SiB) model. Model inputs included micrometeorological measurements made at a reference height (36 m) just above the canopy and site parameters such as the tree canopy leaf area index of seven. Half-hourly diurnal courses of modelled H and λE were generally in good agreement with eddy covariance flux measurements (±30 Wm −2 on average) over six late-summer days of variable weather conditions. The most important model variables determining these fluxes were the bulk leaf boundary-layer resistance ( r b), proportional to leaf size, for H and canopy (stomatal) resistance ( r c), regulated by radiation interception and air saturation deficit, for λE . Recent developments in the modelling of r c for different vegetation types are discussed. Maximum daily ground (forest floor) evaporation rate ( E g) was 0.5 mm day −1, accounting for up to 20% of forest evaporation. Initial model estimates of E g in the forest were nearly 50% less than those of lysimeter measurements. However, agreement between measured and modelled E g was only about ± 0.05 mm day −1 after reduction of the trunk space eddy diffusive resistance ( r d) based on a comparison with other values in the literature.

Genetic variation for root-shoot relationships among cotton germplasm

Twenty-five cotton ( Gossypium spp.) genotypes were used to evaluate the genetic variability in partitioning of biomass into roots and shoots when plants were grown under conditions of declining soil water and different atmospheric evaporative demands. The entries ranged from primitive race stocks to modern cultivars and were selected on field observations of growth under water stress conditions in the field. Seeds of each genotype were planted in soil (56 kg) in large containers in the greenhouse. Water was added to the soil to bring the water content to field capacity and the plants were allowed to grow with no additional water until they reached the permanent wilting point. Large fans were utilized in the second experiment to reduce the leaf boundary layer resistance and increase the evaporative demand. When the plants of each entry had reached the permanent wilting point, the plants were harvested, the roots washed free of the soil, and the dry weights of both roots and shoots were determined. Information on total water used and days to permanent wilting were also collected for each genotype. Differences were observed in partitioning of total biomass between roots and shoots between experiments and genotypes. There was no significant interaction, however, between entries and experiments. Root-shoot ratios increased in plants grown in the more stressful environment resulting from a significant increase in root dry weights with little change in shoot dry weights. The distribution for root-shoot ratios coincided in general with the distribution for root weights.among the entries, with a 59% decrease from the highest to lowest value. The exotic strains also in general had higher root-shoot ratios than the commercial varieties, the herbaceum species and the experimental strain (Lubbock dwarf). There was no direct relationship between shoot weights and root weights among the genotypes for either environment. Those plants that grew large tops did not necessarily grow correspondingly large root systems (e.g. T141 has a large root system with a very small shoot compared to the herbaceum species, which has a relative large shoot and a small root system). The lack of a correlation between shoot and root growth along with genotypic differences in changes in root-shoot ratios in response to environmental demand may provide an opportunity to exploit the observed variability to improve production for a wide range of growth conditions by altering the root development and function independent of shoot development.

The effect of wind velocity, air temperature and humidity on NH 3 and SO 2 transfer into bean leaves ( phaseolus vulgaris L.)

The influence of wind velocity, air temperature and vapour pressure deficit of the air (VPD) on NH 3 and SO 2 transfer into bean leaves ( Phaseolus vulgaris L.) was examined using a leaf chamber. The measurements suggested a transition in the properties of the leaf boundary layer at a wind velocity of 0.3–0.4 ms −1 which corresponds to a Re crit value of about 2000. At higher wind velocities the leaf boundary layer resistance ( r b) was 1.5–2 times lower than can be calculated from the theory. Nevertheless, the assessed relationships between r b and wind velocity appeared to be similar to the theoretical derived relationship for r b. The NH 3 flux and in particular the SO 2 flux into the leaf strongly increased at a VPD decline. The increase of the NH 3 flux could be attributed to an increase of the stomatal conductance ( g s). However, the increase of the SO 2 flux could only partly be explained by an increase of g s. An apparent additional uptake was also observed for the NH 3 uptake at a low temperature and VPD. The SO 2 flux was also influenced by air temperature which could be explained by a temperature effect on g s. The results suggest that calculation of the NH 3 and SO 2 flux using data of g s gives a serious understimation of the real flux of these gases into leaves at a low temperature and VPD.

HNO3 deposition to a deciduous forest

The deposition velocity (Vd) of nitric acid vapor over a fully leafed deciduous forest was estimated using flux/gradient theory. HNO3 deposition velocities ranged from 2.2 to 6.0cm/s with a mean Vdon the order of 4.0cms-1. Estimates of Vdfrom a detailed canopy turbulence model gave deposition velocities of similar magnitude. The model was used to investigate the sensitivity of Vdto the leaf boundary-layer resistance and leaf area index (LAI). Although modeled deposition velocities were found to be sensitive to the parameterization of the leaf boundary-layer resistance, they were less sensitive to the LAI. Modeled Vd's were found to peak at LAI = 7.

The sensitivity of modeled SO2 fluxes and profiles to stomatal and boundary layer resistances

A higher-order closure model for canopy/surface exchange is presented and applied to an oak-hickory forest to model SO2 deposition for typical summertime conditions. The model is then used as a tool to investigate the sensitivity of modeled fluxes (the deposition velocity) and concentration profiles to the parameters used to compute the leaf boundary layer and stomatal resistances. Both the deposition velocity and concentration profiles show little sensitivity to variations in the leaf boundary-layer resistance (r b) since it generally comprises only a small fraction of the total resistance to diffusion from the air to the sub-stomatal cavity. The deposition velocity (V d ) is more sensitive to variations in the minimum stomatal resistance (r s ,min) and light response coefficient (β) than r b .It was found that 50% variations in β give a maximum difference of 0.2 cm s−1 in V d while 30% variations in r s , min produce a maximum difference of near 0.3 cm s−1.

Morphological changes along an altitude gradient and their consequences for an andean giant rosette plant.

Selected morphological features were measured in five populations of the giant rosette plant Espeletia schultzii occurring along an elevation gradient from 2600 to 4200 m in the Venezuelan Andes. Pith volume per amount of leaf area increases with elevation resulting in significantly larger water storage capacity at higher elevations. Thickness of leaf pubescence and, therefore, leaf boundary layer resistance, also increases with elevation resulting in both potentially higher leaf temperatures relative to air temperature and higher leaf to air vapor pressure gradients. The net effect on transpiration rate would depend on ratios of stomatal to boundary layer resistance and leaf energy balance. At higher elevations the central rosette leaves are more vertically oriented and the leaf bases show a pronounced curvature as the intersection with the main axis is approached. This gives these rosettes a distinctly paraboloid appearance and probably enhances capture and retention of incident long and shortwave radiation by the apical bud and expanding leaves. Features which result in enhanced water storage capacity and higher plant temperatures relative to air temperature without greatly increasing water loss are adaptive in high altitude paramo habitats where water availability and growth are limited by year round low temperatures (mean 2-3° C).

Studies on the Effect of Leaf-Boundary Layer Resistance on the Matter Production of Crop

Studies on the Effect of Leaf-Boundary Layer Resistance on the Matter Production of Crop

Gas exchange in open-top field chambers—I. Measurement and analysis of atmospheric resistances to gas exchange

Factors controlling rates of gas exchange between the atmosphere and plants growing in open-top field chambers, commonly used for studying crop responses to air quality, are measured and analysed. Restrictions on gas transfer by incursion through the open top, by air movement from the ventilation fan and by transfer through the leaf boundary layer are described by a resistance analogue. A method of measuring incursion resistance r i , using sulphur hexafluoride is described and applied to chambers of a standard design and of a design with a truncated conical top. There was a well-defined variation of r i with windspeed for the modified design and r i , decreased for a given windspeed as crop height increased. Leaf boundary layer resistance r b was measured in both chamber designs and was analysed as a function of incursion and fan ventilation rate. Values of r b in chambers may be lower than in crop canopies in the field, and possible consequences of this difference are discussed.

Studies on the Effect of the Leaf-Boundary Layer Resistance on the Matter Production of Crop

The diffusion resistance of the leaf boundary layer is different locally with the individual leaves, and it has an effect on the photosynthetic rate (P). Therefore, it is expected that the local difference of the boundary layer resistance makes individual leaves produce the distribution patterns of P. Then, bush beans were used as an example crop and experiments were carried out under various air flow conditions.The distribution patterns of P were determined by the difference of concentration of assimilatory starch in leaves. Starch was made to appear brown color by means of the iodine test method, and its concentration was quantitatively measured by means of Photo-Pattern Analyzer and divided into 4 levels. The results are as follows;1) When the attack angle of air to a leaf (α) is 0°, higher parts of P are distributed at both the leading edge and side tip of leaves.2) The distribution patterns of P are almost uniform when α is positive, in other words when the air flow runs against the leaf's abaxial surface.3) When α is negative, higher parts of P are distributed around the course of the air flow which turns around slant leading edges of the delta shaped leaves, while for rectangular shaped leaves they distributed around the side and trailing edges like a horseshoe shape.4) When α is ±60°, distribution of P is low at the center of leaves.5) When the leaves vibrate freely in the turbulent air flow, higher parts of P are distributed at the front margine of leaves, which correspond to the thinner parts of equivalent viscous sublayers.As mentioned above, the distribution of P of a single leaf agrees well with that expected from the detailed analysis of the boundary layer structure for model leaves. From these results it has been confirmed that the slight difference of the leaf-boundary layer produces a great effect on the photo-synthetic rate of a single leaf.

Studies on the Effect of Leaf-Boundary Layer Resistance on the Matter Production of Crop

Studies on the Effect of Leaf-Boundary Layer Resistance on the Matter Production of Crop