Leaf Dispersion Research Articles

Simulation on Different Patterns of Mobile Laser Scanning with Extended Application on Solar Beam Illumination for Forest Plot

Light detection and ranging (LiDAR) technology has become a mainstream tool for forest surveys, significantly contributing to the improved accuracy of forest inventories. However, the accuracy of the scanned data and tree properties derived using LiDAR technology may differ depending on the occlusion effect, scanning configurations, various scanning patterns, and vegetative characteristics of forest plots. Hence, this paper presents a computer simulation program to build a digital forest plot composed of many tree models constructed based on in situ measurement information and two mobile scanning patterns, i.e., airborne laser scanning (ALS) and ground-based mobile laser scanning (MLS). Through the adjustment of scanning parameters and the velocity of vehicle loading LiDAR sensors, the points scanned using two scanning patterns were compared with the original sampling points, derived from the constructed digital forest plots. The results show that only 2% of sampling points were collected by LiDAR sensors with the fastest vehicle speed (10 m/s) and coarsest scanning angular resolution (horizontal angular resolution 0.16° and vertical angular resolution 1.33°), and approximately 50% of sampling points were collected by LiDAR sensors with slow vehicle velocity (1.25 m/s) and a finer scanning angular resolution (horizontal angular resolution 0.08° and vertical angular resolution 0.33°). Meanwhile, the potential extended application of the proposed computer simulation program as a light model of forest plots was discussed to underpin the creation of the forest digital twin. Three main conclusions are drawn: (1) the collected points from airborne laser scanning (ALS) are higher than those collected from ground-based mobile laser scanning (MLS); (2) reducing the vehicle velocity is more efficient at improving the high density of the point cloud data than by increasing the scanning angular resolution; (3) the lateral extension of crown area increasing the light beams’ receptor area and the clumped leaf dispersion augmenting the light penetration with vertical elongation are the two paramount factors influencing the light transmittance of tree crowns.

Morphological acclimation to agronomic manipulation in leaf dispersion and orientation to promote “Ideotype” breeding: Evidence from 3D visual modeling of “super” rice (Oryza sativa L.)

Food security is confronted by major threats from crop yield stagnation and global climate change. The benefits of phenotypic plasticity across environments for given crop genotypes are thought to be imperative for high-yielding cropping systems. Given that 3D modeling is increasingly recognized for dissecting crop phenotypic plasticity, it requires an assessment of the potential benefits of architectural adaptation of super rice to different agronomic practices. In this study, we focused on a comprehensive evaluation of the phenotypic plasticity of super rice on the aspects of 3D architectural “reoptimization,” photosynthetic productivity, nitrogen economy, and grain yield. A super rice phenotype in superhigh-yielding practice (SH) displays a “reoptimized” morphogenesis in the leaf vertical dispersion and orientation in comparison to that in Farmer's practice (FP). Specifically, a super rice phenotype in SH is provided with a high cumulative rate and peaks of leaf area, increasing the distribution of high leaf inclination angles in comparison to that in FP, particularly in the upper parts of the canopy. These “reoptimizations” sustained profits in light environment within a canopy, leaf area duration, photosynthetic light harvest, and light utilization efficiency and were coordinated with improving nitrogen uptake and assimilation. The current literature indicates that the agronomic plasticity of super rice in architectural “reoptimization” is a promising perspective for high yield formation. Our results suggest that more emphasis should be placed upon agronomic adaptation strategies for super rice across diverse genotypes and environments to further improve crop establishment and photosynthetic productivity.

Leaf canopy architecture determines light interception and carbon gain in wild and domesticated Oryza species

Oryza sativa has many wild relatives that exhibit a wide range of canopy architectures. Two fast-growing ecotypes of an Australian wild rice (O. meridionalis) were compared with cultivated O. sativa to analyze the efficiency of light interception in mature vegetative canopies and how well light capture predicts biomass production. Canopies of individual plants were digitized and analyzed with YplantQMC. Plants were also grown at 700 ppm CO2 as a means of increasing the canopy density and assessing the relative impact of rising CO2 levels on photosynthesis, light interception and biomass. Simulations of light interception for these morphologically diverse canopies were combined with photosynthetic light response curves to estimate theoretical carbon gain. Both O. meridionalis genotypes had more tillers and leaves, and denser canopies, than O. sativa but one (‘Howard Springs’) intercepted more light than the other two genotypes, resulting in the highest simulated carbon gains. Photosynthesis per unit leaf area (Amax) was enhanced by 20–50% in 700 ppm CO2 in all genotypes but the proportion of total leaf area intercepting light decreased as a result of denser leaf canopies in these plants. Estimated total shoot carbon gain increased strongly at elevated CO2 despite these denser canopies and increased self-shading, reaching a peak in ‘Howard Springs’ at 700 ppm CO2. Thus, while high crown density and low leaf dispersion reduced the efficiency of light interception by the canopy, elevated CO2 still enhanced growth through consistently higher assimilation rates. Simulations showed that during the Summer Solstice, carbon gain was predicted to be greater than at the Equinox through superior light capture, particularly in ‘Howard Springs’. However, the relationship between light interception and carbon gain was broadly maintained, regardless of genotype, CO2 concentration and season.

Cytosolic APX2 is a pleiotropic protein involved in H2O2 homeostasis, chloroplast protection, plant architecture and fertility maintenance.

Rice cytoplasmic APX2 is a pleiotropic protein, densely distributed around chloroplasts. It plays key roles in H2O2 homeostasis and chloroplast protection, and is related to plant architecture and fertility regulation. Ascorbate peroxidases (APXs) catalyze the conversion of H2O2 into H2O. In this report, we systematically investigated the function of cytosolic APX2 using a T-DNA knockout mutant. Loss of OsAPX2 altered rice architecture including shoot height and leaf inclination, resulting in shoot dwarfing, leaf dispersion and fertility decline. Sixty-five differentially expressed proteins were identified in flag leaves of the milk-ripe stage, mainly involved in photosynthesis, glycolysis and TCA cycle, redox homeostasis, and defense. The absence of APX2 severely impacted the stability of chloroplast proteins, and dramatically reduced their expression levels. Subcellular localization showed that APX2 was enriched around each chloroplast to form a high concentration sphere, highlighting chloroplasts as key targets protected by the protein. Accumulation of H2O2 was suppressed in the KO-APX2 mutant, which may benefit from increased CAT activity and functional complementation of APX family members. Unexpectedly, the accumulation of soluble sugar, especially sucrose increased significantly, suggesting that APX2 was involved in regulation of sugar metabolism. Obviously, roles of the cytosolic APX2 are very profound and complex in rice. It can be concluded that the cytosolic APX2 is a pleiotropic protein and an important regulator in ROS homeostasis, chloroplast protection, carbohydrate metabolism as well as plant architecture and fertility maintenance.

Ontogenetic changes in crown architecture and leaf arrangement: effects on light capture efficiency in three tree species differing in leaf longevity

AbstractPronounced strategy shifts along ontogeny have been observed in several tree species, mainly because of the trend to maximize growth during the seedling stage, which constitutes the most vulnerable part of the tree's life cycle. Our aim here was to analyze the ontogenetic changes in crown characteristics and light capture patterns in three Quercus species: the evergreens Quercus ilex and Quercus suber and the deciduous Quercus faginea co‐occurring in a Mediterranean open woodland. The seedlings were distributed in the large clearings among the adults and received full sunlight. We constructed three‐dimensional models of the aerial parts of seedlings and mature trees of the three species, using the YplantQMC program. Large differences between growth stages were observed for all variables. The seedlings exhibited smaller branch sizes and crown densities than those observed in the adult trees. Leaf angles to horizontal also tended to increase during ontogeny, whereas leaf dispersion and the observed distances between leaves tended to decrease. The amount of photosynthetic radiation absorbed per unit leaf area throughout the growing season was lower in adult specimens than in young specimens. Changes in absorption efficiency during ontogeny were more intense for the species with longer leaf life span at maturity. We conclude that more intense ontogenetic shifts in species with longer leaf life span reflect the priority change from the maximization of short‐term productivity at the seedling stage to maximizing leaf longevity during the adult stage.

Neighborhood structure influences the convergence in light capture efficiency and carbon gain: an architectural approach for cloud forest shrubs.

Although plant competition is recognized as a fundamental factor that limits survival and species coexistence, its relative importance on light capture efficiency and carbon gain is not well understood. Here, we propose a new framework to explain the effects of neighborhood structures and light availability on plant attributes and their effect on plant performance in two understory shade-tolerant species (Palicourea padifolia (Roem. & Schult.) C.M. Taylor & Lorence and Psychotria elata (Swartz)) within two successional stages of a cloud forest in Costa Rica. Features of plant neighborhood physical structure and light availability, estimated by hemispherical photographs, were used to characterize the plant competition. Plant architecture, leaf attributes and gas exchange parameters extracted from the light-response curve were used as functional plant attributes, while an index of light capture efficiency (silhouette to total area ratio, averaged over all viewing angles, STAR) and carbon gain were used as indicators of plant performance. This framework is based in a partial least square Path model, which suggests that changes in plant performance in both species were affected in two ways: (i) increasing size and decreasing distance of neighbors cause changes in plant architecture (higher crown density and greater leaf dispersion), which contribute to lower STAR and subsequently lower carbon gain; and (ii) reductions in light availability caused by the neighbors also decrease plant carbon gain. The effect of neighbors on STAR and carbon gain were similar for the two forests sites, which were at different stages of succession, suggesting that the architectural changes of the two understory species reflect functional convergence in response to plant competition. Because STAR and carbon gain are variables that depend on multiple plant attributes and environmental characteristics, we suggest that changes in these features can be used as a whole-plant response approach to detect environmental filtering in highly diverse tropical forest communities.

Light interception efficiency explained by two simple variables: a test using a diversity of small‐ to medium‐sized woody plants

• Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. • We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). • The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. • These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.

A three-dimensional light transfer model based on the vertical point-quadrant method and Monte-Carlo simulation in a Fagus crenata forest canopy on Mount Naeba in Japan

We divided the canopy of a 50-year-old secondary Fagus crenata stand into an array of small three dimensional (3D) rectangular cells (termed voxels, 0.25 m × 0.25 m × 0.20 m in height), and determined the leaf area density (LAD) of all voxels using the vertical point quadrat method. The estimated LAD values showed good agreement, at both voxel and stand scales, with measured values obtained by destructive sampling and litter trapping, respectively. The 3D distribution of LAD was then imported into a ray-tracing program based on a turbid medium analogy to simulate photosynthetic photon flux density at leaf surface (PPFD leaf) within a voxel at 30-min intervals. To include finite leaf size and non-random leaf dispersion within a voxel in the light calculation, the angular dependency of the shoot silhouette to projection area ratio (SPAR), with the foliage volume the same as the voxel, was used as the light extinction coefficient in the analogy. In addition, the frequency distribution of PPFD leaf within a voxel was simulated at each interval for all leafy voxels in the canopy, by integrating a Monte-Carlo simulation of the voxel-shading fraction of a solar disc in the light calculations. The light simulation provided reasonable estimates of both the mean PPFD leaf and the frequency distribution of PPFD leaf in a voxel for evaluating canopy photosynthesis ( P canopy), when comparing them to the measured values. Ignoring the within-voxel variation in PPFD leaf caused a 21.8% overestimation of P canopy during the summer period (July to September). The results demonstrate the potential importance of a detailed description of light heterogeneity in P canopy calculations.

Simple equations to estimate light interception by isolated trees from canopy structure features: assessment with three‐dimensional digitized apple trees

* Simple models of light interception are useful to identify the key structural parameters involved in light capture. We developed such models for isolated trees and tested them with virtual experiments. Light interception was decomposed into the projection of the crown envelope and the crown porosity. The latter was related to tree structure parameters. * Virtual experiments were conducted with three-dimensional (3-D) digitized apple trees grown in Lebanon and Switzerland, with different cultivars and training. The digitized trees allowed actual values of canopy structure (total leaf area, crown volume, foliage inclination angle, variance of leaf area density) and light interception properties (projected leaf area, silhouette to total area ratio, porosity, dispersion parameters) to be computed, and relationships between structure and interception variables to be derived. * The projected envelope area was related to crown volume with a power function of exponent 2/3. Crown porosity was a negative exponential function of mean optical density, that is, the ratio between total leaf area and the projected envelope area. The leaf dispersion parameter was a negative linear function of the relative variance of leaf area density in the crown volume. * The resulting models were expressed as two single equations. After calibration, model outputs were very close to values computed from the 3-D digitized databases.

Comparing PAR transmission models for forest understorey vegetation

AbstractQuestion: Do Beer's Law models, multi‐layer scattering models, and a semi‐empirical model for predicting PAR transmission through understorey vegetation give comparable results? Do different driving variables (LAI, PLAI and percentage cover) give different results? How do the models vary when fit with species‐specific, species‐average and the ‘default’ parameters recommended in the literature?Location: Upland boreal forests of western North America.Methods: In calibration and validation plots, PAR transmission was measured, total cover visually estimated, and leaf dispersion, PLAI and cover estimated for each species using a point‐frame. Leaf inclination was measured by clinometer. PAR transmission was modelled using empirically‐fit Beer's Law models, a semi‐empirical model based on hemispherical gap fraction and first‐order scattering, and a multi‐layer model allowing multiple scattering. All models were modified to use leaf area index (LAI), vertically projected leaf area index (PLAI), or percentage cover data.Results: The empirical Beer's Law models had the least bias and best precision in predicting PAR transmission. The semi‐empirical model also had little bias and good precision, since the scattering coefficient compensated for problems in the estimation of gap fraction. The multi‐layer model consistently underestimated transmission. There was little benefit in accounting for species separately. LAI and PLAI‐based models were the most precise, but percentage cover models also provided reasonable predictions of PAR transmission.Conclusions: PAR transmission through forest understories can be simply modelled with Beer's Law using one empirical coefficient representing the average understorey species. More complex scattering models are less effective, likely because they fail to account for the complexity of the dispersion of this vegetation layer and its effect on radiation scattering.

Comparing PAR transmission models for forest understorey vegetation

Abstract Question: Do Beer's Law models, multi-layer scattering models, and a semi-empirical model for predicting PAR transmission through understorey vegetation give comparable results? Do different driving variables (LAI, PLAI and percentage cover) give different results? How do the models vary when fit with species-specific, species-average and the ‘default’ parameters recommended in the literature? Location: Upland boreal forests of western North America. Methods: In calibration and validation plots, PAR transmission was measured, total cover visually estimated, and leaf dispersion, PLAI and cover estimated for each species using a point-frame. Leaf inclination was measured by clinometer. PAR transmission was modelled using empirically-fit Beer's Law models, a semi-empirical model based on hemispherical gap fraction and first-order scattering, and a multi-layer model allowing multiple scattering. All models were modified to use leaf area index (LAI), vertically projected leaf area index (PLAI), or per...

Leaf dispersion and light partitioning in three‐dimensionally digitized tall fescue–white clover mixtures

AbstractA three‐dimensional digitizing method was used to assess the canopy structure of six Festuca arundinacea (FA)–Trifolium repens (TR) mixtures during the installation stage. Virtual canopy images were synthesized and used to derive light interception and partitioning between species. Computations from images were compared with a simple light model based on Beer’s law, in order to analyse within‐ and between‐species foliage dispersion. The total leaf area index of the mixtures ranged from 0·6 to 4·5. The fraction of FA foliage overtopping TR was 9–30%. The mean inclination of FA and TR was 66 and 57°, respectively. Within‐species dispersion parameters of FA and TR were about 0·8 and 1·0, namely clumped and random foliage dispersion, respectively. Although FA was sown in rows, between‐species dispersion was random. Lower leaf inclination and lesser clumping in TR compensated foliage overtopping by FA, so that light partitioning between FA and TR (about 80 and 20%, respectively) was similar to the species contribution to total canopy foliage. Since between‐species dispersion was random, a simple two‐layer light model based on Beer’s law provided correct estimations of light partitioning (RMSE = 0·05), although light interception by FA was slightly overestimated because of its clumped dispersion.

PAR extinction in shortgrass ecosystems: effects of clumping, sky conditions and soil albedo

The amount of photosynthetically active radiation (PAR) absorbed by a canopy (APAR) is an important driving variable for vegetation processes such as photosynthesis. PAR extinction in clumped canopies of shortgrass ecosystems is the focus of this paper. Directional gap fractions estimated at peak biomass on several Mexican shortgrass ecosystems with a hemispherical radiation sensor (Li-Cor, LAI-2000) were higher than those predicted by a Poisson model assuming a random leaf dispersion (RLD). LAI-2000-estimated gap fractions, together with independent estimations of plant area index (PAI), and leaf and stem angle distribution (LSAD) were used for estimating the angular course of a leaf dispersion parameter λ( θ). Radiation extinction coefficients simulated for all solar zenith angles using Markov chain processes and estimated λ( θ) were subsequently incorporated in a simple radiative transfer model for estimating the efficiencies of instantaneous and daily integrated PAR interception and absorption, and for studying the effects of clumping, sky conditions and soil albedo on PAR absorption. For clear sky condition, instantaneous PAR absorption showed marked directional effects, therefore indicating that using a constant extinction coefficient in canopy photosynthesis models working at hourly time step would be inaccurate. The effects of clumping, sky conditions and soil albedo were all found to be significant for low PAI, and decreased with higher PAI. As shortgrass ecosystems are characterized by low PAI, neglecting these effects would give inaccurate estimations of PAR absorption. Daily PAR absorption was found to be significantly higher than PAR interception for low PAI, especially when soil albedo was high, and lower than PAR interception for high PAI. These results indicate that in canopy photosynthesis models where APAR is estimated from simple exponential-like relationships calibrated using PAR interception measurements, the PAR available for photosynthesis might be significantly underestimated in the first stages of the growth, and may be overestimated in the later stages of the growing season.

Modelling and measuring vertical light absorption within grass–clover mixtures

The vertical profile of leaf area, leaf dispersion, and light partitioning and absorption were studied during two regrowth periods in mixtures of perennial ryegrass and two white clover cultivars differing in leaf size under cutting management without fertiliser nitrogen. A triangular leaf area density function with height gave a good description of the vertical leaf area profile for both species. Leaf dispersion was studied by analysing inclined point quadrat data. Calculated leaf dispersion factors of both species were linearly correlated with downward cumulative LAI of the mixture and appeared to be the principal cause for variation in the extinction coefficient ( k) with canopy height. This relationship was negative for clover and positive for grass, corresponding with a shift from regular leaf dispersion in the top layers to a clumped dispersion in the bottom layers for clover and the reverse pattern for grass. Measured light profiles could be exactly mimicked with a modified version of a general multi-layer light competition model by incorporating for both species leaf dispersion as a function of downward cumulative LAI in combination with fitted dispersion-free values of k which only reflect the leaf-angle distribution. Competitive success of clover over grass for light absorption in this study was, next to its greater contribution to total LAI and a more planofile leaf-angle distribution, related to its higher position in the mixture where maximum leaf area density occurred and regular leaf dispersion in the top layers of the canopy. These last two characteristics were especially manifest in the large-leaved clover.

Model simulation of spatial distribution of photosynthesis in structurally differing plant communities in the Central Caucasus

The aim of the present study was to investigate the significance of structural properties for whole canopy photosynthesis ( A can). Effects of both classical parameters of canopy structure (vertical distribution of leaf area, plant area and leaf inclination) and of leaf dispersion on A can were analysed by means of a relatively simple canopy photosynthesis model. This model was designed for photosynthetic input parameters based on measured field parameters. Eight structurally different species-rich seminatural and natural plant communities (used and abandoned pastures, hay meadows, tall herbs and dwarf shrubs) in the subalpine belt of the Central Caucasus were investigated. The validation by means of a micrometeorological approach showed that the estimate of A can in the model corresponds well with the measurements in very differently structured, species-rich plant communities. Simulations showed that the significance of canopy properties for canopy photosynthesis essentially depends on the vertical distribution of the leaf area. Therefore this parameter was used for classifying the canopies investigated. Three types of canopies could be distinguished: type 1 with the leaf area amassed near the soil surface and the structural shape of a pyramid with a broad base; type 2 with a gradual increase in leaf area from the canopy surface to the ground (pyramid with a narrow base) and type 3 with a concentration of the leaf area in the upper canopy (inverted pyramid). The significance of the structural properties for A can increases from type 1 to type 3 and is higher when the stand is dominated by a single species. In stands of type 1 canopy structure did not limit A can via light climate, but rather by temperature effects. In stands of type 2 and type 3, mutual shading is an important limiting factor for canopy photosynthesis, but the structural properties of type 3 stands are better optimized for A can. Simulations with assumed equal leaf area index and photosynthetic characteristics showed that in canopies of type 3 the structural properties (leaf inclination, leaf dispersion) are such that the major part of the leaf area in the canopy is supplied with intermediate photosynthetic photon flux densities, thus increasing daily total A can. Sensitivity analysis of A can to non-random leaf dispersions showed effects especially in stands of type 2 and type 3. Clumped leaf dispersions resulted in better light utilization and higher A can. When compared with the leaf dispersions measured in the canopies, the simulation results show that the canopies optimize leaf dispersion with respect to A can.

Leaf orientation and sunlit leaf area distribution in cotton

The diurnal leaf orientation behaviour of row-planted cotton plants ( Gossypium hirsutum L. cv. ‘DES 119’) and its relationship to sunlit leaf area distribution at three stages of development were studied in the field. Electromagnetic digitizing was used for plant geometrical structure measurement for three periods of 2 h during the day. Cotton leaves showed a diaheliotropic response throughout the day. This heliotropic behaviour varied according to growth stage. In addition to changes in orientation, leaves also moved in space. The distance moved by a leaf between two observation times increased with stage of development, in agreement with petiole and blade lengths. Sunlit leaf area distribution varied according to stage of development. Analysis of interception showed that probability of light interception was greater in the morning and in the afternoon than at noon. Without a diurnal change in canopy structure, cotton plants would intercept less direct radiation in the morning and in the afternoon. Leaf dispersion was regular during the first and last stage of development, but it was clumped during the intermediate stage. Leaf dispersion was more regular in the morning and the afternoon than at noon.. Leaf dispersion, however, changed primarily with sun direction and not with canopy structure. This would indicate that small changes in leaf location do not significantly affect light interception. Finally, the ecological significance of diaheliotropism and the implications for modelling light interception in cotton plants are discussed.

Evaluation of structure description requirements for predicting gap fraction of vegetation canopies

Gap fraction in homogeneous crop canopies is usually computed using the Poisson law. For heterogeneous canopies, gap fraction may be computed by assuming the canopy to be made of contiguous cells, in which the Poisson law is used. However, the implicit hypothesis of random phyto-element position is frequently violated, because of plant structure and canopy planting pattern. This may empirically be taken into account by introducing an additional dispersion parameter. However, we are unable, from a deterministic approach, to predict such a parameter. Most experimental works to study the relations between gap fraction and canopy structure suffer from the limited accuracy of the measurements. A laboratory experiment was therefore conducted on artificial canopies, which enabled an accurate description of both structure and gap fraction. The usual turbid medium approach, with random leaf dispersion, but an accurate description of spatial variations of canopy structure, enabled us to predict the directional gap fraction of the artificial canopies with a maximum error of about 0.06. The accuracy of simulations remained fairly good when using simplified descriptions of structure, provided that they took into account most of the spatial variations of leaf area index. The residual error of 0.06 indicates however a significant departure from the Poisson law. Analysis of spatial and directional dependence of gap fraction measurements showed that fitting simulated and experimental data required the introduction of a dispersion parameter incorporating spatial, zenith, and azimuth variations.

A theoretical analysis of radiation interception in a two-species plant canopy

Classical radiation interception laws for monospecific canopies cannot be used directly for bispecific canopies. They are always based on the gap frequency concept (i.e., the probability of no interception), which does not provide any information about the sharing of intercepted radiation between species. A theoretical analysis is reported that relates the radiation interception probabilities to the geometrical structure of the crop (i.e., the leaf area density and the leaf angle distribution of each component) and the foliage dispersion. The leaf dispersion globally describes the spatial relations between the leaf elements; it may be regular if the leaves avoid mutual shading, random, or clumped if they tend to overlap. For such two-species canopies, the leaf dispersions within each component (WSLD: within-species leaf dispersion) and between two species (BSLD: between-species leaf dispersion) are distinguished. Using bivariate multinomial distributions, general expressions for the gap frequency and the interception probabilities of a homogeneous vegetation layer were set as exponential functions of the foliage thickness, taking into account a number of dispersion parameters as small as possible. First, one WSLD for each species describes the rate of foliage overlap between the leaves of this species; it is quite similar to the leaf dispersion of single-species canopies. Second, the rate of foliage overlap between species is characterized by one BSLD. As in monospecific canopies, this parameter is positive, zero, or negative, respectively, for regular, random, or clumped BSLD. Third, another BSLD parameter has to be used if the foilage overlap between species is more than random (i.e., in the case of clumped BSLD); the latter shows the direction of overlap between species and may be taken as the probability of finding a leaf element of the first species in the case of marked overlapping. Suggestions for estimating the leaf dispersion parameters and possible uses of such relations are also discussed.

A critical review of light models for estimating the shortwave radiation regime of plant canopies

A general review of geometrical and statistical light models is presented in this paper. In the geometrical approach plant shapes are simulated by various geometrical forms described by characteristic dimensions. Geometrical models may be divided into two classes — those which consider individual shapes and those which consider an arrangement of shapes. In the statistical approach the location of plant elements is parameterized by various distributions. The type of leaf dispersion in space is the most important consideration in statistical models. Four different types of leaf dispersion are considered in this review: regular leaf dispersion, clumped leaf dispersion, random leaf dispersion, and variable leaf dispersion. Hypotheses which underlie the various random and more generalized types of statistical light models are presented. Several models in the literature are discussed in terms of these assumptions. Although, in many cases, required plant data and actual light measurements in the field are grossly inadequate for experimental verification of light models, it appears that the light regime in plant canopies can be adequately described by those models already available. However, for the most part, these models are very complex and a synthesis of these fundamental models into workable expressions that can be used by agronomists, crop ecologists and others concerned with breeding plants for more efficient interception of light is needed.

The effect of gibberellic acid on apparent photosynthesis and dark respiration of simulated swards of Pennisetum clandestinum Hochst

Gibberellic acid (GA3 at 40 g/ha equivalent was applied to simulated swards of Pennisetum clandestinum (kikuyu grass) at the fourth leaf stage in a glasshouse held at a 25°C maximum and 15° minimum diurnal temperature cycle. Continuous 24-hr recordings of apparent photosynthesis and dark respiration in relation to normal variation in solar radiation were made on the whole swards by using an open chamber system monitored by infrared gas analysis. No effect was noted until 9 days from spraying, by which time the GA3 application stimulated the apparent photosynthetic rate of the sward and reduced the dark respiration rate. These effects became greater with time. The stimulation of apparent photosynthesis increased as the level of radiation increased. At 21 days from spraying, while large increases in apparent photosynthesis and reductions in dark respiration rate were being recorded, a botanical separation of the swards was made. GA3 increased the total dry weight (tops plus roots) of the sward and the dry matter above soil level. The latter effect was due to a greater amount of stem dry matter in the GA3-treated sward. GA3 reduced the dry weight of senescent leaf blade and leaf sheath tissue. GA3 had no effect on leaf area index. Area per leaf was increased, but GA3 stimulated apical dominance reduced the number of leaves per sq metre of sward. It is postulated that stimulation of apparent photosynthesis by GA3 is related to leaf age and a change in canopy structure through an improvement in leaf dispersion.