Cumulative Leaf Area Research Articles

The response of yield, number of clusters, and cluster weight to meteorological factors and irrigation practices in grapevines: A multi-experiment study

Knowledge of the yield components of wine grapevines is essential to achieve target production and effectively design vineyard management. Therefore, it is necessary to map and analyze the factors that control the yield components to optimize wine production and profitability. The objective of this study was to identify the ranked effects of environmental and agrotechnical factors (e.g., meteorological, physiological, vegetative, and irrigation) on grapevine yield, number of clusters, and cluster weight, and to quantify their patterns. To achieve this objective, a multi-experiment database was used, assembling data collected during four long-term irrigation experiments including different varieties and diverse climatic regions across Israel. First, a paired analysis of the yield components was conducted to define the relationships among them. Then, prediction models for the three variables (i.e., yield components) were designed using the support vector machine (SVM) algorithm to explore the ranked effects and patterns of the controlling factors. Validation was conducted using a test set, consisting of 20% of the dataset. The relationships between yield and cluster weight (0 < r < 0.73) and between yield and the number of clusters were always positive (0.67 < r < 0.77), while negative relationships were found between cluster weight and number of clusters (-0.61 < r < 0). The accuracy of the prediction models was high, with mean absolute errors (normalized to the range) ranging between 4.1%-8.11%. The most dominant factors affecting yield components were cumulative leaf area index, accumulated growing degree days (GDD), cumulative irrigation, and evapotranspiration. Leaf area showed an optimum point, below and above which the values of the yield components decrease; GDD negatively affected yield and cluster weight but had a positive effect on the number of clusters; irrigation was positively associated with yield components but leveled out above 348-395 mm per season. Reference evapotranspiration had a negative effect on yield and cluster weight, and a generally positive effect on the number of clusters. Utilizing data from multiple experiments and applying multivariate analysis, allow to define general patterns of relationships and interactions between controlling factors and yield components. Extracting this knowledge from the data and managing the growing amounts of information can be achieved through complex modeling and the implementation of standard protocols for data collection and monitoring.

Variación fenológica y enverdecimiento de la Reserva de la Biósfera Mariposa Monarca (2000-2019)

Introduction: The Abies forest of the Monarch Butterfly Biosphere Reserve (MBBR) is a key ecosystem for the butterfly Danaus plexippus L. The study of this habitat is essential to understand the global effects of environmental transformation. Objectives: To study the impact of climate change over the last 20 years (2000-2019) on the MBBR, to (1) describe variations in climate and phenology; (2) analyze the relationships between LAI (leaf area index) and climate; and (3) estimate greening Materials and methods: Monthly mean temperature and precipitation values from the NASA Giovanni database and the Copernicus LAI series were analyzed. Using the TIMESAT algorithm, 11 phenological variables per pixel and year were determined and the annual spatial average of each one was calculated for each type of land cover (pine, oyamel, shrubs and crops) and the cumulative increase of the extended integral (greening). Results and discussion: Mean temperature increased 0.87 °C in 2000-2019. Beginning, middle, and end of the season have occurred progressively earlier, and cumulative leaf area, as well as minimum and maximum LAI, have increased. Greening was recorded in 53.21 % of the MBBR and browning in 33.97 %. Conclusion: The increase in mean temperature affects phenology of MBBR, causing longer growing seasons and net greening of the area.

Dynamic Simulation of the Crown Net Photosynthetic Rate for Young Larix olgensis Henry Trees

Numerical integration of the instantaneous net photosynthetic rate (An) is a common method for calculating the long-term CO2 uptake of trees, and accurate dynamic simulation of the crown An has been receiving substantial attention. Tree characteristics are challenging to assess given their aerodynamically coarse crown properties, spatiotemporal variation in leaf functional traits and microenvironments. Therefore, the variables associated with the dynamic variations in the crown An must be identified. The relationships of leaf temperature (Tleaf), the vapor pressure deficit (VPD), leaf mass per area (LMA) and the relative depth into the crown (RDINC) with the parameters of the photosynthetic light-response (PLR) model of Larix olgensis Henry were analyzed. The LMA, RDINC and VPD were highly correlated with the maximum net photosynthetic rate (Amax). The VPD was the key variable that mainly determined the variation in the apparent quantum yield (AQY). Tleaf exhibited a significant exponential correlation with the dark respiration rate (Rd). According to the above correlations, the crown PLR model of L. olgensis trees was constructed by linking VPD, LMA and RDINC to the original PLR equation. The model performed well, with a high coefficient of determination (R2) value (0.883) and low root mean square error (RMSE) value (1.440 μmol m−2 s−1). The extinction coefficient (k) of different pseudowhorls within a crown was calculated by the Beer–Lambert equation based on the observed photosynthetically active radiation (PAR) distribution. The results showed that k was not a constant value but varied with the RDINC, solar elevation angle (ψ) and cumulative leaf area of the whole crown (CLA). Thus, we constructed a k model by reparameterizing the power function of RDINC with the ψ and CLA, and the PAR distribution within a crown was therefore well estimated (R2 = 0.698 and RMSE = 174.4 μmol m−2 s−1). Dynamic simulation of the crown An for L. olgensis trees was achieved by combining the crown PLR model and dynamic PAR distribution model. Although the models showed some weakened physiological biochemical processes during photosynthesis, they enabled the estimation of long-term CO2 uptake for an L. olgensis plantation, and the results could be easily fitted to gas-exchange measurements.

Vertical distributions of chlorophyll and nitrogen and their associations with photosynthesis under drought and rewatering regimes in a maize field

In this study, we characterize the vertical leaf distribution of chlorophyll (Chl) and nitrogen (N) content and their associations with leaf photosynthetic responses in Zea mays L. under field watering regimes. We simulated five precipitation patterns, including a drought-rewatering sequence using an electric-powered, rainproof shelter. The results indicate the vertical leaf Chl and N distribution versus the cumulative leaf-area index (LAIc) fit well into a significant quadratic function. The simulated precipitation patterns significantly influenced the parabolic curve trajectory patterns and their parameters. Chlorophyll and N contents had the same trend, with a close and positive relationship. Drought stress followed by rewatering increased the slopes of the linear equations but narrowed the parabolic opening of the quadratic functions. This finding implies that the relationship between Chl and N content can be used to estimate responses to drought and rewatering. The findings also suggested that the relationship patterns between Chl and N levels could be an assessment tool for N-fertilizer managements under different drought conditions to maintain high yields in maize production. Principal component analysis indicated the correlations between functional traits in maize leaves and the responses to drought and rehydration. These findings help to improve drought management and cultivar selection, which will be important in coping with the severe intensity and high frequency of episodic drought events expected from climate change.

Rate of photosynthetic induction in fluctuating light varies widely among genotypes of wheat.

Crop photosynthesis and yield are limited by slow photosynthetic induction in sunflecks. We quantified variation in induction kinetics across diverse genotypes of wheat for the first time. Following a preliminary study that hinted at wide variation in induction kinetics across 58 genotypes, we grew 10 genotypes with contrasting responses in a controlled environment and quantified induction kinetics of carboxylation capacity (Vcmax) from dynamic A versus ci curves after a shift from low to high light (from 50 µmol m–2 s–1 to 1500 µmol m–2 s–1), in five flag leaves per genotype. Within-genotype median time for 95% induction (t95) of Vcmax varied 1.8-fold, from 5.2 min to 9.5 min. Our simulations suggest that non-instantaneous induction reduces daily net carbon gain by up to 15%, and that breeding to speed up Vcmax induction in the slowest of our 10 genotypes to match that in the fastest genotype could increase daily net carbon gain by up to 3.4%, particularly for leaves in mid-canopy positions (cumulative leaf area index ≤1.5 m2 m–2), those that experience predominantly short-duration sunflecks, and those with high photosynthetic capacities.

Estimation of the vertical leaf area profile of corn (Zea mays) plants using terrestrial laser scanning (TLS)

It remains challenging to estimate a crop’s leaf area profile accurately and automatically. Terrestrial laser scanning (TLS) offers a potential solution, since it has a small footprint and can be used to rapidly and accurately resolve the canopy’s 3D structure. To improve the accuracy of vertical leaf area profile estimation, we selected point clouds from three corn plants scanned in two TLS experiments and used an improved voxel-based leaf area profile estimation method based on Wilson’s contact-frequency theory. The improvement has three aspects: First, it separates corn leaf points from stalk points using the difference-of-normals (DoN) method, which is commonly used to separate different surfaces of manmade objects such as tables or walls. The DoN method captures the geometrical difference between corn leaves and stalks. Second, it determines the optimal voxel size based on the calculated contact frequency. Third, it applies Wilson’s voxel-based contact-frequency method to the LiDAR point clouds. Comparing the estimated and measured leaf area profiles showed that separating leaf and stalk points improved the leaf area accuracy, which depended on the voxel size. The optimal voxel sizes were 10 cm × 10 cm × 10 cm and 5 cm × 5 cm × 5 cm, respectively, in the two study years based on irradiation of the corn plant with a single laser beam. The estimated vertical and cumulative leaf area profiles agreed well with the measured profiles; mean absolute errors for the samples ranged from 15.7 to 18.9%.

Ecophysiological Responses of White Clover - Hybrid Ryegrass Mixture to Foliar Fertilisation

The rationale of the study was to check whether the association of hybrid ryegrass with white clover owing a moderate competition capacity, and optimal foliar fertilization will provide high and stable yields by reducing conventional fertilization rates. The experiment was carried out during two years of cropping with six growth cycles in a randomized block design with three replicates in Targoviste Piedmont Plain, South of Romania. In the first year, the plots received no nitrogen fertilisers in order to allow the biological potential development. In the second year, foliar fertilisation was applied six times in the fertilised variants during the vegetation season. The comparison between the six growth cycles showed that the biological potential of white clover in pure stand without foliar fertilisation was higher in the first cropping year than in the second one. Application of foliar fertilisation treatments enhanced the competition capacity of the hybrid ryegrass in the first production cycle of the second year. In white clover pure stand, the fertilisation increased the assimilatory areas from 3.6 to 4.8 (p&lt;0.05). The contribution of hybrid ryegrass to the cumulative Leaf Area Index of species in the binary mixture was significant (4.7 no fertilisation; 4.9 foliar fertilisation), which revealed a low competition capacity of white clover in mixture (0.69 no fertilisation; 1.03 foliar fertilisation). In white clover-hybrid ryegrass mixtures, foliar fertilisation showed benefits in the first two growth cycles according to the phenophase particularities and weather conditions.

Fertilization effects with P on accumulated leaf area duration, biomass and yield of three cultivars of maize in Toluca, Mexico

The effect of six phosphorus levels (0, 40, 80, 120, 160 and 200 kg/ha) on the duration of cumulative leaf area, biomass and agronomic yield was determined in the maize cultivars: Amarillo Almoloya, Cacahuacintle and Condor in 2010 and 2011. Such cultivars were sown in the Cerrillo Piedras Blancas Mexico. A completely randomized complete block design with factorial arrangement was utilized. High phosphorus levels (120, 160 and 200 kg/ha) positively affected the duration of cumulative leaf area; greatest values were obtained in Cacahuacintle. A greater duration of accumulated leaf area contributes to determine high values of biomass accumulation and grain yield in this cultivar. Leaf area duration appeared to be a useful tool for evaluating different genotypes in a given environment.

Simplification of a light-based model for estimating final internode length in greenhouse cucumber canopies.

Light quantity and quality affect internode lengths in cucumber (Cucumis sativus), whereby leaf area and the optical properties of the leaves mainly control light quality within a cucumber plant community. This modelling study aimed at providing a simple, non-destructive method to predict final internode lengths (FILs) using light quantity and leaf area data. Several simplifications of a light quantity and quality sensitive model for estimating FILs in cucumber have been tested. The direct simplifications substitute the term for the red : far-red (R : FR) ratios, by a term for (a) the leaf area index (LAI, m(2) m(-2)) or (b) partial LAI, the cumulative leaf area per m(2) ground, where leaf area per m(2) ground is accumulated from the top of each plant until a number, n, of leaves per plant is reached. The indirect simplifications estimate the input R : FR ratio based on partial leaf area and plant density. In all models, simulated FILs were in line with the measured FILs over various canopy architectures and light conditions, but the prediction quality varied. The indirect simplification based on leaf area of ten leaves revealed the best fit with measured data. Its prediction quality was even higher than of the original model. This study showed that for vertically trained cucumber plants, leaf area data can substitute local light quality data for estimating FIL data. In unstressed canopies, leaf area over the upper ten ranks seems to represent the feedback of the growing architecture on internode elongation with respect to light quality. This highlights the role of this domain of leaves as the primary source for the specific R : FR signal controlling the final length of an internode and could therefore guide future research on up-scaling local processes to the crop level.

Canopy photosynthesis in a mangrove considering vertical changes in light-extinction coefficients for leaves and woody organs

The light-penetration pattern for a mangrove Kandelia obovata canopy was analyzed considering the vertical changes in the light-extinction coefficients for leaves and woody organs. Furthermore, canopy photosynthesis and foliage respiration were estimated on the basis of the constructed light-penetration models. The canopy structure and light-penetration pattern were investigated by use of a destructive method. Seasonal changes in photosynthetic light-response curves and dark respiration of leaves were measured at different canopy depths. The main findings of this study were: (1) the ratio of the cumulative woody silhouette area density C to the cumulative leaf area density F increased downward within the canopy; (2) the light-extinction coefficient for leaves increased from 0.30 to 0.72 downward and the light-extinction coefficient for woody organs was almost constant at 0.77; and (3) ignoring the woody organs and the vertical change in the C/F ratio and light-extinction coefficients for leaves caused ca. ±10 and ±20% errors in annual canopy gross photosynthesis and surplus production, respectively.

Voxel-Based 3-D Modeling of Individual Trees for Estimating Leaf Area Density Using High-Resolution Portable Scanning Lidar

A method for accurate estimation of leaf area density (LAD) and the cumulative leaf area index (LAI) profiles of small trees ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Camellia sasanqua</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Deutzia crenata</i> ) under different conditions was demonstrated, which used precise voxel-based tree models produced by high-resolution portable scanning lidar. In this voxel-based canopy profiling (VCP) method, data for each horizontal layer of the canopy of each tree were collected from symmetrical azimuthal measurement points around the tree using optimally inclined laser beams. The data were then converted into a voxel-based three-dimensional model that reproduced the tree precisely, including within the canopy. This precise voxel model allowed the LAD and LAI of these trees, which have extremely dense and nonrandomly distributed foliage, to be computed by direct counting of the beam-contact frequency in each layer using a point-quadrat method. Corrections for leaf inclination and nonphotosynthetic tissues reduced the estimation error. A beam incident zenith angle near 57.5deg offered a good correction for leaf inclination without knowledge of the actual leaf inclination. Non-photosynthetic tissues were removed by image-processing techniques. The best LAD estimations showed errors of 17% at the minimum horizontal layer thickness and of 0.7% at the maximum thickness. The error of the best LAI estimations was also 0.7%

Influence of brick air scrubber by-product on growth and development of corn and hybrid poplar

Studies were conducted to determine the effects of spent reagent from air pollution control scrubbers used at a brick manufacturing facility on emergence, growth, and physiological responses of corn and hybrid poplar plants. Scrubber by-product was obtained from General Shale Brick, Louisville, KY. Potting substrate was weighed and quantities of scrubber by-product were added to the substrate to obtain treatments of 0%, 6.25%, 12.5%, 25%, 50%, 75%, and 100% scrubber by-product (w:w) for the corn study. Each treatment mix was potted into nine replicate polyethylene pots and four corn seeds were sown per pot. The pots were randomized in a greenhouse at Clemson University and the number of seedlings emerging from each treatment, dark-adapted leaf chlorophyll a fluorescence, and shoot heights were measured at the end of a 21-day growth period. Then, dry shoot biomass was determined for plants from each treatment and plant tissues were analyzed for selected constituents. For the poplar study, nine-inch cuttings of hybrid poplar clone 15–29 ( Populus trichocarpa × P. deltoides) and clone OP367 ( P. deltoides × P. nigra) were planted in treatments of scrubber by-product-potting soil mixes of 0% , 5% , 10% , and 25% w:w. Leaf chlorophyll a fluorescence was measured over six weeks and cumulative leaf area, dry biomass, and nutrient content of tissues were determined upon harvest. Results of these studies indicate that percent seedling emergence for corn plants decreased with increasing scrubber by-product application rates. Application rates up to 12.5% scrubber by-product w:w had no adverse effect on corn seedling emergence. Shoot elongation, biomass production, and the status of the photosynthetic apparatus of the seedlings were also not severely impaired at applications below this level. A critical value of 58.2% w:w scrubber by-product was estimated to cause 25% inhibition of seedling emergence. Biomass production, cumulative leaf area, and chlorophyll a fluorescence of hybrid poplar plants were not affected by scrubber by-product applications of up to 5% w:w.

RELATIONSHIP BETWEEN MODIFIED NORMALIZED DIFFERENCE VEGETATION INDEX AND LEAF AREA INDEX FOR PROCESSING TOMATOE

Remote sensing has the potential for obtaining large amounts of soil and plant data inexpensively and rapidly.Vegetation growth monitored using normalized difference vegetation index (NDVI) tends to be affected by the backgroundsoil reflectance. Modified normalized difference vegetation index, which is related to the soil adjusted vegetation index(SAVI), has the capability to overcome this effect. Modified NDVI is related to the slope of vegetation isolines and is basedon the assumption that all vegetation isolines and soil line intersect at a single point. Leaf area index obtained through fieldand laboratory measurements for a processing tomato crop correlated well with the slope of the vegetation isolines with aR2 value of 0.92. A neural network model was developed and trained to predict daily LAI values based on six sets of ModifiedNDVI values derived from biweekly aerial images obtained during the tomato growing season. The trained network was ableto predict LAI values with R2 values of 0.96 or higher. The cumulative leaf area index (CLAID) predicted by the neural networkmodel correlated well with the measured CLAID values (R2 = 0.83).

Variation in Crown Light Utilization Characteristics among Tropical Canopy Trees

Light extinction through crowns of canopy trees determines light availability at lower levels within forests. The goal of this paper is the exploration of foliage distribution and light extinction in crowns of five canopy tree species in relation to their shoot architecture, leaf traits (mean leaf angle, life span, photosynthetic characteristics) and successional status (from pioneers to persistent). Light extinction was examined at three hierarchical levels of foliage organization, the whole crown, the outermost canopy and the individual shoots, in a tropical moist forest with direct canopy access with a tower crane. Photon flux density and cumulative leaf area index (LAI) were measured at intervals of 0.25-1 m along multiple vertical transects through three to five mature tree crowns of each species to estimate light extinction coefficients (K). Cecropia longipes, a pioneer species with the shortest leaf life span, had crown LAI <0.5. Among the remaining four species, crown LAI ranged from 2 to 8, and species with orthotropic terminal shoots exhibited lower light extinction coefficients (0.35) than those with plagiotropic shoots (0.53-0.80). Within each type, later successional species exhibited greater maximum LAI and total light extinction. A dense layer of leaves at the outermost crown of a late successional species resulted in an average light extinction of 61% within 0.5 m from the surface. In late successional species, leaf position within individual shoots does not predict the light availability at the individual leaf surface, which may explain their slow decline of photosynthetic capacity with leaf age and weak differentiation of sun and shade leaves. Later-successional tree crowns, especially those with orthotropic branches, exhibit lower light extinction coefficients, but greater total LAI and total light extinction, which contribute to their efficient use of light and competitive dominance.

Water-use Efficiency of One-year-old Sweet Cherry (Prunus avium L.) cv. `Rainier' on Dwarfing and Standard Rootstocks, under Well-watered and Water Deficit Conditions

This study was conducted to determine whether standard and dwarfing sweet cherry rootstocks under water deficit conditions respond differently relative to plant growth and gas exchange parameters, water-use efficiency, and leaf carbon isotope composition. One-year-old potted sweet cherry cv. `Rainier' grafted on the standard rootstock `Mazzard' and on the dwarfing rootstock `Gisela 5' were compared under two different water treatments: 1) well-watered, which received daily 100% of the amount of water lost by ET, and 2) a water deficit treatment, which received 50% of the water applied to the control. Relative shoot growth rate, leaf emergence rate and cumulative leaf area were recorded every three to seven days during the experiment. Leaf net carbon dioxide assimilation rate, stomatal conductance, transpiration rate, internal CO2 concentration, and WUE were measured daily for the duration of the experiment. At the end of the experiment, leaf samples were collected to determine leaf carbon isotope composition. The growth parameters measured were affected similarly in the two rootstocks indicating a similar degree of sensitivity to water deficit in the genotypes tested. Cumulative leaf area was affected earlier by water deficit than relative shoot growth, and leaf emergence rate. Gas exchange parameters were affected earlier than growth parameters. Overall, WUE was not significantly different between dwarfing and standard rootstocks, and did not appear to increase under water deficit condition, indicating that irrigation should be considered as an important practice in sweet cherry orchards, especially when dwarfing rootstocks are selected.

Kinetin-induced modification in growth criteria, ion contents and water relations of sorghum plants treated with cadmium chloride

The objective of this study was to investigate the effect of soil drench with three different concentrations of CdCl2 on growth criteria, ions content and water relations of Sorghum bicolor plants. Also particular interest was focused on the effect of grain presoaking with kinetin in order to ameliorate the toxicity effects exerted by the different levels of CdCl2. In general, the results showed, that the observed suppression in growth criteria (i.e. root length, root fresh and dry weights, plant height, shoot fresh and dry weights, cumulative leaf area as well as root/shoot ratio), ions content (i.e. K+, Na+, Ca2+ and Cd2+), water relations (i.e. total leaf conductivity, transpiration rate and relative water content) in response to CdCl2 was relieved either partially or completely when grains were presoaked in kinetin (50 ppm). The alleviating effect of kinetin for Cd2+ toxicity was more pronounced in response to 1 mM CdCl2 treatment.

SAPWOOD AND INNER BARK QUANTITIES IN RELATION TO LEAF AREA AND WOOD DENSITY IN DOUGLAS-FIR

The relationships between leaf area and sapwood and inner bark quantities (widths, areas, and volumes) were studied in an attempt to understand the design criteria for sapwood quantity in eighteen 34-year-old Douglas-fir (Pseudotsuga menziesii) trees with a wide range of leaf areas, sapwood areas, and dry masses of leaf, xylem, bark, and branch. Cumulative leaf area increased from the tip to the base of the crown, and then was constant; none of the other variables had the same distribution, and so whereas there were many significant correlations, none of the factors can be related to leaf area in a simple, causal manner. Leaf area /sapwood area was extremely variable from tree to tree at a given height, and within a tree from height to height. Sapwood width was relatively constant from the tip down the stem, supporting the hypothesis that sapwood quantity in this species is related to radial gas diffusion causing either a lethal buildup of CO2 or a lethal depletion of O2 at the sap/heart boundary. However, there was no significant correlation between leaf area and either total sapwood density (dry weight/green volume) or the average latewood density in the sapwood which were used as proxies for radial diffusion rate; further research on actual radial gas diffusion in green wood may be informative.

Effects of leaf area profiles and canopy stratification on simulated energy fluxes: the problem of vertical spatial scale

We investigated the effects of the shape of leaf area profiles and the number of canopy layers on simulated sensible and latent heat fluxes using a gradient diffusion-based biometeorological model. Three research questions were addressed through simulation experiments: (1) Given the same amount of cumulative leaf area in the vertical direction, how does the shape of the leaf area profile affect simulation results? (2) For a given leaf area profile, how does the number of layers influence the simulation results? (3) How do these two factors interact with each other in affecting the simulated energy fluxes? Our results demonstrated that the scheme of canopy stratification could substantially affect the simulated energy fluxes, and that the effect exhibited a consistent pattern — an S-shaped response curve. There existed a minimal number of layers for achieving a required degree of accuracy. The turning point of the S-shaped curve represented the optimal number of layers, indicating a desirable balance between the accuracy of the model and demands for computation and data collection. Our results also showed that differences in the shape of leaf area profiles alone could significantly alter the simulated energy fluxes even if the total amount of leaves in the canopy and the values of all other model parameters remained the same. Furthermore, the shape of leaf area profiles interacted with the number of layers in affecting the simulated energy fluxes. While considerable impacts were observed for all leaf area profiles, complex (nonlinear) shapes exacerbated the effects of changing the number of layers.

Statistical treatment of sunfleck length inside willow coppice

Different sunfleck characteristics, length and number of sunflecks per metre and sunfleck fractional area, were measured at different depths inside willow ( Salix viminalis) canopy. All these characteristics depend on the pathlength of the direct solar radiation beam into the canopy, τ= L/sin h, where L is the downward cumulative leaf area index and h is the solar elevation. The mean sunfleck length l S decreases exponentially with τ. The maximum length of short sunflecks (length <6 cm) decreases linearly with τ, but not exponentially. Like the number of short sunflecks, the total number of sunflecks increases with τ, reaches a maximum value of 10–12 sunflecks m −1 at τ=2.5 and then decreases slowly to zero at τ=6. The vertical profiles of the sunfleck fractional area are different for long (length >6 cm) and short sunflecks. The fractional area of long sunflecks decreases with τ exponentially; the fractional area of short sunflecks, k SS, can be approximated by the formula k SS=0.19 τ exp(−0.15 τ 2), and the fractional area of all sunflecks k S, by the formula k S=exp(−0.18 τ 2). Within the canopy, direct solar radiation is present in two areas — in sunflecks and in penumbra. The total flux of direct solar radiation in penumbra is smaller than it is in sunflecks and reaches its maximum value of 50% at the pathlength τ=2. There exists a good correlation with R 2=0.94 between the fractional area of umbra, k U, and the fractional area of sunflecks, k S, which can be fitted by the exponential formula k U=0.63 exp(−2.30 k S).

Estimation of leaf area and its vertical distribution during growth period

A statistical interpolation method is proposed which allows calculation of the leaf area index LAI( t), the downward cumulative leaf area index L( z, t) and the canopy leaf area density u( z, t) as the functions of height and time for any day of the growth period. The method is based on three functions: probability density of stem height, interrelationship between stem height and stem foliage area, and the shape function of stem foliage vertical distribution. The method was elaborated in detail using phytometrical measurements carried out on willow coppice ( Salix viminalis L.) during the growth period of 1998 in Tõravere, Estonia. The estimated values of u( z, t) and those calculated by the method differ by about 10–15%. The interpolation model can be used when phytometrical data are available for certain days throughout the growing season. It could be applied to pure stands: young forests, orchard plantations, and maize, sorghum and cotton crops, etc.