Simulating the grazing of a white clover 3-D virtual sward canopy and the balance between bite mass and light capture by the residual sward.

BACKGROUND AND AIMS: The productivity and stability of grazed grassland rely on dynamic interactions between the sward and the animal. The descriptions of the sward canopies by standard 2-D representations in studies of animal-sward interactions at the bite scale need to be improved to account for the effect of local canopy heterogeneity on bite size and regrowth ability. The aim of this study was to assess a methodology of 3-D digitized canopies in order to understand the balance between bite mass and light interception by the residual sward. METHODS: 3-D canopy structures of four white clover swards were recorded using a POLHEMUS electromagnetic digitizer and adapted software (POL95). Plant components were removed after digitizing to determine aerial dry matter. Virtual canopies were synthesized and then used to derive canopy geometrical parameters, to compute directional interception and to calculate bite mass. The bit masses of cattle and sheep were simulated according to their form, depth and placement on the patch, taking account of explicit sward architecture. The resulting light interception efficiency (LIE) of each organ was then calculated using a projective method applied to the virtual residual sward. This process enabled an evaluation of light interception based on Beer's law at the bite scale. KEY RESULTS: The patterns of the vertical profiles of LAI appeared as bimodal, triangular or skewed parabolic functions. For a single bite of similar area and depth, the lowest mass was observed with half-spherical form and the highest for the cylindrical form, whatever the initial sward structure. The differences between the actual LIE and that calculated by Beer's law were marked for residual swards shorter than 8 cm. Bite mass and LIE values after grazing were more strongly affected by the initial structure of the sward than by bite form and placement. CONCLUSIONS: 3-D digitizing techniques enabled a definition of the geometry of each component in sward canopies and an accurate description of their vertical and horizontal heterogeneities. The discrepancy between Beer's law results and actual light interception was reduced when the sward regrew rapidly and if the rest period was long. Studies on the biting process would greatly benefit from this method as a framework to formulate and test hypotheses in a quantitative manner.

Light interception and partitioning between shoots in apple cultivars influenced by training.

The effect of two training systems (Central Leader with branch pruning versus Centrifugal Training with minimal pruning, i.e., removal of fruiting laterals only) on canopy structure and light interception was analyzed in three architecturally contrasting apple (Malus domestica Borkh.) cultivars: 'Scarletspur Delicious' (Type II); 'Golden Delicious' (Type III); and 'Granny Smith' (Type IV). Trees were 3D-digitized at the shoot scale at the 2004 and 2005 harvests. Shoots were separated according to length (short versus long) and type (fruiting versus vegetative). Leaf area density (LAD) and its relative variance (xi), total leaf area (TLA) and crown volume (V) varied consistently with cultivar. 'Scarletspur Delicious' had higher LAD and xi and lower TLA and V compared with the other cultivars with more open canopies. At the whole-tree scale, training had no effect on structure and light interception parameters (silhouette to total area ratio, STAR; projected leaf area, PLA). At the shoot scale, Centrifugal Training increased STAR values compared with Central Leader. In both training systems, vegetative shoots had higher STAR values than fruiting shoots. However, vegetative and fruiting shoots had similar TLA and PLA in Centrifugal Trained trees, whereas vegetative shoots had higher TLA and PLA than fruiting shoots in Central Leader trees. This unbalanced distribution of leaf area and light interception between shoot types in Central Leader trees partly resulted from the high proportion of long vegetative shoots that developed from latent buds. These shoots developed in the interior shaded zone of the canopy and therefore had low STAR and PLA. In conclusion, training may greatly affect the development and spatial positioning of shoots, which in turn significantly affects light interception by fruiting shoots.

Evaluation of a turbid medium model to simulate light interception by walnut trees (hybrid NG38 × RA and Juglans regia) and sorghum canopies (Sorghum bicolor) at three spatial scales.

Light is one of the most important components to be included in functional-structural plant models that simulate the biophysical processes, such as photosynthesis, evapotranspiration and photomorphogenesis, involved in plant growth and development. In general, in these models, light is treated using a turbid medium approach in which radiation attenuation is described by the Beer-Lambert law. In the present study, we assessed the hypothesis of leaf random dispersion in the Beer-Lambert law at the whole-canopy, horizontal-layer and local scales. We compared two calculation methods of radiation attenuation: a 3D turbid medium model using the Beer-Lambert law and the other based on a projective method. The two models were compared by applying the calculations to two walnut trees and two sorghum canopies, which have contrasting structural characteristics. The structures of these canopies were measured in 3D to take into account the arrangement and orientation features of the plant elements. The assumptions made by the Beer-Lambert law allowed adequate simulation of light interception in a structure with little overlapping at the horizontal-layer and whole-canopy scales. At the local scale, discrepancies between the turbid medium model and the model based on a virtual plant were reduced with an adequate choice of structural parameters, such as the leaf inclination distribution function.

A double-digitising method for building 3D virtual trees with non-planar leaves: application to the morphology and light-capture properties of young beech trees (Fagus sylvatica).

We developed a double-digitising method combining a hand-held electromagnetic digitizer and a non-contact 3D laser scanner. The former was used to record the positions of all leaves in a tree and the orientation angles of their lamina. The latter served to obtain the morphology of the leaves sampled in the tree. As the scanner outputs a cloud of points, software was developed to reconstruct non-planar (NP) leaves composed of triangles, and to compute numerical shape parameters: midrib curvature, torsion and transversal curvature of the lamina. The combination of both methods allowed construction of 3D virtual trees with NP leaves. The method was applied to young beech trees (Fagus sylvatica L.) from different sunlight environments (from 1 to 100% incident light) in a forest in central France. Leaf morphology responded to light availability, with a more bent shape in well-lit leaves. Light interception at the leaf scale by NP leaves decreased from 4 to 10% for shaded and sunlit leaves compared with planar leaves. At the tree scale, light interception by trees made of NP leaves decreased by 1 to 3% for 100% to 1% light, respectively.

Six-year time course of light-use efficiency, carbon gain and growth of beech saplings (Fagus sylvatica) planted under a Scots pine (Pinus sylvestris) shelterwood.

Two-year-old Fagus sylvatica L. saplings were planted under the cover of a Pinus sylvestris L. stand in the French Massif Central. The stand was differentially thinned to obtain a gradient of transmitted photosynthetically active radiation (PAR(t); 0-0.35). Eighteen Fagus saplings were sampled in this gradient, and their growth (basal stem diameter increment) was recorded over six years. Over the same period, morphological parameters (leaf area, number and arrangement in space) were monitored by 3D-digitization. Photosynthetic parameters were estimated with a portable gas-exchange analyzer. Photosynthesis was mainly related to light availability, whereas sapling morphology was mainly driven by sapling size. Annual stem diameter increment was related to the amount of light-intercepting foliage (silhouette to total leaf area ratio (STAR) x total sapling leaf area (LA)) and light availability above the saplings (PAR(t)). However, light-use efficiency, i.e., the slope of the relationship between STAR x LA x PAR(t) and stem diameter increment, decreased over time as a result of a relative decrease in the proportion of photosynthetic tissues to total sapling biomass.

Architecture of the pruned tree: impact of contrasted pruning procedures over 2 years on shoot demography and spatial distribution of leaf area in apple (Malus domestica).

BACKGROUND AND AIMS: Demography and spatial distribution of shoots are rarely studied on pruned trees. The present 2-year study deals with the effect of pruning strategies on shoot demography and development, and consequences on the spatial distribution of leaf area in three architecturally contrasted - from type II to IV - apple cultivars: 'Scarletspur Delicious', 'Golden Delicious' and 'Granny Smith'. METHODS: All trees were initially subjected during 5 years to Central Leader training with winter heading on all long shoots. For 2 years, half of the trees were further trained with Centrifugal training, where removal of flowering shoots - called extinction pruning - was carried out along the trunk and at the bottom of branches at flowering time. During these 2 years, shoot type (vegetative, inflorescence) and length, and the three-dimensional spatial distribution of all shoots were assessed with an electromagnetic digitizer. KEY RESULTS: Shoot demography, frequency of transitions toward an inflorescence from either an inflorescence (bourse-over-bourse) or a vegetative shoot (trend toward flowering), and the number of bourse-shoots per bourse were strongly affected by cultivar, with little influence of tree manipulation. In contrast, the proportion of vegetative long shoots developing from previous year latent buds was significantly lower in Centrifugal-trained trees for the three cultivars. Canopy volume showed large variations between cultivars, but only that of 'Granny Smith' was affected by tree manipulation in the 2 years. Spatial distribution of shoots varied significantly according to cultivar and manipulation. In 'Scarletspur Delicious' and, to a lesser extent 'Golden Delicious', the distribution of vegetative and flowering shoots in the outer and the inner parts, respectively, was not affected by tree manipulation. In contrast, in 'Granny Smith', vegetative shoots were stimulated in the periphery of Central Leader trees, whereas flowering shoots were stimulated in the periphery of Centrifugal-trained trees. CONCLUSIONS: In apple, the variability of responses to contrasted pruning strategies partly depends on the genetically determined growth and flowering habit of the cultivar.

Regional climate modulates the canopy mosaic of favourable and risky microclimates for insects.

1. One major gap in our ability to predict the impacts of climate change is a quantitative analysis of temperatures experienced by organisms under natural conditions. We developed a framework to describe and quantify the impacts of local climate on the mosaic of microclimates and physiological states of insects within tree canopies. This approach was applied to a leaf mining moth feeding on apple leaf tissues. 2. Canopy geometry was explicitly considered by mapping the 3D position and orientation of more than 26 000 leaves in an apple tree. Four published models for canopy radiation interception, energy budget of leaves and mines, body temperature and developmental rate of the leaf miner were integrated. Model predictions were compared with actual microclimate temperatures. The biophysical model accurately predicted temperature within mines at different positions within the tree crown. 3. Field temperature measurements indicated that leaf and mine temperature patterns differ according to the regional climatic conditions (cloudy or sunny) and depending on their location within the canopy. Mines in the sun can be warmer than those in the shade by several degrees and the heterogeneity of mine temperature was incremented by 120%, compared with that of leaf temperature. 4. The integrated model was used to explore the impact of both warm and exceptionally hot climatic conditions recorded during a heat wave on the microclimate heterogeneity at canopy scale. During warm conditions, larvae in sunlight-exposed mines experienced nearly optimal growth conditions compared with those within shaded mines. The developmental rate was increased by almost 50% in the sunny microhabitat compared with the shaded location. Larvae, however, experienced optimal temperatures for their development inside shaded mines during extreme climatic conditions, whereas larvae in exposed mines were overheating, leading to major risks of mortality. 5. Tree canopies act as both magnifiers and reducers of the climatic regime experienced in open air outside canopies. Favourable and risky spots within the canopy do change as a function of the climatic conditions at the regional scale. The shifting nature of the mosaic of suitable and risky habitats may explain the observed uniform distribution of leaf miners within tree canopies.

Herbivory mitigation through increased water-use efficiency in a leaf-mining moth-apple tree relationship.

Herbivory alters plant gas exchange but the effects depend on the type of leaf damage. In contrast to ectophagous insects, leaf miners, by living inside the leaf tissues, do not affect the integrity of the leaf surface. Thus, the effect of leaf miners on CO2 uptake and water-use efficiency by leaves remains unclear. We explored the impacts of the leaf-mining moth Phyllonorycter blancardella (Lepidoptera: Gracillariidae) on light responses of the apple leaf gas exchanges to determine the balance between the negative effects of reduced photosynthesis and potential positive impacts of increased water-use efficiency (WUE). Gas exchange in intact and mined leaf tissues was measured using an infrared gas analyser. The maximal assimilation rate was slightly reduced but the light response of net photosynthesis was not affected in mined leaf tissues. The transpiration rate was far more affected than the assimilation rate in the mine integument as a result of stomatal closure from moderate to high irradiance level. The WUE was about 200% higher in the mined leaf tissues than in intact leaf portions. Our results illustrate a novel mechanism by which plants might minimize losses from herbivore attacks; via trade-offs between the negative impacts on photosynthesis and the positive effects of increased WUE.

Does shade improve light interception efficiency? A comparison among seedlings from shade-tolerant and -intolerant temperate deciduous tree species.

Here, we tested two hypotheses: shading increases light interception efficiency (LIE) of broadleaved tree seedlings, and shade-tolerant species exhibit larger LIEs than do shade-intolerant ones. The impact of seedling size was taken into account to detect potential size-independent effects on LIE. LIE was defined as the ratio of mean light intercepted by leaves to light intercepted by a horizontal surface of equal area. Seedlings from five species differing in shade tolerance (Acer saccharum, Betula alleghaniensis, A. pseudoplatanus, B. pendula, Fagus sylvatica) were grown under neutral shading nets providing 36, 16 and 4% of external irradiance. Seedlings (1- and 2-year-old) were three-dimensionally digitized, allowing calculation of LIE. Shading induced dramatic reduction in total leaf area, which was lowest in shade-tolerant species in all irradiance regimes. Irradiance reduced LIE through increasing leaf overlap with increasing leaf area. There was very little evidence of significant size-independent plasticity of LIE. No relationship was found between the known shade tolerance of species and LIE at equivalent size and irradiance.

Three-dimensional reconstruction of partially 3D-digitized peach tree canopies.

A simplified method for building three-dimensional (3D) mock-ups of peach trees is presented. The method combines partial digitizing of tree structure with reconstruction rules for non-digitized organs. Reconstruction was applied at two scales: leaves on current-year shoots (CYS) and shoots on 1-year-old shoots (OYOS). Reconstruction rules make use of allometric relationships, random sampling of shoot attribute distribution and additional hypotheses (e.g., constant internode length). The method was quantitatively assessed for two training systems (tight goblet and wide-double-Y), at a range of spatial scales. For this purpose, light interception properties of reference and reconstructed mock-ups were compared. Mock-up quality depended on scale. Foliage reconstruction on CYS was unsuitable for generating a given CYS. Similarly, CYS reconstruction on OYOS was unsuitable for generating a given OYOS. This is because generic rules derived at the population scale do not consider specific foliage or shoot attributes of a given CYS or OYOS. In contrast, foliage reconstruction on CYS was able to generate OYOS mock-ups having light properties similar to the reference mock-ups. The same held for CYS reconstruction on OYOS for light capture properties at the tree scale. The CYS reconstruction on OYOS was also suitable for deriving OYOS distribution as a function of light interception ability. Reconstruction rules were successfully used to build the vegetation neighborhood of a reference shoot. The proposed method could therefore be used to make 3D tree mock-ups usable for a range of some, but not all, light computations. Because the simplified method allows large time savings, it could be used in virtual experiments requiring large numbers of replicates, such as comparative studies of tree genotypes or training systems.

A method for describing the canopy architecture of coppice poplar with allometric relationships.

A multi-scale biometric methodology for describing the architecture of fast-growing short-rotation woody crops is used to describe 2-year-old poplar clones during the second rotation. To allow for expressions of genetic variability observed within this species (i.e., growth potential, leaf morphology, coppice and canopy structure), the method has been applied to two clones: Ghoy (Gho) (Populus deltoides Bartr. ex Marsh. x Populus nigra L.) and Trichobel (Tri) (Populus trichocarpa Torr. & A. Gray x Populus trichocarpa). The method operates at the stool level and describes the plant as a collection of components (shoots and branches) described as a collection of metameric elements, themselves defined as a collection of elementary units (internode, petiole, leaf blade). Branching and connection between the plant units (i.e., plant topology) and their spatial location, orientation, size and shape (i.e., plant geometry) describe the plant architecture. The methodology has been used to describe the plant architecture of 15 selected stools per clone over a 5-month period. On individual stools, shoots have been selected from three classes (small, medium and large) spanning the diameter distribution range. Using a multi-scale approach, empirical allometric relationships were used to parameterize elementary units of the plant, topological relationships and geometry (e.g., distribution of shoot diameters on stool, shoot attributes from shoot diameter). The empirical functions form the basis of the 3-D Coppice Poplar Canopy Architecture model (3-D CPCA), which recreates the architecture and canopy structure of fast-growing coppice crops at the plot scale. Model outputs are assessed through visual and quantitative comparisons between actual photographs of the coppice canopy and simulated images. Overall, results indicate a good predictive ability of the 3-D CPCA model.

Spatial distribution of leaf nitrogen and photosynthetic capacity within the foliage of individual trees: disentangling the effects of local light quality, leaf irradiance, and transpiration.

There is presently no consensus about the factor(s) driving photosynthetic acclimation and the intra-canopy distribution of leaf characteristics under natural conditions. The impact was tested of local (i) light quality (red/far red ratio), (ii) leaf irradiance (PPFD(i)), and (iii) transpiration rate (E) on total non-structural carbohydrates per leaf area (TNC(a)), TNC-free leaf mass-to-area ratio (LMA), total leaf nitrogen per leaf area (N(a)), photosynthetic capacity (maximum carboxylation rate and light-saturated electron transport rate), and leaf N partitioning between carboxylation and bioenergetics within the foliage of young walnut trees grown outdoors. Light environment (quantity and quality) was controlled by placing individual branches under neutral or green screens during spring growth, and air vapour pressure deficit (VPD) was prescribed and leaf transpiration and photosynthesis measured at branch level by a branch bag technique. Under similar levels of leaf irradiance, low air vapour pressure deficit decreased transpiration rate but did not influence leaf characteristics. Close linear relationships were detected between leaf irradiance and leaf N(a), LMA or photosynthetic capacity, and low R/FR ratio decreased leaf N(a), LMA and photosynthetic capacity. Irradiance and R/FR also influenced the partitioning of leaf nitrogen into carboxylation and electron light transport. Thus, local light level and quality are the major factors driving photosynthetic acclimation and intra-canopy distribution of leaf characteristics, whereas local transpiration rate is of less importance.

Effects of crown development on leaf irradiance, leaf morphology and photosynthetic capacity in a peach tree.

The three-dimensional (3-D) architecture of a peach tree (Prunus persica L. Batsch) growing in an orchard near Avignon, France, was digitized in April 1999 and again four weeks later in May 1999 to quantify increases in leaf area and crown volume as shoots developed. A 3-D model of radiation transfer was used to determine effects of changes in leaf area density and canopy volume on the spatial distribution of absorbed quantum irradiance (PAR(a)). Effects of changes in PAR(a) on leaf morphological and physiological properties were determined. Leaf mass per unit area (M(a)) and leaf nitrogen concentration per unit leaf area (N(a)) were both nonlinearly related to PAR(a), and there was a weak linear relationship between leaf nitrogen concentration per unit leaf mass (N(m)) and PAR(a). Photosynthetic capacity, defined as maximal rates of ribulose-1,5-bisphosphate carboxylase (Rubisco) carboxylation (V(cmax)) and electron transport (J(max)), was measured on leaf samples representing sunlit and shaded micro-environments at the same time that the tree crown was digitized. Both V(cmax) and J(max) were linearly related to N(a) during May, but not in April when the range of N(a) was low. Photosynthetic capacity per unit N(a) appeared to decline between April and May. Variability in leaf nitrogen partitioning between Rubisco carboxylation and electron transport was small, and the partitioning coefficients were unrelated to N(a). Spatial variability in photosynthetic capacity resulted from acclimation to varying PAR(a) as the crown developed, and acclimation was driven principally by changes in M(a) rather than the amount or partitioning of leaf nitrogen.