Claude Varlet-Grancher

Comparison of models for daily light partitioning in multispecies canopies.

A simulation model of light partitioning in horizontally homogeneous multispecies canopies is proposed. The model is based on the Kubelka‐Munk equations (KM) applied to a mixture of N vegetation components. Only two hemispherical fluxes, i.e. downwards and upwards, are considered. The exact solution of KM equations was then simplified in such a way that the model can be easily extended to multispecies canopies including several vegetation layers. The simplified KM model (KMS) was compared to two other light models dealing with mixed canopies: the more detailed model SIRASCA [Sinoquet, H., Moulia, B., Gastal, F., Bonhomme, R., Varlet-Grancher, C., 1990. Modeling the radiative balance of the components of a well-mixed canopy: application to a white clover‐tall fescue mixture. Acta Oecol. 11, 469‐486], and the simpler model ERIN [Wallace, J.S., 1997. Evaporation and radiation interception by neighbouring plants. Q. J. R. Meteorol. Soc. 123, 1885‐1905]. All three models were applied to theoretical two-species monolayer canopies, and to actual mixed canopies, the geometry of which was retrieved from the literature. In the PAR waveband, the model KMS gave simulation results very similar to those of SIRASCA in case of contrasted canopy structures. In conditions of high leaf and soil scattering, deviations between SIRASCA and KMS outputs were higher and reached maximum values of ‐0.08 for erectophile species. Comparison between SIRASCA and ERIN outputs showed that ERIN largely underestimated light competition in a two-component canopy in several conditions, due to light partitioning only based on height differences between components. Simulations also showed the significant effect of the vertical distribution of leaf area on light partitioning in the case of mixtures where components had equal or different heights. Finally it appears that the model KMS could be a candidate for inclusion in growth models for multispecies canopies, since all KMS parameters have physical meaning and it is very easy to implement. ©2000 Elsevier Science B.V. All rights reserved.

Carbon partitioning in forage crops

The paper describes the conceptual models used to understand the processes determining plant growth rates in response to environmental changes. A series of experiments and growth models were used at three organizational levels: the specific plant organs, the whole plant and the plant canopy. The energy conversion efficiency and the total plant carbon balance were first examined. The carbon partitioning amongst the plant parts was then studied. The energy conversion efficiency is generally understood. In modelling carbon partitioning it was first necessary to establish the carbon demand for each plant organ. The carbon partitioned amongst plant organs was then calculated in two ways. The first one based on empirical data consisted in defining which organ received the carbon prior to other organs. The second one was based on the relationship between the carbon mass of specific organs and their trophic activity. This hypothesis allowed the optimization of the carbon partitioning in order to maximize the whole plant growth rate. The opportunities to use these theoretical approaches in plant growth modelling are discussed.

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.

Rythme d'apparition des racines primaires du maïs (Zea mays L.). III: Variations observées au champ

La variabilite du nombre de racines primaires par entre-noeud ou groupe d’entre-noeuds induite par les facteurs du milieu a ete caracterisee a partir d’un dispositif experimental multilocal et d’une etude a partir de semis echelonnes. Des differences significatives ont ete mises en evidence, tant au niveau des nombres moyens de racines qu’au niveau de la forme des distributions racinaires. La variabilite augmente avec le rang de l’entre-noeud considere. Le nombre de racines a niveau donne depend peu de l’emission aux entre-noeuds inferieurs. De plus, il n’y a pas de relation chez les plantes matures entre le nombre total de feuilles et le nombre total de racines. L’echelonnement de la date de semis ne modifie pas la cinetique de l’emission racinaire mais a une influence sur le nombre de racines portees par les derniers entre-noeuds.

A Quantitative Analysis of the Three‐Dimensional Spatial Colonization by a Plant as Illustrated by White Clover (Trifolium repens L.)

A new method is described for measuring spatial colonization by plants from three‐dimensional (3‐D) records of their aerial morphology. Nonparametric indices were devised to quantify the space colonized by the plant by measuring the spatial dispersion of sets of points representing relevant morphological features of its architecture, such as the nodes and petiole tips. These indices are defined as the square root of the second‐order moments of these sets, i.e., as dispersion radii. They can be split into the components that characterize the major directions of colonization and those that analyze the contributions of the various morphological features. Horizontal dispersion can be visualized as an indicator ellipse (to characterize the anisotropy), and vertical dispersion as equivalent heights. The kinetics of spatial colonization are then characterized as the changes in these dispersion radii during the development of the plant. This method is illustrated using isolated white clover plants. Two phases can be distinguished in the growth of this species: (i) a phase of rapid horizontal colonization involving the petioles and (ii) a phase in which the rate of horizontal colonization becomes slower and steady and involves both internodes and petioles. Petioles therefore make a major contribution to both vertical and horizontal colonization. We also show that these conclusions could not have been reached using only organ lengths and that the usual assumptions about the function of petioles in white clover should be reconsidered. We argue that this is a general method that can be applied to most species and growth forms of plants. It should be particularly useful for analyzing plastic morphogenetic responses to light conditions and their contribution to light foraging as a complement to estimating light interception.

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 Beers 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 Beers 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 Beers 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.