Impact of shadow on crop growth and productivity, prospects for agroforestry in temperate zones
In June 2017, Sidonie Artru defended her doctoral thesis at Gembloux Agro-Bio Tech - ULiège under the supervision of Professor Sarah Garré and Ludivine Lassois, her co-promoter. The objective was to better understand the biological mechanisms underlying the development of winter wheat and sugar beet, and to quantify the final productivity of these crops under shade conditions of an agroforestry system in Belgium.
To meet this objective, a field experimentation approach and a modelling approach were conducted in parallel.
In situ, an artificial shading system has been developed to isolate the light component from other possible competitions between trees and crops (water, nutrients, etc.). Furthermore, the design of this system aims to reproduce the heterogeneous light environment observed in agroforestry systems composed of late phenology trees such as hybrid walnut. For winter wheat, the application of late shading during the season leads to a significant reduction in grain yield (number of grains per m² and grain weight), partially offset by an increase in protein content. Sugar beet meets shade conditions by morphological adaptations of its aerial part, as well as by a significant reduction in root biomass and final sugar yield. Overall, for both crops, the decrease in final yield under shade varies with the amount and duration of light received and the phenological stage of the crop during which shade is applied.
In parallel, an experiment was carried out on a plot bordered by poplars in order to evaluate the growth and productivity of winter wheat in a context where light would no longer be the only potentially limiting resource. The presence of trees induces a decrease in the final grain yield along a gradient from the tree to the centre of the plot. Even when other interactions may occur, in addition to light competition, the maximum yield loss under trees never reaches the level of yield reduction observed under artificial shading systems.
In order to extrapolate the results observed in the field and predict the long-term productivity of winter wheat in shaded conditions, a modelling approach was conducted with the STICS culture model. Overall, the STICS model satisfactorily simulates the total production of dry above-ground wheat biomass under shade. However, despite the adjustment of some parameters, the model overestimates the final grain yield under shaded conditions.
Finally, a transversal reflection was conducted proposing agroecology as a model for the development of agroforestry projects in temperate regions. This agroecological approach aims to integrate all stakeholders from fork to fork in agroforestry project design and highlights the importance of the natural, social and economic context to ensure the sustainability of such systems.
