Impact of spatio-temporal sahde on crop growth and productivity, perspectives for temperate agroforestry

Currently, silvoarable agroforestry is receiving renewed interest in Europe, as a land use system that allows for combining the production of commodities with a range of non-commodity outputs, such as environmental protection. Despite the potential of this practice, it remains rarely implemented in Northwestern Europe. One of the obstacles in the adoption of silvoarable agroforestry systems is the lack of quantitative knowledge on the long term performance of different crops when they are competing for resources with trees. In the face of a wide range of possibilities, it remains difficult to obtain a clear overview of overall system functioning. In this thesis, we simplify this complexity by focusing our research questions on the resource of light, based on the assumption that in Belgian climatic conditions light is likely to be the predominant constraint for understorey crops in a silvoarable agroforestry system. With regard to this resource, we develop our research in order to gain insights into the growth mechanisms and final yield of shaded winter wheat and sugar beet crops. We address these questions using an artificial shade system, which has been developed to reproduce the effect of the heterogeneous spatio-temporal pattern of light observed under lateflushing trees in an agroforestry system, isolated from the competition effects for water and nutrients. The shade structures recreate two shade environments: continuous and periodic. The continuous shade treatment leads to shade throughout the entire day, while the periodic shade treatment induces an intermittent shade period, which varies during the day and according to structure orientation. Winter wheat responded to the late application of both shade treatments with a significant decrease in grain yield, which was partly compensated for by an increase in grain protein content. When shaded, sugar beet compensated through morphological adaptations of the aboveground part of the plant, and by a decrease in the final root dry matter and sugar yield. Overall, for both crops, the magnitude of the final yield repercussion varied with the level and period of shade application. Additionally, an arable plot bordered by a row of poplar trees was selected to evaluate the effect of real trees on the winter wheat. The reduction in the final grain yield follows a gradient, from underneath the trees to the centre of the field. Notwithstanding that interactions other than light competition may have occurred, the maximum yield reduction observed under the trees never reaches the level of decrease which is observed under the continuous shade treatment simulated by the artificial shade arrangement. This experimental approach with winter wheat was complemented by a modelling study, in which we evaluate the ability of the STICS crop model to simulate crops growing under dynamic shade. The results highlight the limits of the STICS model when it is used to simulate crop growth under contrasted shade conditions. Finally, we propose agroecology as a conceptual framework for developing sustainable and profitable agroforestry systems in Europe, and reflect on agricultural practices, food systems, and research methodologies.