Impact of spatio-temporal shade dynamics on wheat growth and yield, perspectives for temperate agroforestry

A stumbling block to the adoption of silvoarable agroforestry systems is the lack of quantitative knowledge on the performance of different crops when competing for resources with trees. In North-Western Europe, light is likely to be the principal limiting resource for understorey crops, and most agronomic studies show a systematic reduction of final yield as shade increases. However the intensity of the crop response depends on both the environmental conditions and the shade characteristics. This study addressed the issue by monitoring winter wheat (Triticum aestivum L.) growth, productivity and quality under artificial shade provided by military camouflage shade-netting, and using the Hi-sAFe model to relate the artificial shade conditions to those applying in agroforestry systems. The field experiment was carried out over two consecutive years (2013–14 and 2014–15) on the experimental farm of Gembloux Agro-Bio Tech, Belgium. The shade structures recreated two shade conditions: periodic shade (PS) and continuous shade (CS), with the former using overlapping military camouflage netting to provide discontinuous light through the day, and the latter using conventional shade cloth. The experiment simulated shading from a canopy of late-flushing hybrid walnut leaves above winter wheat. Shading was imposed 16 (2013–14) and 10 (2014–15) days before flowering and retained until harvest. The crop experienced full light conditions until the maximum leaf area index stage (LAImax) had been reached. In both years, LAI followed the same dynamics between the different treatments, but in 2013–2014 an attack of the take-all disease (Gaeumannomyces graminis var. tritici) reduced yields overall and prevented significant treatment effects. In season 2014–15 the decrease in global radiation reaching the crop during a period of 66 days (CS: – 61% and PS: – 43%) significantly affected final yield (CS: – 45% and PS: – 25%), mainly through a reduction of the average grain weight and the number of grain per m2. Grain protein content increased by up to 45% under the CS treatment in 2015. Nevertheless, at the plot scale, protein yield (t/ha) did not compensate for the final grain yield decrease. The Hi-sAFe model was used to simulate an agroforestry plot with two lines of walnut trees running either north-south or east-west. The levels of artificial shade levels applied in this experiment were compared to those predicted beneath trees growing with similar climatic conditions in Belgium. The levels used in the CS treatment are only likely to occur real agroforestry conditions on 10% of the cropped area until the trees are 30 years old and only with east-west tree row orientation.