Landsharing vs landsparing: how to reconcile crop production and biodiversity ? A simulation study focusing on weed impacts

Résumé En

Weeds are harmful for crop production but essential for biodiversity. They are affected by cultural practices, their seeds survive for years in the soil and disperse among fields. Here, the weed dynamics model FLORSYS (Colbach et al., 2014) was used to analyse whether weed-related biodiversity and crop production can be reconciled in each field or whether separate fields should be used to maximise either biodiversity or production. Materials and Methods FLORSYS is a virtual field cluster on which cropping systems can be tested, including seed dispersal (Thomson et al., 2011). It predicts indicators of weed impact on biodiversity and production. Here, a cluster of four fields was simulated with pedo-climatic conditions from South-Western France. First, a medium-production system with a soybean/maize/wheat/maize rotation, superficial tillage, glyphosate in maize and conventional herbicides in other crops was tested with four annual crop-patterns (Fig. 1.A). Then, five combinations of a high-production system with a high-biodiversity system were simulated (Table 1.B). All systems were chosen based on previous single-field simulations (Bürger et al., 2016). Each scenario was simulated over 28 years and repeated 10 times with randomly chosen weather series from South-Western France. Results and Discussion The more crops are grown each year, the less weed impact varies between years (see example in Fig. 1.B) but the stronger it is in average, with more biodiversity and weed harmfulness, and less crop production (Table 1.A). Effects are larger than in single-field simulations (Bürger et al., 2015) because seed dispersal from weedfavourable crops (here wheat) in year N to neighbour fields grown with favourable crops in year N+1, thus avoiding depressive effects of unfavourable crops (here maize) following favourable crops. The more fields are grown with the high-production system, the higher the production in the cluster and the lower both biodiversity and harmfulness. Even when growing only 25% of high-biodiversity system, biodiversity is higher than for the medium-production system. And even when growing only 25% of highproduction system, production is higher and harmfulness lower than for the medium system.