Résumé
1. Combining several crop species and associated agricultural practices in
a crop sequence has the potential to control weed abundance while
promoting weed diversity in arable fields. However, how the variability of
environmental conditions that arise from crop sequences affects weed
diversity and abundance remains poorly understood, with most studies
to-date simply opposing weed communities in monoculture and in crop
rotation. Here, we describe crop sequences along gradients of disturbance
and resource variability using a crop functional trait and associated
agricultural practices. We tested the hypothesis that variability of
disturbances reduces weed abundance while variability of resources
promotes weed diversity. 2. We used functional Hill’s numbers to compute
crop sequence functional diversity based on sowing date, herbicide
spectrum and crop height - these are the respective proxies of disturbance
timings, disturbance types and light availability. Using a large-scale
weed monitoring database, we assessed crop sequence diversity for 1045
crop sequences of five consecutive cropping seasons. We computed weed
richness and abundance at pluri-annual (pool of weeds observed across five
cropping seasons) and annual (pool of weeds observed during a winter
cereal cropping season preceded by five cropping seasons) scales. We also
accounted for herbicide and tillage intensities to test whether management
intensity affects the response of weed diversity and abundance to crop
sequence diversity. 3. At the pluri-annual scale, weed richness increased
with the diversity of crop height and sowing date while weed abundance
decreased with sowing date diversity. Annual weed richness decreased with
sowing date diversity while annual weed abundance poorly relied on crop
sequence diversity. 4. Synthesis and applications. This study establishes
a scientific basis for designing crop sequences according to specific weed
management goals. We show that farmers may enhance arable weed diversity
on a pluri-annual scale by sequentially sowing crop species that differ in
their competitive ability and sowing date. They may also achieve a better
control of weed abundance by increasing the diversity of crop sowing dates
across the crop sequence.