The response of weed and crop species to shading. How to predict their morphology and plasticity from species traits and ecological indexes?

To assess the competitive ability of plant species, ecologists describe many species from contrasting habitats with traits that are proxies of ecophysiological functions whereas agronomists describe few species from similar habitats with process-based parameters. Here, we combined both approaches and compared many contrasting crop and weed species of temperate European arable crops in terms of competition for light, to understand weed response to shading by crop canopies and to choose light-competitive crop species and varieties. We (1) measured species parameters that drive light-competition processes in 26 crop and 35 weed species of temperate European arable cropping systems, (2) related the parameter values to species features that are easier to measure or available in databases. Early plant-growth parameters (relative growth rate RGR, initial leaf area) were measured in optimal light and nutrient conditions in a greenhouse with automatic non-destructive measurements. Potential plant morphology in unshaded conditions (specific leaf area SLA, leaf biomass ratio LBR, plant height and width per unit biomass HM and WM, vertical leaf distribution) was measured in garden plots in optimal light and nutrient conditions and harvested at 4-5 stages. Shading response was measured by comparing potential morphology to that of plants grown under shading nets. We confirmed well-known relationships (lower SLA and LBR in legumes vs non-legumes ...), included new species features (base temperature, photosynthetic pathway ...), and established relationships for the new shading-response parameters (weeds respond more to shade than crops, by increasing LBR, SLA, HM and WM ...). Some correlations reported in ecology (RGR vs SLA ...) were not verified on our species pool from arable temperate fields. Shade-response parameters explained species responses to habitat described by Ellenberg indexes, e.g., when shaded, shade-loving species (low Ellenberg-L values) increased SLA and HM to increase light interception.