Modelling assimilation rates of 14 temperate arable weed species as a function of the environment and leaf traits

Information on the response of assimilation rate to environmental factors is lacking for many less competitive weed species that need to be considered in the context of increasing farm biodiversity. A pot experiment was sown to parameterize gross assimilation rate at light saturation and initial light use efficiency for 14 common UK annual weeds and winter wheat at four leaf temperatures. Field experiments were also sown to measure inter-specific differences in specific leaf area (SLA), leaf nitrogen content and assimilation rates in the field at near-optimum temperatures. A generic relationship describing the response of assimilation rate to temperature and light using SLA and leaf nitrogen content as conversion factors successfully predicted inter-specific differences in assimilation rates in the field. This relationship could be incorporated into weed-crop competition models to predict the productivity and competitive impact of weed mixtures, including species outside the current data set. Assimilation rates at light saturation in the field were determined largely by SLA. This trait was variable between species and within a species across the growing season and needs to be well described in mechanistic competition models to accurately calculate instantaneous assimilation rates.