Evaluation of the suitability of three autochthonous herbaceous species as cover crops under Mediterranean conditions through the calibration and validation of a temperature-based phenology model

Abstract

This study presents the analysis of the suitability of three native species of annual plants, Bromus rubens, Medicago truncatula and Anthemis arvensis, as cover crops in Andalusia, Southern Spain. We used experimental data from two agricultural years to calibrate and validate degree-day-based phenological models for these cover crop species. From these developed models, we determined the seed maturity dates for the three species for seven locations in Andalusia for two different date assumptions regarding conditions for seed germination: fixed on October 20 or with the first significant rainfall (25 mm on four consecutive days) in late summer or early autumn, using an 18 year dataset of daily temperature and precipitation. This analysis enabled us to determine the probability distribution of seed maturity dates for the three plant species at each location, and determine an empirical linear regression model to calculate the average seed maturity date for each species in the region. This empirical model enabled us to calculate a regional map of maturity dates for the three cover crop species based on long-term average daily temperatures across the region. The analysis showed how the management of a temporary cover crop needs to be regionalised and adapted to the specific edaphoclimatic conditions of each orchard. Thus, whereas in the western part of the Guadalquivir river valley the three species can produce viable seeds in early April, especially A. arvensis and M. truncatula, in the rest of the region management of these cover crops needs to be adapted to combine seed production for self-seeding with minimising the risk of competition for soil water with the trees. This can be done with systems allowing partial killing of the cover crop in late winter, leaving a tiny fraction of the cover alive to produce seed for the next year. Our analysis suggests that new cover crop species with shorter cycles are needed, particularly grasses. In this search another key trait should be ease of seed dispersal. Our thermal time phenological models and approach may be directly applicable to similar areas in the Mediterranean or re-calibrated for different conditions or plant species with relatively simple experiments.