Effects of cattle grazing timing and intensity on soil seed banks and regeneration strategies in a Mediterranean grassland

The relationship between the soil seed bank at the onset of the growing season and the structure of the ensuing vegetation under grazing was investigated in an eastern Mediterranean grassland. Species responses to two contrasting cattle grazing regimes were studied through plant trait analyses. Traits included plant size, phenology, seed size and germination fraction. Changes in species composition of the soil seed bank and the vegetation were analyzed using a plant functional group approach. The results showed that seed bank and relative cover oftall annual and perennial grasses increased when grazed late in the growing season. The opposite was noted for short annual grasses, annual legumes, annual thistles, annual crucifers and other annual forbs. Grazing treatments little affected plant cover of herbaceous perennials. Tall annual and tall perennial grasses have large seeds and germination rates over 90%. Short annual grasses showed similarly high germination rates but with smaller seeds. Annual legumes were characterized by medium size seeds and lower germination fractions (<50%). Tall annual grasses showed high competition capabilities at late grazed paddocks due to a combination of regeneration traits that included: high germination fraction, larger seed and seedling size, and inflorescence with morphological defenses. It is proposed that in addition to plant size and palatability, regeneration traits such as seed dormancy, seed and seedling size play an important role in determining the vegetation structure under different grazing regimes, thus contributing to the high plant species diversity characteristic to Mediterranean grasslands.