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 of tall 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. we consider the characteristic mixed composition of annual and perennial species (mainly hemicryptophytes) in these communities. The seed bank in these grasslands represents the potential species composition of the ensuing vegetation and, therefore, may determine their productivity. However, there is no general consensus about the relationship between seed bank characteristics and vegetation structure, since it can vary across plant communities, according to present local environmental conditions, land use and long-term history of disturbances (Ortega et al. 1997, Marañon 1998, Jutila 1998, Egan et al. 2000). Studies in Mediterranean grasslands have shown that responses of the vegetation to grazing are frequently associated to plant traits (e.g., life-cycle, growth form, height, palatability, morphological and chemical defenses) (Gomez Sal et al. 1986, Noy-Meir et al. 1989, Lavorel et al. 1999, Sternberg et al. 2000). However, few studies have focused on the relationships between species response to grazing and reproduction traits, such as timing of flowering and seed-set, seed size and germination fraction, even though Mediterranean grasslands have a large component of annual plants that depend on these traits for regeneration (Peco 1989, Noy-Meir and Briske 1996, Azcárate et al. 2002, Diaz et al. 2007). In the present work we studied the relationships between grazing, timing of flowering and seed traits (i.e., seed size, germination fraction in the field), using a plant functional group approach based on life-form, plant size, palatability and taxonomic affiliation (Noy-Meir et al. 1989, Gitay and Noble 1997, Lavorel et al. 1999, Diaz and Cabido 2001). These relationships were examined in a Mediterranean grassland in northeastern Israel, under two contrasting cattle grazing systems. They were characterized by very heavy stocking rates, early onset of grazing vs. heavy grazing pressure, but late in the grazing season (Sternberg et al. 2000).