Experiments were conducted during the 2006/2007 season at two sites in Wadi Shuayb (Fig. 1), a steep valley running from east to west, perpendicular to the Jordan River. It is located 20 km north-east of Amman, Jordan. The two sites were 10 km apart. The climate at the first site (850 m a.s.l.; 32°1′0·42″ N, 35°43′11·43″ E) is typically Mediterranean, while that of the second site (90 m a.s.l.; 31°56′24·94″ N, 35°39′43·27″ E) is semi-arid. Average annual precipitations were 417 and 195 mm for the Mediterranean and the semi-arid site respectively and were 475 and 249 mm the study year. The two sites differ also in length of the growing season. The first and last significant rains (< 10 mm) were registered the 29 October 2006/11 April 2007 and the 27 December/15 March 2007 for the Mediterranean and the semi-arid site respectively. The two sites have similar bedrock and aspect (i.e. limestone and north facing). The soil is Terra Rossa in the first site and desert lithosol in the second site. The floristic composition in the two sites is highly diverse (c. 250–300 species, personal observation). Vegetation in the Mediterranean site is composed of oak trees (Quercus calliprinos), shrubs (Ononis natrix, Calicotome villosa), and annual herbaceous vegetation including mainly tall grasses (Hordeum spontaneum, Avena sterilis) and legumes (Vicia sp., Ononis sp., Trifolium sp., Medicago sp.) as dominant species. Vegetation in the semi-arid site is composed of shrubs (Phlomis sp., Astragalus spinosus, Teucrium polium, Retama raetam), geophytes (Anemone coronaria, Ranunculus asiaticus very abundant, see Fig. 1) and a large variety of small annual grasses (e.g. Crithopsis delileana, Bromus fasciculatus, B. lanceolatus, B. scoparius, Brachypodium distachyon), forbs (Plantago sp., Torilis sp.), and legumes (Trifolium sp., Ononis sp., Trigonella sp., Medicago sp.). The average annual plant biomass per area was 300 ± 48 and 169 ± 29 g m−2 in the Mediterranean and the semi-arid sites, respectively, for the 2006/2007 season (the study season).
The two target species chosen for this study, Bromus fasciculatus C. Presl. and Brachypodium distachyon (L.) Beauv. (Poaceae), are both winter annual grass species (Bromus and Brachypodium hereafter). They are suitable focal species for our study since they have a large distribution range from Mediterranean to desert environments and they are abundant and co-occur all along Wadi Shuayb. Ecotypic differences have been observed for these two species for nearby populations in Israel (Aronson et al. 1992, 1993).