干旱区生态环境知识资源中心

We followed plant cover and soil seed density of shrubs and herbs in two markedly contrasting years with regard to annual rainfall in two opposite-facing slopes (mesic vs xeric) and an intervening, relatively wet, ravine of a typical creek in semi-arid Chile. During the ENSO year 1997 cover of ephemerals increased in all three sites; 43% vs 8% on the xeric slope 75% vs 26% on the mesic slope and 57% vs 32% in the ravine. The number of species was almost three times higher. On the xeric slope (27 vs 10 species), increased by 47% on the mesic slope (28 vs 19) and by 14% in the ravine (24 vs 21). Cover of shrubs plus perennial grasses increased from 52% in 1996 to 59% in 1997 on the xeric slope, but no significant cover changes were found on the mesic slope or the ravine. On the xeric slope peak density of seeds was 4500/m(2) in 1996, while during the ENSO year it was 24000/m(2). On the mesic slops equivalent values were 3000 and 17000 seeds/m(2) while in the ravine figures were 8500 and 27000 seeds/m(2). The strong responses of ephemerals and seed bank to the ENSO-driven wet year of 1997 demonstrate the importance of this phenomenon in replenishing, recovering and probably maintaining ephemeral vegetation in this region.