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Vegetation and seed bank

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On spatiotemporal variation of vegetation and seed bank, as affected by ENSO (El Niño Southern Oscillation) in Auco

We monitored plant cover and soil seed density of shrubs and herbs in two markedly contrasting years with regard to annual rainfall (the dry 1996 vs the ENSO-driven, wet year of 1997), in two opposite-facing slopes (mesic v/s xeric), and the intervening --relatively wet-- ravine of a typical creek of Auco. During the ENSO year, cover of ephemerals increased over five times its previous level (43% vs 8%) in the xeric slope; almost three times (75% vs 26%) in the mesic slope; and almost doubled (57% vs 32%) in the ravine. The number of species was almost three times higher in the xeric slope (29 vs 10 species), increased by 57% in the mesic slope (33 vs 21), and by 19% in the ravine (25 vs 21). Cover of shrubs plus perennial grasses increased from 52% in 1996 to 59% in 1997 in the xeric slope, from 48% to 49% in the mesic slope, and remained in 43% in the ravine. Number of shrub plus perennial species remained constant between years: 13 in the xeric slope, 17 in the mesic slope, and 20 in the ravine. In the xeric slope, peak density of seeds was 4,500 seeds/m2 in 1996, while during the ENSO year it was 24,000 seeds/m2 , over five times higher. In the mesic slope, equivalent figures were 3,000 and 17,000 seeds/m2, almost six times higher. In the ravine, figures were 8,500 and 27,000 seeds/m2, over three times higher. The strong responses of ephemerals and of the seed bank to the ENSO-driven wet year of 1997 demonstrate the importance of this phenomenon in replenishing and recovering, and likely maintaining, ephemeral vegetation in the semiarid region of Chile.

Summary of plant and seed responses to the ENSO-driven wet year of 1997 in comparison to the previous dry year, in three landscape features of Auco. Xeric slope Dry 1996 Wet 1997 %Difference Ephemeral herb cover 8% 43% 438 Ephemeral herb species 10 29 190 Shrub and perennial cover 52% 59% 13 Shrub and perennial species 13 13 0 Total seed bank (seeds/m2) 4,500 24,000 433 Mesic slope Dry 1996 Wet 1997 %Difference Ephemeral herb cover 26% 75% 188 Ephemeral herb species 21 33 57 Shrub and perennial cover 48% 49% 2 Shrub and perennial species 17 17 0 Total seed bank (seeds/m2) 3,000 17,000 467 Ravine Dry 1996 Wet 1997 %Difference Ephemeral herb cover 32% 57% 78 Ephemeral herb species 21 25 19 Shrub and perennial cover 43% 43% 0 Shrub and perennial species 20 20 0 Total seed bank (seeds/m2) 8,500 27,000 218

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