Factors controlling the establishment of Helianthemum squamatum, an endemic gypsophile of semi-arid Spain

1 The restriction of some vascular plants to soils high in gypsum under arid or semi-arid climates has been reported by many authors in different parts of the world. Factors controlling the presence of gypsophytes on these soils are far from being definitively explained. 2 The establishment of Helianthemum squamatum, an Iberian gypsophyte, was investigated during a 2-year period in a typical semi-arid gypsum gradient landscape in central Spain that included the three community belts where this plant can grow. 3 More than 500 seedlings were tagged. Their growth and survival were monitored periodically (23 census dates). Over 80% of the recorded seedlings emerged during the first year, which followed a severe and long drought period. Germination occurred principally in winter (over 85%), although seeds germinated sporadically even in late May. Emergence was clearly spatially confined, with seedlings appearing only in the summit area, which was characterized by gypsum outcrops (160 per m(2)), and at the boundary between gypsum slope and alluvial piedmont (273 per m(2)). Seedlings were very scarce in lower areas (Lygeum spartum and Artemisia herba-alba communities) and absent from the steeper gypsum slope. 4 Survival was size-dependent: larger plants had a better chance of surviving the first summer, which is the bottle-neck for recruitment. We detected few differences in the final survival percentage after the first year for cohorts or zones, and survival curves were also similar, except at the boundary between gypsum slope and piedmont. However. seedlings emerging in 1997 harl a higher survival percentage than those emerging in 1996. 5 Proximity to a seed source was the most relevant predictor of emergence. This may be related to the presence of mucilaginous coats in H. squamatum seeds, which may enable plants to become established on crusted soils. 6 Our results seem to link gypsophily with some properties of the surface crust, which is thought to contribute to the restrictive behaviour of such soils. We detected seedlings emerging on soils with very high surface strength, but also in alluvial soils. This suggests that H. squamatum can penetrate hard soils but also has the potential to grow on less restrictive soils. Competition with annuals may exclude H. squamatum on bottom flats.