Summary 0The restriction of some vascular plants to soils high in gypsum under arid or semi! arid climates has been reported by many authors in di}erent parts of the world[ Factors controlling the presence of gypsophytes on these soils are far from being de_nitively explained[ 1 The establishment of Helianthemum squamatum\ an Iberian gypsophyte\ was inves! tigated during a 1!year period in a typical semi!arid gypsum gradient landscape in central Spain that included the three community belts where this plant can grow[ 2 More than 499 seedlings were tagged[ Their growth and survival were monitored periodically "12 census dates#[ Over 79) of the recorded seedlings emerged during the _rst year\ which followed a severe and long drought period[ Germination occurred principally in winter "over 74)#\ although seeds germinated sporadically even in late May[ Emergence was clearly spatially con_ned\ with seedlings appearing only in the summit area\ which was characterized by gypsum outcrops "059 per m 1 #\ and at the boundary between gypsum slope and alluvial piedmont "162 per m 1 #[ Seedlings were very scarce in lower areas "Ly`eum spartum and Artemisia herba!alba communities# and absent from the steeper gypsum slope[ 3 Survival was size!dependent] larger plants had a better chance of surviving the _rst summer\ which is the bottle!neck for recruitment[ We detected few di}erences in the _nal survival percentage after the _rst year for cohorts or zones\ and survival curves were also similar\ except at the boundary between gypsum slope and piedmont[ However\ seedlings emerging in 0886 had a higher survival percentage than those emerging in 0885[ 4 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[ 5 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 [

The restriction of vascular plants to gypsum-rich soils under arid or semiarid climates has been reported by many authors in different parts of the world. However, factors controlling the presence of gypsophytes on these soils are far from understood. We investigated the establishment of Lepidium subulatum, a gypsophyte, in a nondisturbed semiarid gypsum-soil landscape in central Spain, both from spatial and temporal perspectives. Over 1400 seedlings were tagged, and their growth and survival were monitored for a 2-yr period. Several biotic and abiotic variables were measured to determine the factors controlling the emergence and early survival. These variables included the cover of annual plants, bryophytes, lichens, litter, gypsum crystals, bare fraction and cover of each perennial plant, and several soil properties (gravel, fine gravel, and fineearth fraction, conductivity, pH, gypsum content, organic matter and penetrometer soil resistance). Our results support the linkage of gypsophily with some physical properties of the surface crust. Seedlings tended to establish on the gypsum surface crust, and their survival was size dependent, probably as a consequence of the necessity of rooting below the surface crust before summer drought arrives. However, once seedlings emerged, a higher survival rate occurred on the alluvial soils of the piedmont-slope boundary where soil crusts are absent or thinner. We conclude that Lepidium subulatum may be considered a refuge model endemic with a distribution range that occupies a reduced fraction of a wider habitat from which it is probably excluded by competition.