Gypsum physical soil crusts and the existence of gypsophytes in semi-arid central Spain

Probably gypsophytes are the most interesting set of edaphic specialists of arid and semiarid climates. Despite they conform a global biodiversity priority, there are almost no information about those adaptive traits that confer such a specialised behaviour. Our broad hypothesis is that gypsophytes are "refuge-endemics" that are able to grow on gypsum soils due to their ability to surpass extremely hard gypsum soil physical crust during emergence. With this in mind we have conducted an experimental approach combining field and greenhouse assays. Seeds from two gypsophytes, genuine and widely distributed in the Iberian Peninsula gypsophytes (Helianthemum squamatum and Lepidium subulatum) and one gypsovag (Teucrium capitatum), a generalist plant that can also grow on gypsum soils were used in our experiments. Two complementary experimental approaches were conducted. The first involved a field experiment in which the presence or absence of the physical crust together with the sowing date were manipulated and a greenhouse experiment in which the irrigation amount and the types of soil were controlled. Variables of interest were the percentage of germination, growth and survival. In the field experiment we found a significant decrease in the final germination of the gypsovag in the plots with intact crusts. On the other hand, H. squamatum is able to grow in the three tested soils, despite higher survival and growth on genuine gypsum soils. Our results confirm the hypothesis that gypsum edaphic specialists base their behaviour to a great extent on the ability to surpass extremely hard gypsum surface crusts, although this seems a marginal adaptive trait as shown by the capability to grow on a complete array of soils and the negative effect of the crust along the earlier development life stages of gypsophytes. Furthermore, a gypsovag such as Teucrium capitatum presents extreme difficulties to emerge on non-disturbed gypsum physical crusts but once surpassed its growth and survival is not limited.