The Edaphism: Gypsum, Dolomite and Serpentine Flora and Vegetation

Abstract

The relationship between plants and the different types of rocks has been known at least since the sixteenth century when Caesalpino linked the existence of the endemic Alyssum bertolonii to the Italian serpentines. In the seventeenth century, Spanish botanists Asso and Cavanilles also point out the same soil-plant connection in the Iberian gypsum, as later did Linnaeus creating the genus Gypsophila. The study of this edaphic phenomenon is part of Geobotany or even, as a discipline within it, Geoecology. The term “edaphism” has been used by European authors almost as a synonym of Geoecology, although it has also been employed as a contraction of “edaphic endemism”. Be it as it may, the best expression of the soil-plant link is the high percentage of endemic plant species that appear on special rocks like serpentines, gypsum and dolomite. These three types of substrates make plants face nutritional imbalances, so that soil chemistry is important to understand them. From this point of view, the main feature that can serve as a link between these 3 types of edaphisms is the Ca/Mg ratio, well above unity in the gypsum soil and near that value, or even below, in dolomites and serpentines. Of the three, serpentines are undoubtedly the most recurrent motif of the botanical-ecological research; several monographs worldwide are evidence of this. At the other extreme are dolomites, of which the research might almost be described nowadays as anecdotal. The research on gypsum has grown enormously in recent years. The most recent advances in edaphism research show that not only chemical (nutritional) factors in the soil help to understand it, but also that the physical factors and biotic interactions are key elements and, especially, those concerning the roots. The roots, associated with different symbiotic organisms, must face not only the inherent chemical imbalances in these soils, but also meet the challenge of growing and spreading in them as well as getting to minimize water stress. The different plant strategies to address these limitations also make these edaphic endemism noteworthy from the ecophysiological point of view. Thus, metal hyperaccumulating plants have great value in both phytomining and phytoremediation. While many serpentine plants are heavy metals hyperaccumulators, some gypsophytes do not only accumulate significant amounts of Ca and Sr, but are able to extract the gypsum structural H₂O molecules, which is why it is legitimate to speak of “biological mining of water”. Moreover, the very special physicochemical conditions of these ecosystems make them particularly interesting as terrestrial analogues (a field site, which bears an analogy or similarity in some way to other planetary bodies of the solar system). The Iberian Peninsula is especially rich in these 3 types of substrates as evidenced by the 25 forest types, the over 56 different types of scrublands and 18 types of grasslands described. Not to mention the cryptogamic crust and microbial communities associated with these rocks, to which some of the oldest organisms on the planet, as Chrooccocidiopsis, belong.