Threshold ionic contents for defining the nutritional strategies of gypsophile flora

Plants that show a strict link with gypsum soils, gypsophytes, constitute a major current research topic in the field of plant ecology. Among the hypotheses proposed to explain this close relationship, the nutritional imbalances that this type of soil implies for plants stands out. According to this hypothesis, gypsophytes would be able to accumulate high contents of certain key minerals such as Ca, Mg and S, especially in leaves. Although the distinction between gypsophile and non-gypsophile species has often been based on the capacity to accumulate the aforementioned mineral elements, objective thresholds have never been established. The main aim of the present work is to establish the levels of Ca, Mg and S above which a species can be considered an accumulator of these elements. To this end, the concentration ranges included in handbooks and papers on plant nutrition with information for a large number of species were used as a starting point. Based on these ranges, thresholds for four Nutritional Content Levels (NCL) for Ca, Mg and S were established: deficient, normal, high and very high. For each level, a numerical value between 1 and 4 was assigned, from lowest to highest. The sum of the values of the levels of accumulation (Additional Nutritional Value, ANV) corresponding to the three elements mentioned served to classify the species studied in groups or categories (N_STRATEGY). These categories were adjusted to the classic strategies that divide the plants into exclusors (values between 3 and 5), indicators (6–9) and accumulators (10–12). The N_STRATEGY predictive power of plants’ gypsophily degree was statistically analysed (Artificial Neural Networks). The categories established by N_STRATEGY showed a high predictive value (up to 98%) to detect gypsophily in plants and proved a more objective and easier criterion than previous functional classifications proposed for gypsophytes. It has an additional advantage in that it can also be applied to other plants linked with special substrates (serpentines, dolomites, etc.). The results regarding the nutritional contents of Ca, Mg and S showed that Ononis tridentata and the Gypsophila genus, both taxa with succulent leaves, are accumulators for all these elements. Ca seems clearly associated with the succulent character of the leaves. Another novel contribution is the role in the interpretation of the gypsophile flora played by Mg, whose importance is likely greater than has been considered to date. This research confirms previous results on S and pinpoints it as the element that best discriminates between gypsophytes and non-gypsophytes or gypsovags.