Riparian vegetation and water chemistry in a basin under semiarid Mediterranean climate, Andarax River, Spain

A study has been made of the relationships between the characteristics of the riparian vegetation (floristic composition, structure and diversity) and the spatial-temporal variation of the quality of the stream waters in a basin under a semiarid Mediterranean climate in the southeastern Iberian Peninsula. The plant communities of the high reaches present greater specific richness and diversity (S-mean = 7.0 +/- 3.4 and H'(mean) = 2.0 +/- 0.7) than do those of the middle and low reaches (S-mean = 4.5 +/- 1.6 and H'(mean) = 1.8 +/- 0.6). One zone reached the highest specific richness (S = 12, H' = 3.2), which, apart from being situated in the intermediate stretch of the basin. represents a transitional state (ecotone) between the Salix and Tamarix communities. The characteristics of the waters analyzed indicate very high rates of erosion and runoff due to the nature of the soils (easily eroded marls) and to agricultural expansion and mining since the 16th century. The present-day riparian vegetation is not adequate to absorb the nitrates added to the basin by crop fertilization, reaching extremely high values, particularly during the dry period (between 1.2 and 42.5 mg/liter). Sewage dumping at three sampling stations did not appear to affect the specific composition of the woody vegetation, in the zones with watercourses, water salinity was low during the period of greater water flow, but considerably higher in the dry season (the upper limit was some 1.2 mS/m), resulting in a predominance of salt cedars over willows. Three types of saltcedar areas were distinguished: subhalophilous, which barely changes its chemical composition over the season; halophilous, which develops over strongly mineralized waters and markedly alters in chemical composition during the dry season; and hyperhalophilous, where salinity is extraordinarily high and quite constant throughout the year. A direct relationship was found between the dominance of Tamarix africana and abundance of NaCl.