Edaphic characterization and soil ionic composition influencing plant zonation in a semiarid Mediterranean salt marsh

Soil characteristics and plant zonation were studied in a semiarid Mediterr anean salt marsh in SE Spain. According to topographic sequences and plants distribution, two transects were established from the border of La Mata la goon to the upland vegetation limit and soils were described and analysed. Regularly spaced plots were established in these transects in accordance wi th the stands of vegetation and surface soil samples were taken every 2 mon ths for 2 years. The following edaphic factors were determined: soil moistu re content, pH and concentration of Cl-, SO42-, Ca2+, Mg2+, Na+ and K+ in t he saturation extract. In addition, the groundwater level was measured and the duration of the flooding periods established in each plot. Soil-plant r elationships were studied by means of canonical correspondence analysis. Ba sed on rainfall data for the study period, dry and wet seasons were separat ed and the habitats of the plant communities were compared for salt quantit y and quality independently for each season. Soils were classified, accordi ng to FAO (1998), as Hypercalcic, Sodic and Mollic Solonchaks and Hypercalc ic Sodic Calcisols. The most important variables which explained plant zona tion were: the flooding period, total salinity, minimum Ca2+/Na+ ratio and the mean sodium adsorption ratio. When the habitats of the main plant commu nities were compared, differences in salt quantity, quality and seasonal va riations were found. Two chenopod shrubs, Arthrocnemum macrostachyum and Sa rcocornia fruticosa, predominated in the most saline areas. More pronounced seasonal variations in soil salinity were found in the A. macrostachyum zo ne than in the Sarcocornia fruticosa zone. The highest value for K+/Na+ and Ca2+/Na+ ratios were measured in the Suaeda vera stand. The Lygeum spartum zone was distinguished by the high Ca2+/Na+ and Ca2+/Mg2+ ratios in the we t period. Among the rushes, Schoenus nigricans predominated in the less sal ine areas, where the K+/Na+ ratio was higher and the Ca2+/Na+ ratio lower t han where Juncus maritimus predominated. Limonium cossonianum communities o ccupied an intermediate position with respect to soil salinity, between the chenopod shrubs and the other communities. Our results suggest that salt marsh plant zonation is influenced by tempora l and spatial edaphic gradients which must be jointly considered if soil-pl ant relationships in saline soils are to be fully understood. (C) 2001 Else vier Science B.V. All rights reserved.