Halophytenvegetation im Chaco Boreal, Paraguay - Pflanzengeographie, Ökologie und Dynamik in Zeiten des Klima- und Landnutzungswandels

Abstract

The South American Gran Chaco with its particular geological and climatic conditions is especially susceptible to salinization of soils and water. The aim of this study was to analyze the phytogeography and the phytosociology of the salt-affected vegetation in the Paraguayan Chaco and to find patterns of species composition and community structure in relation to different ecological and soil properties. The study area is located in the transition zone between the regularly inundated humid eastern Chaco and the dry Central Chaco. In these areas widespread natural salinization occurs in lagoons, depressions and along periodical water courses. These salinisations are connected with a shallow saline ground water table. In total 162 original phytosociological relevés in salt marshes and 236 relevés in salt-affected woody vegetation were collected in 2010-2012 in 14 different study sites. Inventories were associated with the ecological characterization of the habitats. Similar to other inland salt areas in South America, the examined salt habitats present a low species diversity. From the total species diversity in the Paraguayan Chaco, 18.9 % of the species were found in the whole salt-influenced study area and only 2.6 % in the salt marshes. The examined salt-affected vegetation is dominated by pantropical and neotropical genera. At species level neotropical elements in the broader sense are prevailing, in which the typical Chacoan species play a crucial role in the species composition. For the analysis of the life form composition and the distribution along a precipitation gradient the species of the study area have been compared with the checklist of the vascular plants of the Paraguayan Chaco that was prepared and recently published by the author. The Paraguayan Chaco as well as the investigated salt-affected habitats are dominated by phanerophytes, whereas in the salt marshes hemicryptophytes are prevailing. In the salt marshes nine plant communities were identified along a salinity gradient. The plant communities are mostly dominated by only one plant species depending on the degree of soil salinisation and water supply. The communities range from the hydro-halophytic Ruppia maritima community and the hygrophytic Setaria geminata in low saline conditions to the drought and salt resistant Heterostachys ritteriana community. The species distribution in the salt marshes is significantly influenced by sodium content and the organic matter in the topsoil. Setaria geminata grows in humid soils with a higher humus content, whereas Heterostachys ritteriana and Sarcocornia ambigua grow in soils rich in sodium. Minor topographic differences influence the sediment deposition, water supply and duration of inundation considerably. Sandy sediment accumulations reduce the humidity and salinity and facilitate pioneer species such as Sesuvium portulacastrum and Sporobolus pyramidatus. The salt-affected woody vegetation has been classified into ten communities along a gradient of decreasing water supply and salinity. Along this gradient the species diversity and structural diversity increased and the proportion of halophyte abundance declines. The species composition in the shrub and forest communities is greatly influenced by the sodium, sand and silt content in the topsoil. High sodium content and pH in the topsoil are positively correlated and characterize salt habitats with halophilous shrubs such as Cyclolepis genistoides, Lophocarpinia aculeatifolia, Lycium americanum, Lycium boerhaaviaefolium and Prosopis rojasiana. High silt and clay content in the topsoil determine the drainage capacity of the soils and with it the species composition. Characteristic plant communities on poorly drained soils are the Parkinsonia aculeata-Copernicia alba community, Prosopis ruscifolia-Sporobolus pyramidatus community, Prosopis ruscifolia-Evolvulus convolvuloides community and the Prosopis ruscifolia-Tabebuia nodosa community The abundance, frequency, dominance and the importance value index (IVI) were calculated for each woody species with a DBH ≥ 10 cm in each community. In shrub communites Prosopis ruscifolia occurs with high abundance and frequency, while in forest communities Salta triflora is the most abundant and frequent species. Furthermore, the salt-affected plant communities of the Paraguayan Chaco are compared with those found in related studies in the South American context. A syntaxonomical scheme for the plant communities is also presented. The vegetation of the salt marshes as well as the woody vegetation can be classified each into three different syntaxa at class level. Finally, effects of global change and developments of the salt areas in the Paraguayan Chaco in consequence of climatic variation and increasing land use are discussed. Recommendations for future use and conservation measures as well as further research activities are also suggested.