Southernmost South America Steppes: vegetation and its modern pollen-assemblages representation

MANCINI, M.V., ; DE PORRAS, M.E,; BAMONTE, F.P.

Steppe Ecosystems: Dynamics, Land Use and Conservation

Nova Science Publishers, Hauppauge

Lugar: New York; Año: 2012; p. 141 - 156

Abstract At southernmost South America, grass, shrub and dwarf-shrub steppes are the most widely distributed plant communities on the Patagonian plateaus. Their distribution is mainly determined by the prevailing dry conditions (400-100 mm) which are consequence of the rain-shadow effect. The uplift of the westerly air masses over the western slopes of the Andes produces orographic precipitation leading to a maximum there, whereas the forced subsidence over the eastern side of the Andes produces very dry conditions in the Patagonian plateaus. Grass steppes dominated by Festuca sp. are distributed discontinuously to the west, in a latitudinal band between 51° - 52°S to the south and on plateaus over 700 masl. The shrub steppes with tall to medium shrubs such as Nardophyllum obtusifolium, Colliguaja integerrima, Berberis spp., Junellia tridens and dwarf-shrub steppes represented by cushion plants such as Nassauvia, Ephedra frustillata and Azorella sp. are the most widespread plant communities at the Patagonian plateaus. Shrub and dwarf-shrub steppes are both physiognomically and floristically heterogeneous. The patchiness (heterogeneity) within these steppes is related to the rainfall spatial variation, the runoff redistribution and the edaphic diversity. On the other hand, southernmost South America steppes have been disturbed since the end of the 19th century as a consequence of human activities (mainly sheep breeding) leading not only to a decrease in the species diversity but the introduction of exotic species such as Rumex acetosella, Taraxacum officinale and Plantago major. Modern pollen assemblages clearly reflect the steppe plant communities and their regional distribution according the increase of aridity to the east. Modern pollen-vegetation relationships from the southernmost South America steppes were established to be used as modern analogous in a long-term project which aims to understand the steppes dynamics particularly during the Holocene. From the pollen analysis of different sequences it can be seen that the modern steppes were established during the last millennium. Furthermore, pollen records clearly show that during the last century plant communities underwent important changes in response to intensive overgrazing. These changes are evident by over-representation of certain taxa (eg. Asteraceae subf. Cichorioideae, Rumex and Plantago). Grass steppes have been the most affected communities (due to the high quality pastures for sheep breeding) leading to severe changes in floristic composition, physiognomy and plant cover whereas in the shrub and dwarf-shrub communities these changes have not been so evident.