Woody plant invasion in relictual grasslands

Invasion by alien plants is one of the greatest and most widely distributed causes of man-made changes in ecosystems. One of its most conspicuous variants is the invasion of natural grasslands by exotic trees, which not only means the addition of new taxa to the native biota, but also the introduction of completely new life-forms. In Argentina, the Pampean plain is one of the most highly altered regions and its natural vegetation is only partially conserved in the coastal dunes, swampy lowlands, mountainous areas, riparian habitats, field edges, roadside verges and railway tracksides. Nevertheless, all these habitats are undergoing severe invasions by woody aliens that jeopardize their conservation. We present a preliminary survey of the alien woody plants affecting the last remaining natural grasslands of the Argentine Pampas, the history of their colonization and the phase of the invasion process in which they presently occur. We construct an index of invasion severity to evaluate which woody exotic species should be considered invasive and therefore targeted for control, and to set priorities for the application of management practices according to the seriousness of the threat posed to biodiversity conservation.