Biological invasions have important ecological impacts at both local and global levels, affecting biodiversity, ecosystem functioning, and economic sustainability. The study of invasions requires specific methodological approaches to gain rapid insight into the spatial and temporal dynamics of the system. We studied a recent invasion by the exotic herb Hieracium pilosella L. (Mouse-ear hawkweed, Asteraceae) in the Argentinean portion of the northern part of Tierra del Fuego Island in Southern Patagonia. To assess the extent of this invasion and the related ecological and land use factors, we performed an extensive field sampling at a regional scale and used a spatial pattern approach using geostatistical techniques to build a map of the invasion. Our results showed that the invading species is widely distributed across the entire Fuegian steppe, in general with low cover (\2%) and a particular spatial structure having hot spots (10-70%) related to specific land uses. We found regionallatitudinal and also local-community level variability in the frequency, cover and spatial distribution of the invasive species, each of them associated with particular ecological factors, and no association with regular domestic animal grazing. Region-wide invasion may be accounted for by wind dispersal of the seeds and the latitudinal variability of the precipitation; while localvariability was associated with the susceptibility of different plant communities to invasion. Hot spots were found to be related to massive soil disturbances (e.g., road building, shrub removal, physical impacts of long-term and intense domestic animal use, etc.), probably due to the presence of bare soil favoring colonization by the invading species. Scrublands, lawns and grasslands were more invaded than wetlands, heathlands or salt grasslands. Since the invasion is apparently at initial stages due to the low cover values found, we believe that information about the extent and environmental or management factors involved in this exotic plant's expansion should be used to design appropriate control and mitigation tools. In this sense, our results point to management actions focused on reducing the presence of hot spots of invasion by means of quick revegetation of bare soil after a massive disturbance event takes place.

Our main objective was to classify vegetation and soils of wetlands in northern Tierra del Fuego (Argentina) along a latitudinal precipitation gradient within the Magellanic Steppe Zone. We presented the first detailed ecological characterization of these wetlands by relating floristic composition to local site conditions, bedrock and climate. The survey consisted of 125 phytosociological censuses and 52 soil profile descriptions. Soils were classified according to FAO, and vegetation samples were explored by applying numeric methods such as cluster analysis and indirect ordination that included a post-hoc correlation with environmental variables. Floristic composition of freshwater communities was strongly related to a latitudinal gradient, and to soil pH, base cations and C/N ratio. Most eutrophic marshes were found on organic soils situated in springs on Tertiary sediments at mid-latitudes. Most acidic marshes occurred within Pleistocene catchment areas at higher latitudes (higher precipitation). Wet grasslands on mineral soils of low C/N ratio within formerly glaciated areas at lower latitudes (lower precipitation) represented the dry end of the moisture gradient covered by this study. Saltwater communities predominated on playas of Tertiary sediments and in estuarine systems. They were characterized by alkaline soils and high Na concentrations. Magellanic wetlands form a floristic continuum from the semiarid north to the subarid south with composition not necessarily related to landscape position and soil units, but strongly related to soil variables of secondary taxonomic level. We conclude that at least at the resolution level of our study, floristic composition along the observed gradient is strongly influenced by regional climate.

The semiarid Espinal in central Argentina, being recently transformed from natural semiarid grasslands into agriculture, represents an interesting scenario to understand the early stages of weed community assembly and its relationship with crop identity and management. Our aim was to characterize the weed communities in corn (Zea maysL.) and soybean [Glycine max(L.) Merr.], the main crops of the Espinal region, under the dominant rainfed conditions. Weed surveys were carried out in 53 fields, and farmers were interviewed to collect information about crop management. Floristic composition was compared within and between crops by calculating the additive partition of the abundance-based Bray-Curtis dissimilarity. We compared the frequency and mean cover of functional groups between crops through generalized linear models. Finally, canonical correspondence analysis was carried out to analyze the associations between floristic composition and agronomic variables. Mean alpha and gamma diversity was greater in corn (10.0 and 80 species, respectively) than in soybean (7.6 and 46 species, respectively). Furthermore, species composition of weed communities was more similar among soybean fields than among either cornfields or fields of both crops. Hence, floristic differences between crops are potentially the result of different microenvironmental heterogeneity above- and belowground, with corn likely to be more permissive to weed establishment compared with soybean. The higher frequency of annual, dicotyledonous, and native species, and the high proportion of rare species, mostly native, suggest a strong legacy of the original vegetation that thrived in these recently cultivated systems. The functional composition was also affected by agronomic management, with sulfur, nitrogen, and grass herbicide application being the most important factors related to the floristic composition of weed communities. This early description can be used as a starting point for studies concerning trajectories, mechanisms, and processes of weed communities related to environment and management.

Plant invasions have important ecological impacts on biodiversity, the functioning of ecosystems and economic sustainability. In this study, we evaluated the effects of four control measures (pasture sown + fertiliser, fertiliser and selective ⁄ non-selective herbicide applications) in two different grazing conditions (grazed and ungrazed) during a recent invasion of the exotic herb Hieracium pilosella in northern grasslands of Tierra del Fuego Island in Southern Patagonia, Argentina. As response variables, we measured the cover of the invasive species, the dominant growth forms of other plant species, litter and bare soil at patch scales (m 2 ) during two consecutive growing seasons. The effects of fertilisation depended on the grazing conditions; H. pilosella cover decreased by more than 92% and was replaced by dicotyledonous herbs in the ungrazed ⁄ fertilised subplots, while it exhibited no decrease in the grazed ⁄ fertilised subplots after the second growing season. Both herbicides (selective and non-selective) reduced H. pilosella cover by c. 63% compared with the untreated subplots independently of grazing. However, the non-selective herbicide application resulted in an increase in bare soil and litter cover in the treated grazed and ungrazed subplots respectively. In contrast, such effects were not observed with the selective broad-leaved herbicide application. A control strategy based on the local application of selective herbicides and ⁄ or NP fertilisers, in conjunction with a transient ban on sheep grazing, reduced the invaderÕs cover in the short term and at a local scale and also reduced the cover of bare soil through the restoration of native vegetation. An economic assessment of this strategy supported the profitability of these control measures.