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Questions: It is well documented that invasive alien plants can reduce species richness and diversity and modify the composition of invaded communities. However, our knowledge on the impacts of invasive alien plants on the community functional structure and ecosystem properties and the mechanisms underlying these changes is more limited. We evaluated how the community functional structure and ecosystem properties change along a gradient of Ligustrum lucidum abundance. Location: Subtropical mountain forest in NW Argentina. Methods: We conducted tree vegetation sampling in 164 plots distributed along a gradient of L. lucidum abundance. For L. lucidum and for the most abundant tree species in these plots we estimated six functional traits (specific leaf area, leaf tensile strength, leaf nitrogen concentration, leaf water potential, wood density and maximum plant height), as well as their relative growth rates. Then, we calculated indices of community functional structure (community‐weighted means for each trait; richness, evenness, divergence and dispersion). We explored the extent to which the invasive success of L. lucidum and its impact may be attributed to niche differentiation or to fitness superiority. Finally, we explored how L. lucidum abundance alters ecosystem properties. Results: Some indicators of community functional structure changed linearly while others tended to saturate along the invasion gradient. L. lucidum has a higher growth rate than resident species and it lays close to the periphery, but within the limits of the functional space defined by resident trees. L. lucidum tends to displace more functionally similar resident species. Soil moisture and N content declined, while soil C:N increased with L. lucidum abundance. Conclusions: Our findings suggest that both niche differentiation and fitness superiority contribute to explain the invasion success of L. lucidum and the changes in functional structure of the community along the gradient of L. lucidum abundance with evident impacts on ecosystem properties. [ABSTRACT FROM AUTHOR]

Aims Ligustrum lucidum, native to China, is one of the dominant exotic and invasive tree species in secondary forests in Argentina. (1) We assessed its invasion during 20 yr in post-agricultural secondary forests dominated by native tree species, and (2) evaluated tree demographic rates of native individuals under and away from L. lucidum trees that invaded these forests. Locations Four 1-ha permanent plots of secondary forests, subtropical Andes of Tucumán, Argentina. Methods We quantified the invasion of L. lucidum individuals (≥10-cm diameter) from 1991 to 2011 within four 1-ha plots of secondary forests and analysed annual growth and mortality rates of native tree individuals (≥10-cm diameter) under and away from L. lucidum trees. For this, we compared tree demographic rates of all native individuals together (community level) under and away from individuals of L. lucidum, referred to as exotic and native neighbourhoods, respectively. Then, we described the relative growth rate of native tree species in exotic/ native neighbourhoods (species level) and related them to their demographic life histories and morpho-physiological traits. Results Both density and basal area of L. lucidum increased from 0.1% to 4% between 1991 and 2011. Native tree individuals grew 65% less in exotic neighborhoods compared to native neighborhoods. Relative growth rate showed negative values under L. lucidum individuals for the abundant species Ocotea porphyria, Parapitadenia excelsa, Juglans austalis and Allophylus edulis, indicating competition; while less abundant species Terminalia triflora, Cupania vernalis and Blepharocalix saliscifolius showed positive growth values under L. lucidum, suggesting a facilitation process. Thus, L. lucidum competes efficiently with some native tree species, particularly with those abundant species that dominate the canopy. Conclusions Ligustrum lucidum is invading post-agricultural secondary forests dominated by native tree species. While invading, L. lucidum competes with native abundant tree species that dominate the main structure of the forest canopy. As this invasive trend will likely increase in the near future, L. lucidum may alter forest structure, composition and dynamics, as well ecosystem functioning. [ABSTRACT FROM AUTHOR]

Species loss and invasion of exotic species are two components of global biodiversity change that are expected to influence ecosystem functioning. Yet how they interact in natural settings remains unclear. Experiments have revealed two major mechanisms for the observed increase in primary productivity with plant species richness. Plant productivity may rise with species richness due to the increased amount of resources used by more diverse communities (niche complementarity) or through the increased probability of including a highly productive, dominant species in the community (sampling effect). Current evidence suggests that niche complementarity is the most relevant mechanism, whereas the sampling effect would only play a minor and transient role in natural systems. In turn, exotic species can invade by using untapped resources or because they possess a fitness advantage over resident species allowing them to dominate the community. We argue that the sampling effect can be a significant biodiversity mechanism in ecosystems invaded by dominant exotic species, and that the effect can be persistent even after decades of succession. We illustrate this idea by analyzing tree species richness-productivity relationships in a subtropical montane forest (NW Argentina) heavily invaded by Ligustrum lucidum, an evergreen tree from Asia. We found that the forest biomass increased along a natural gradient of tree species richness whether invaded by L. lucidum or not. Consistent with the sampling effect, L. lucidum invasion tripled total tree biomass irrespective of species richness, and monocultures of L. lucidum were more productive than any of the most species-rich, uninvaded communities. Hence, the sampling effect may not be restricted to randomly assembled, synthetic communities. We emphasize that studying invaded ecosystems may provide novel insights on the mechanisms underlying the effect of biodiversity on ecosystem function. [ABSTRACT FROM AUTHOR]

Question: How are plant communities of the Flooding Pampa grasslands spatially distributed? How do canopy dynamics of the different communities vary among seasons and years? Location: Buenos Aires province, Argentina. Methods: We characterized the distribution of communities through a supervised classification based on four Landsat 5 TM images. We sampled species composition of 200 sites, with 130 of them corresponding to natural communities. Of the sampling areas 60% were used to classify, and the remaining areas to assess classification accuracy. We characterized the seasonal and interannual variability of canopy dynamics using NDVI (Normalized Difference Vegetation Index) data provided by MODIS /Terra images. Results: Overall accuracy of the classification was satisfactory. The resulting maps showed a landscape formed by a matrix of extended lowlands with small patches of mesophytic and humid mesophytic meadows. The October scene (near the peak of productivity) was particularly important in discriminating among communities. The seasonal pattern of NDVI differed among communities and years. Mesophytic meadows had the highest NDVI mean and the lowest interannual coefficient of variation, halophytic steppes had the lowest mean, and vegetated ponds were the most variable. Conclusions: These grasslands have a fine-grained heterogeneity at the landscape scale. Each plant community has distinct seasonal and interannual canopy dynamics. These two features of grassland structure and functioning represent key information for rangeland management that may be obtained through a combination of minor field sampling and remote sensing. [ABSTRACT FROM AUTHOR]

Question: What factors limit woody plant recruitment in a mosaic landscape where former agricultural lands are dominated by the invasive tree Ligustrum lucidum (Oleaceae)? Location: Subtropical northwestern Argentina. Methods: In secondary forest patches, we measured (I) tree, shrub and liana abundance in different size classes; (2) seed rain of Ligustrum and two native trees and (3) topographic, soil and light variables. We used spatial autoregressive models to test for effects of Ligustrum dominance and environment on native plant abundance in each size class. We used multiple regression on resemblance matrices to quantify the relative importance of spatial (e.g. dispersal) and environmental effects on native species composition. Results: Native tree abundance in the smallest size class was unrelated to Ligustrum canopy dominance, while native tree abundance in larger size classes and native liana abundance were negatively correlated with Ligustrum dominance. Native species composition was both environmentally and spatially structured, suggesting that some species are dispersal limited. Seed rain was spatially correlated with conspecific basal area for one of two native species, but not for Ligustrum. Conclusions: Native tree recruitment appears to be limited primarily by sapling mortality in patches dominated by the invasive Ligustrum. Ligustrum does not appear to be dispersal limited in our study area and is likely to continue spreading. Invaded patches may persist for hundreds of years. [ABSTRACT FROM AUTHOR]

Successional patches are a large component of forest ecosystems throughout the world and their vegetation composition is conditioned by multiple factors such as land use history, disturbances, environmental conditions and landscape context. We investigated the relative contribution of historical, environmental, biotic and spatial factors in determining vegetation composition and invasion by exotic species in secondary forest patches of Sierra de San Javier, Tucumán, Argentina. We estimated canopy cover for shrub, vine and tree species distributed over 51 patches with known land use history. We also recorded environmental, historical and spatial variables and used multivariate techniques to explore the relationship between forest composition and explanatory variables. Land use, time since abandonment, altitude, slope and cover of different strata were related to the vegetation pattern in the study site, and they were all significantly structured over space. Exotic species appeared to differ from natives in their response to explanatory variables. Overall, exotic species were dominant on the edges of young patches originated from herbaceous crops, but the total number of exotic species was related to the distance to urban areas and small farms identified as potential sources of exotic propagules. Vegetation composition of secondary forests in NW Argentina was related to historical and environmental factors, but spatial variables strongly influenced vegetation composition as well as the variation in explanatory variables. [ABSTRACT FROM AUTHOR]