Altitudinal gradients are expected to heavily influence the general performance of mountain tree species. For this study we evaluated vitality, radial growth and reproductive efficiency in Polylepis australis (Rosaceae) throughout its complete altitudinal range in central Argentina. We selected seven forest fragments ranging from 900 to 2700 m a.s.l. In the field, we subjectively assigned vitality and seed productivity indexes to 12 P. australis trees per fragment. In addition, we cored and assessed for radial growth 10–18 trees per altitudinal level. In the laboratory, we individually weighed seeds from four or five trees from each altitudinal level and divided into two seed mass classes (light and heavy). We then tested for germination and monitored their seedlings for survival. Seedlings of three trees per altitudinal level were harvested after 40 days and assessed for dry mass. Seedlings of one to two trees per altitudinal level were monitored for survival until both leaves and roots had died. Our main results showed two types of responses to altitude: a unimodal relationship with an optimum at intermediate altitudes of around 1800 m a.s.l. for tree vitality, radial growth, seed productivity and seed mass; and an increase with altitude for in‐vitro seed germination and seedling survival. A rise in temperature due to climate change could restrict P. australis to the upper most altitudes, where conservation of these forests will be a priority.

Mountain forests and their soils provide ecological services such as maintenance of biodiversity, provision of clean water, carbon capture and forage for livestock rearing, which is one of the principal economic activities in mountain areas. However, surprisingly little is known about livestock impact in South American mountain forest soils. With the aim of understanding how livestock and topography influence patterns of forest cover, soil compaction, soil loss and soil chemical properties, we analysed these parameters in 100 Polylepis australis woodland plots situated in the humid subtropical mountains of Central Argentina. We used distance from the nearest ranch as an objective index of historical livestock impact and measured standard topographic variables. Our main results reveal that distance from ranch in all cases partly explains tree canopy cover, soil loss, soil compaction and soil chemical properties; suggesting a strong negative effect of livestock. Intermediate altitudes had more tree canopy cover, while landscape roughness -a measure of the variability in slope inclination and aspect -was negatively associated to soil impedance and acidity, and positively associated to soil organic matter content. Finally, flatter areas were more acid. We conclude that livestock has had a substantial influence on forest soil degradation in the Mountains of Central Argentina and possibly other similar South American mountains. Soil degradation should be incorporated into decision making when considering long-term forest sustainability, or when taking into account retaining livestock for biodiversity conservation reasons. Where soil loss and degradation are ongoing, we recommend drastic reductions in livestock density.

To help determine the major factors associated with alien plant in a newly invaded mountain range; we analyzed the distribution patterns of woody alien species along the altitudinal gradient of the Córdoba mountains, in relation to biotic, abiotic and anthropogenic factors. We selected 303 plots using a Geographic Information System (GIS) covering all the variability of these factors. In the field we registered woody alien occurrence in these 303 and in 303 additional neighbor plots. We used 12 biotic, abiotic and anthropogenic variables from the GIS to predict the probability of presence of alien species through a logistic model. Then, we analyzed if neighbor alien occurrences could explain some additional variance. We created a probability map with 4 categories of alien occurrence which was then validated by new field sampling. Occurrence of woody aliens was highest in the eastern slope (with longer history of species introduction), at low altitudes, near sources of propagule pressure (human settlements, roads and neighbor sites with established alien plants), and associated to sheltered topographies. In the upper belt of the Córdoba Mountains woody invasion is incipient and thus in a transient stage. Accordingly, propagule pressure seems to be the major factor at play, while the relevance of disturbance and biotic interactions is less clear.

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