Paula Marcora

Factors associated with woody alien species distribution in a newly invaded mountain system of central Argentina

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.

Age–size–habitat relationships for Polylepis australis : dealing with endangered forest ecosystems

Assessing tree ages is important for the understanding of forest recruitment patterns and tree growth. However, little experience exists in the subtropics and accurate aging usually involves potentially damaging techniques such as tree coring or even the use of entire cross sections, which is not recommendable in endangered species or ecosystems. We provide an example of how age may be predicted on the basis of tree size and site conditions, using Polylepis australis of Central Argentina. Our study was conducted in two entire river basins where 96 independent trees were successfully cored for age analysis. Site and tree characteristics were registered and tree age determined through standard dendrochronological techniques. The multiple regression procedure selected tree circumference and proportion of rock under the tree canopy as significant variables explaining 48% of the variation in number of growth rings. Trees surrounded by rock clearly grew slower than those surrounded by a higher proportion of soil and vegetation. A comparison with a one site study shows that the ability to predict P. australis age is clearly reduced when geographical range is extended but the explanatory power of our model is still high enough for certain applications and within the range of other one site studies. We suggest that tree size and site characteristics may be used to predict age in other subtropical mountain forests with a well marked growth season.