Paula A. Tecco

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.

Plant invasions in undisturbed ecosystems: The triggering attribute approach

Abstract The invasion of a target community by a non-indigenous plant species includes the stages of arrival, establishment and spread, which tend to depend on different characteristics of the invasive species and its context. While the mechanisms behind the invasion of highly disturbed ecosystems are well known, our understanding of the invasion process in undisturbed or weakly disturbed ecosystems is much more limited. Here we propose that, once a non-indigenous species has arrived to a new ecosystem and become established, the likelihood that it spreads, and thus becomes invasive, may depend on just one or very few characteristics, called ‘triggering attributes’ (TA). We propose that a TA is a vegetative or regenerative attribute discontinuously distributed in comparison to the resident community. This attribute allows the species to benefit from a resource that is permanently or temporarily unused by the resident community. We present an original study case and examples from the literature to illustrate our approach, and we also propose some ways to test it in different ecosystems. Abbreviation: RGR = Relative growth rate; SLA = Specific leaf area; TA = Triggering attribute.

Changes in floristic composition and physiognomy are decoupled along elevation gradients in central Argentina

Question(s) Most vegetation descriptions tacitly assume that floristic composition and physiognomy are tightly linked. However, both vegetation properties may not respond in a similar way to environmental and disturbance gradients, leading to uninformed management planning and difficulties when attempting to restore degraded ecosystems. In this context, we addressed two main questions: how close are relations between floristic and physiognomic types as defined by numerical vegetation classification in mountain ecosystems? How are floristic and physiognomic types distributed along the elevation gradient?. Location Central mountains of Argentina, 31° 27′, 64° 54′, between 500 and 1700 m a.s. l. Methods We selected 437 sites where we performed complete floristic and physiognomic releves. We classified eight physiognomic and eight floristic types. We tested the relationship between both classifications through a chi2 analysis. We tested the association between elevation and each physiognomic and floristic type performing random permutations. Results In general, floristic types were significantly and positively associated with more than one physiognomic type and vice versa. Physiognomic and floristic types responded differently to the elevation gradient. Floristic types were restricted to different sections of the gradient, though having large overlap among them. In contrast, seven out of the eight physiognomic types did not show elevation restriction, being distributed along the complete elevation gradient. The open low woodland with shrubs was the only restricted physiognomy, significantly absent from the upper part of the gradient. Conclusions We highlight the importance of considering both vegetation properties independently when characterizing vegetation patterns in heterogeneous systems, since they show decoupled responses to environmental gradients. We notice that the assumption of a direct link between floristic composition and physiognomy may induce bias into the understanding of vegetation patterns and processes. For that reason, we encourage managers and restoration practitioners to consider the complete range of possible physiognomic types under each floristic type. This article is protected by copyright. All rights reserved.

Testing alien plant distribution and habitat invasibility in mountain ecosystems: growth form matters

Most studies focused on understanding habitat invasibility use the current levels of invasion as a direct proxy of habitat invasibility. This has shown to be biased by the influence of propagule pressure and climate. We suggest that plant growth forms need to be considered as an extra factor, as habitat preferences might not be equal for all potential invaders. We test the influence of propagule pressure, climate and habitat characteristics on the current level of invasion and habitat invasibility, specifically addressing whether an analysis focused on growth forms evidence different patterns than the total pool of alien species. We used 499 floristic vegetation plots located in Córdoba Mountains. We used proportional alien richness of the total pool and for each growth form as response variables. We identified models that best explained current levels of invasion. We used the residuals of the models with propagule pressure and climate as the response variable. Then, we performed linear models to test the relationship between habitat characteristics and the residuals of the models. We found different drivers of current alien distribution patterns for the total pool and each growth form. Habitat invasibility was not equal when quantified for the total pool or growth forms. Shrublands and outcrops were recorded as less susceptible to woody invasion, while grasslands and native woodlands were resistant to the invasion of grasses and none habitat type was resistant to the invasion of forbs. We highlight that the current level of invasion and habitat invasibility are highly growth form dependent.

Impact of Ligustrum lucidum on the soil seed bank in invaded subtropical seasonally dry woodlands (Córdoba, Argentina)

The impact of invasive species on below ground flora may differ from that on the above ground vegetation. Recent reviews of invaded and native communities emphasize the need for more comprehensive information on the impacts of plant invasion on soil seed banks. Ligustrum lucidum is one of the most important invasive woody species in several ecosystems of Argentina; however, its impact on soil seed bank communities has not been studied. Here we analyzed differences in species richness, total seed density and species composition (total, native and exotic species) in the soil seed bank of native and invaded woodlands, in two different seasons. We also analyzed differences in similarity between standing vegetation and soil seed banks of both woodland types. The study was carried out in the Chaco Serrano woodlands of Córdoba, central Argentina. Our main results indicate differences in L. lucidum woodland composition and a reduction in both richness and total density of species in the soil seed bank compared to the native woodlands, independently of the sampling season. Moreover, a higher abundance of certain exotic species in the soil seed bank was observed in L. lucidum woodlands, particularly in spring. Finally, low similarity between soil seed bank and the established vegetation was observed in both woodland types. From a management perspective, it seems that passive restoration from soil seed banks of L. lucidum might be coupled with active addition of some native woody species and control of other exotic species.

Similarities in recruitment but differences in persistence in two related native and invasive trees: relevance of regenerative and vegetative attributes.

Ecological theory predicts that the success of exotic plants in new environments depends on a combination of both regenerative and vegetative attributes. Identifying those attributes may benefit from approaches that specifically compare related exotic and native species, thereby overcoming strong habitat dependence and phylogenetic bias. Gleditsia triacanthos L. (Fabaceae) is described as an aggressive woody invader in a broad range of ecosystems of Argentina, where it coexists with other leguminous trees. In the present study, we assessed whether the success of G. triacanthos in the lower areas of Chaco woodlands of central Argentina is determined (1) by differences from the dominant native, Prosopis alba, in its combination of several attributes that enhance the invader’s competitive ability, (2) by differences from P. alba in few key attributes that facilitate its spread, or (3) by strong functional similarities in response to environmental filtering by the invaded system. Our comparison of 19 functional traits of both species showed that regenerative and vegetative trends may vary independently, and that two contrasting strategies seem to underlie the success of G. triacanthos in this region. On the one hand, the invader might benefit by sharing regenerative attributes with the native relative that dominates the community, which is evidently a successful way of recruiting in the system. On the other hand, as recruitment is ensured, the species differs in several vegetative attributes, which may enhance its competitive ability in terms of growth rates and nutrient uptake.