Functional traits underlying the invasion success of IAS (Invasive Alien Species)

The issue of alien species, especially invasive aliens (IAS), is the current focus of the international scientific debate owing to the negative effects that such species have caused and continue to cause to ecosystems, economic activities and human health. In Italy, there was an increase of 96% of alien species (over 3,000) over the last thirty years. The reason for the widespread diffusion of alien species lies in some features that make them particularly competitive, such as the resistance to pests, the ability to adapt to different environmental conditions, including anthropized habitats, and the competitive superiority in gaining resources compared to native species. Climate change and biological invasions are two important drivers affecting biodiversity and ecosystem services. However, their effect on biodiversity has usually been assessed independently, despite good scientific reasons to expect the rate and extent of biological invasions to be influenced by climate change. Resident species can become increasingly poorly adapted to the local environment, whereas newcomers might be better adapted and, thus, more competitive under the new conditions. Populations of alien plants are considered more likely to survive if they are introduced to areas with climatic conditions similar to those in their native distribution range. Changes in climatic conditions resulting in a prolonged growing and reproductive period often provide alien species with exploitable opportunities. Indeed, there is increasing evidence that global warming can enable expasion into areas where the species were previously kept in check by climate. Over recent decades, an increasing number of studies have analysed patterns of biological invasions to disentangle the underlying ecophysiological processes. Under this perspective, the recent focus on the functional traits or the syndrome of traits enhancing the invasive potential of IAS, holds promise to develop predictions on which species are more likely to become successful invaders in different habitats. Functional trait ecology represents a relatively novel approach to investigate this phenomenon. Functional traits provide tools for measuring functional diversity, as they impact plant fitness via effects on growth, reproduction and survival, and reflect the trade-offs among different physiological and ecological functions. In particular, through the ecophysiological processes it is possible to understand how the different processes of growth, reproduction and distribution of IAS are influenced by the interaction with ecological factors (abiotic and biotic). Many factors may contribute to the invasion success, including seed production and germination, high relative growth rate (RGR), seedling recruitment, carbon allocation, life history characteristics of species, and susceptibility to disease and/or herbivores. The aim of the present thesis was to identify the functional traits that underline the invasion process of three widespread IAS in Italy: Ailanthus altissima (Mill.) Swingle, Robinia pseudoacacia L. and Phytolacca americana L.. In particular, the focus has been directed to three fundamental aspects, i.e., seed germination traits, seedlings growth dynamics and physiological and morphological traits of seedlings under drought condition. The focus was to understand which traits allow a better performance to IAS and if there are common strategies of considered species. The results presented through the three Chapters of the thesis provide a significant contribution to characterize ecophysiological behaviour of three important IAS to expand knowledge on functional traits underline the invasion process of IAS. In particular, the results from: - Chapter 2 demonstrate that a global temperature increase could be considerably increase germination percentages of some species and future warmer conditions could naturally break seed dormancy, extending the favourable period for seedling establishment; - Chapter 3 provide compelling evidence that certain traits, such as rapid growth, high leaf production rate and high leaf area production rate are considered intrinsic characteristics that can give species some advantages for the colonization of new sites and therefore increase their distribution; in addition, it highlights not all invasive species respond in the same way at temperature and seedling growth reflects the climatic conditions of the native habitat. The considered traits can be useful to help disentangle the trait dimensions by which IAS cope with air temperature change; - Chapter 4 explore how water stress affect the functional traits related to gas exchange in each considered species. The results shows a species dependent effect of the water stress at a physiological and morphological level as well as an interaction between species and stress duration. In particular, the study reveal the considered IAS differ mainly in their capacity to control photosynthesis through stomatal control, highlighting a different species responsiveness in coping with water stress. Overall, the present PhD Thesis by analysing the response to temperature and water availability variations at different plant life stages (i.e., germination and seedling) contribute to the scientific debate on the IAS capacity to face the current climatic changes. In particular, the results reveal that the extent of the response to environmental factor variations, differs among IAS due to different capacity to maximize their performance under limiting environmental conditions. Accordingly, the results highlight the importance of integrating physiological functional information to better understand the invasion strategies of these IAS in the perspective of planning more effective measures of their management and control.