Regional- and Local-scale Drivers of Establishment and Invasion Success of Alien Plants

Abstract

Due to intensive human activities across the globe, a vast number of alien plants are introduced to new biogeographical areas. A major theme in ecology is to understand why only some of them manage to become established and further invasive in new ranges, where they threaten biodiversity and ecosystem functioning. This is a challenging theme because plant invasion is a complex process occurring across ecological scales at different invasion stages. In this thesis, I explored both regional- and local-scale drivers of alien plants establishment and invasion success using dataset analysis, and common-garden and greenhouse experiments.First, I explored how regional-scale establishment success and spread of 449 Chinese woody species in Europe were interactively associated with a broad range of factors. I found that the establishment success and spread significantly increased with residence time (only in spread), planting frequency and climatic suitability. Although none of species traits had significant main effects on the establishment, having a longer flowering period and compound leaves favored the spread of established species. The role of some traits (fruiting duration, leaf retention and leaf type) in the establishment and/or spread was dependent on other factors (residence time, planting frequency, climatic suitability and other species traits). These findings explicitly show how the role of some factors in the invasion process is context-dependent.Second, I explored whether the ability of alien woody plants to cope with shading explained their regional-scale establishment success. Overall, the established and non-established species did not differ greatly in biomass production, morphological characteristics and CO2 assimilation, across five light intensities (from 100% to 7% of ambient light). However, the established species grew taller and reduced total leaf area more than the non-established ones in response to shading. Furthermore, the established species maintained a greater low-light CO2-assimilation capacity under shading. Overall, these differences were small, and therefore I conclude that the response to shading is not important for the successful establishment of alien woody plants.Last, I explored local-scale direct and indirect interactions among alien and native plant species. In particular, I tested whether a native species that is phylogenetically closely related rather than distantly related to an alien species can directly suppress the alien, and as a consequence can indirectly reduce competitive effects of the alien on other co-occurring native species. Overall, competition was more severe between more closely-related alien species and native species, although this pattern was partly dependent on the regional-scale commonness of the alien. However, the presence of distantly-related rather than closely-related native species indirectly offset strong competitive effects of alien species on target native species. These interactions were explained by functional traits (e.g. plant height, SLA, leaf area ratio, root length ratio and shoot weight ratio) rather than by phylogenetic/functional distances of the interacting species. This implies that functional traits could help to disentangle complex interactions among plants.To sum up, my thesis explored different mechanisms across ecological scales in the process of plant invasions. My findings may shed lights on the future studies to systematically integrate regional-scale processes (by considering complex interactions among various factors) with local-scale processes, such as adaptation to local environments and species interactions to ultimately explain and predict success of alien plants outside their native ranges.