From seed to seedlingExploring the early life-cycle stages in alien conifer species

Abstract

Invasion biology is a relatively new discipline that has increased the attention of ecologists due to the current human-mediated biodiversity crisis as a consequence of human trade worldwide. Two key issues it deals with are whether some species are inherently equipped to become invasive when moved to new areas and whether there are differences in the vulnerability of different habitats to be invaded. In this vein, ecological factors affecting the early life-cycle stages are crucial to understand the establishment success/failure of alien plants. Conifers have been a study model of invasiveness research, but relatively few studies have addressed the ecological barriers from seed to seedling that limit their establishment and whether these barriers are site specific. The general aims of this thesis are to provide information about the current invasion status of alien conifers in Europe; to screen which early life-cycle stage limits the establishment of alien conifers and to compare differences in natural enemy resistance among native and alien conifer species. As a first approach we reviewed the current invasion status of alien conifers in Europe through a literature review (Chapter 2). Then, we conducted field and greenhouse experiments to compare the early life-cycle stages between native and alien conifers in Europe to reveal which stages and which ecological processes might act as barriers against plant establishment (Chapter 3, 4 & 5). Seed mass, germination, seedling survival and seedling chemical defenses were considered as the main factors influencing the success of the early plant life-cycle. While processes that act as a main barrier in the introduced area were native fauna, such as seed removers (Chapter 3 & 4), or seedling attacks by native generalist insect herbivores (Chapter 5). In summary, our literature review illustrates there is little evidence of invasion by alien conifers in Europe. The low introduction effort of alien conifers, long lag-time from plantation to establishment that is necessary, and the phylogenetic proximity between alien and native conifers are possible reasons for their low expansion in Europe to date. The relatedness between alien and native species can result in high colonization rates of natural enemies on alien trees. In our experimental approaches, we have elucidated some processes that affect the establishment phase of alien conifers in Europe. On overall, our results reinforce the hypothesis that native generalist enemies act as a barrier against the establishment of alien conifers in the introduced range (Biotic Resistance Hypothesis) where coexist with congener conifer species, and this might be one reason why there is a low incidence of invasion by these aliens in Europe.