The effects of future climate change on physiological traits and competitive abilities of invasive riparian plants Impatiens glandulifera and Heracleum mantegazzianum

Abstract

Over the last few centuries, human activity has greatly contributed to different forms of global change, such as climate change and biological invasions, with great consequences for biodiversity and ecosystem services. The interactions between these factors need to be taken into consideration more carefully in order to better predict and mitigate negative impacts. Many recent studies have speculated about the possible effects of climate change on plant invasions. Riparian habitats have been repeatedly described in the literature as being highly susceptible to invasions by non-indigenous plant species. Due to the dynamic nature of rivers, it is likely that river systems will be very responsive to changing conditions, such as water temperature and altered hydrology. This thesis focuses on two of the most well-known and widespread riparian invasive species in Europe, Impatiens glandulifera Royle and Heracleum mantegazzianum Sommier & Levier.

I investigated the ways in which the performance and competitive abilities of these species might be affected by increased temperatures and climate-induced changes in soil moisture, respectively, as well as the future distributions of these species in Europe. Firstly, increased temperatures directly impacted the physiological and life-history traits of both invasive and native plants and, thus, competition outcomes for the invaded plant communities. Results suggested that warming might hinder invasion success in I. glandulifera, by quickening its life cycle and increasing growth of stems at the expense of biomass accumulation in roots and leaves, as well as by promoting the suppression of invader root growth by native species. Secondly, soil moisture seemed to have clear effects on the competition between H. mantegazzianum and co-occurring native species. Changes in soil moisture affected native species to a greater extent than the invader, increasing the community’s susceptibility to invasion. Notably, excessive soil moisture experienced at the beginning of the growth season favoured invasion by H. mantegazzianum through reducing the native community’s biomass, while simultaneously having little effect on the invader’s biomass. However, drought conditions inhibited the growth and competitive abilities of H. mantegazzianum as well, even when biotic resistance from the native community was also low. Finally, species distribution models revealed that future warming could reduce the ranges of these species in Europe, as they shift northwards to Scandinavia and to higher altitudes. Nevertheless, as observations about competitive outcomes between invasive and co-occurring native species are highly context-dependent and can vary depending on the scale of the study and the species involved, it is crucial to examine multiple facets of these complex relationships. Riparian invasions could interact with other forms of global change in the near future in complex, species-specific and scale-dependent ways which need to be targeted by future researchers.