Plant invasions in high-UV-B environments : Patterns, mechanisms and projections in the context of Global Change
Progressive globalization and continuing human-mediated transports of plant material over long distances facilitated the introduction of a large number of non-native plant species all over the world. The resulting successful plant invasions might have substantial negative impacts on native natural ecosystems and cultural landscapes and, therefore, imply significant ecological and economic harms. To better predict and limit these consequences for humans and ecosystems, a deeper understanding of mechanisms underlying plant invasions is important. Thus, one central question of invasion ecology research refers to traits and conditions that facilitate plant invasion success of species. Environmental matching of non-native species with novel habitats is known to be strongly dependent on macroclimatic conditions as they predominantly determine the species’ physiological niche. Especially temperature and precipitation have been previously discussed as most important abiotic filters for species distribution. Radiation, however, displays a so far neglected abiotic environmental factor in the context of plant invasions, although fundamental differences occur at a global scale and plant metabolism strongly relies on light conditions. Nevertheless, biologically active high-energy UV-B radiation might even have negative effects on plant growth and development and, thus, might act as limiting environmental factor during plant invasions. Local surface UV-B radiation intensities mainly depend on elevation, latitude, daytime and season. Beside the seasonal differences between hemispheres, higher maximum and annual mean UV-B intensities are measured in comparable latitudes of the southern hemisphere. This difference results from the elliptical Earth’s orbit around the sun leading to a smaller Sun-Earth distance during the southern hemisphere summer. Several hotspots of plant invasions are located on the southern hemisphere and consequently offer suitable conditions to investigate plant invasions in high UV-B environments. Moreover, UV-B radiation intensity is also affected by human impact and ongoing climate change that will further change local UV-B levels worldwide. This thesis investigates the importance of high UV-B radiation for plant invasions based on 27 invasive species of New Zealand grasslands. The conducted studies contain common garden experiments comparing native and invasive origins in different UV-B environments, as well as macroecological approaches including species distribution data, trait data and global UV-B satellite data. All approaches aimed at assessing the importance of UV-B as selective force during plant invasions and tested for potential UV-B preadaptation of native population, as well as local adaptation of invasive populations as consequence of evolutionary processes in the invaded range. The results revealed no evidence for genetic post-introduction adaptation to UV-B in high-UV-B environments. However, both origins of the study species responded to UV-B with quantifiable physiological and phenotypic changes that hint at a pronounced plasticity maintaining plant growth in response to UV-B. Moreover, higher UV-B intensities in the native range turned out to be of advantage for alien species under UV-B exposure in novel habitats and indicate an existing UV-B preadaptation of some study species by previous UV-B experience. There was only a medium directly limiting effect of UV-B radiation in comparison to other common environmental stressors of grasslands, e.g. drought. Nevertheless, greater importance of UV-B during plant invasions might originate from the ability to induce physiological crossprotection to oxidative stress caused by other biotic and abiotic environmental factors. This effect constitutes the relevance of UV-B for invasion processes in the context of predicted global and climate change. Therefore, consideration of UV-B radiation in future species distribution models might be especially recommendable for predictions of potential suitable habitats and associated risk assessment.
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