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Do extreme climatic events facilitate plant invasions?

Periodic Reporting for period 1 - EXTREME (Do extreme climatic events facilitate plant invasions?)

Reporting period: 2016-04-01 to 2018-03-31

EXTREME aimed to explore how climate change affects plant invasions, specifically focusing on the role of increased climatic variability and extreme events. Extreme events are projected to increase in magnitude and frequency with ongoing global warming. Recent studies have shown that global warming influences the probability of species invasions, enabling alien species to expand into regions in which they previously could not survive and reproduce. However, we have little understanding to what extend increased climatic variability and extreme events may promote species invasions. The overall objective of EXTREME was to increase this understanding. We focused on drylands which are predicted to be highly sensitive to future climate change, especially to changes in rainfall events. Drylands are also among the systems where the impact of invasions is predicted to rapidly increase over the coming century. They occupy 40% of the global land area and harbour nearly 2 billion people in the poorest regions of the world. Understanding how biological invasions in these systems will be facilitated by increased climatic variability and extreme events will be essential for our ability to mitigate future impacts on ecosystems as well as on people’s livelihoods.
We reviewed the current state-of-the-art of the effect of extreme events on plant invasions and initiated a project on climate-vegetation feedbacks. Climate change does not only affect plants, but plants affect climate as well in a set of intricate feedbacks. These feedbacks can be related to carbon, for example plants take up CO2 from the atmosphere thereby reducing atmospheric CO2 levels, so-called carbon sequestration. But climate-vegetation feedbacks can also be related to biophysical processes, such as hydrology and radiation. We reviewed the relative importance of these different climate-vegetation feedbacks in drylands. Secondly, we developed experiments testing the response of plant to climate change. We studied the role of biotic resistance under different water availabilities. We tested how alien invasive plants responded to drought in a South African savanna reserve and we developed a long-term experiment in Afromontane grasslands testing the effect of experimental warming and fire on grassland vegetation.
Our study on the role of biotic resistance to invasion was published in 2018. We studied how native grassland communities affected growth, reproduction and survival during the adult life-stage of the high-impact woody invader Chromolaena odorata under different water availability treatments. We found that savanna grasslands have a strong competitive effect on invasion by the exotic woody shrub C. odorata, reducing growth and reproduction, but not survival. We found no evidence that biotic resistance was stronger under more unfavourable abiotic conditions, as C. odorata was equally impacted in all water treatments. The high survival rates of C. odorata suggest that competitive interactions are not likely to prevent invader persistence in the landscape. Invader persistence is important in determining longer term invasion success as well as invader impact and the concept of persistence should not be overlooked in studies on invasive species.

Our project on climate-vegetation feedbacks in African grasslands and savannas shows that these systems provide important benefits for climate regulation that are often overlooked, but could have large implications for successful climate change mitigation. These include their radiative properties, high infiltration rates and high soil carbon pools, among others. Current climate change mitigation policies have a strong focus on tree planting and large afforestation programs are currently unrolled over Africa. We draw parallels between the tundra biome and these tropical grassy biomes to argue that afforesting grasslands will not reduce global warming. Moreover, it would have devastating effects on biodiversity and counteract mitigation efforts. We discuss the different climate benefits of tropical grassy biomes and why we need to consider their importance for climate change mitigation, not only in terms of biogeochemical feedbacks (i.e. carbon storage), but also in terms of biophysical feedbacks (i.e. hydrological processes, longwave radiation), taking into consideration fire and herbivory as important and essential drivers of these systems.

The final results of these projects will contribute to Europe’s knowledge-based society by providing important knowledge on climate change and invasive species that is of the highest priority in the EU.
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