Ecological drivers at the warm edge of species’ distribution

Why are species not evenly distributed all over the globe but are restricted to certain areas? This question has fascinated biologists for a long time. By studying ecology and evolution, biologists try to understand the limits in species geographic distribution and the restriction of adaptation to environmental stresses. Past research has shown that the low latitudinal range edges are often characterised by abiotic environmental factors. The aim of my thesis was to systematically study two pivotal environmental factors, temperature and precipitation, of the model species, Arabidopsis lyrata. In a transplant experiment, I showed that the increase in temperature and variability of precipitation patterns, caused by climate change, negatively affect plant performance. Specifically, germination and flowering success, as well as survival rates were reduced. A greenhouse experiment focusing on the effects of heat, drought, and their combination was conducted to study the adaptation strategies to the different stresses. Even though exposure to heat or drought alone did not affect survival rates much, the combination of both stresses caused a high mortality rate. Populations originating at the warm range edge displayed a slightly better performance and plants had a higher specific leaf area and root-to-shoot ratio. However, plant performance is also dependent on the interaction with the rhizosphere microbiome. Therefore, I analysed how different plant traits correlate with microorganisms under different environmental conditions. I compared the correlations of plant traits with different primary root exudate compounds and rhizosphere bacteria and fungi under different watering treatments. The composition of root exudates and bacteria changed significantly between a moist and a drought treatment. Essentially, more high positive correlations were found under drought, confirming the importance of plant-microbe interactions under stress conditions. In summary, my thesis emphasises the threat of global warming to population persistence at the warm range edge, even if populations originating at the warm edge are adapted to a certain degree. With ongoing climate change, the range optima are likely to shift northwards, despite the beneficial interactions in the rhizosphere.