Climate change will mediate shifts in the range sizes of plant species. In Europe, northern regions can be assumed to gain new harmful weed species in future since several noxious species are missing there currently. The assessment of the establishment risk of these species requires information on their ability to maintain populations in the new region. The present study aimed at exploring the potential success of two annual arable weeds – Amaranthus retroflexus and Echinochloa crus-galli – in Southern Finland as a consequence of climate change, and to identify the most critical life-history traits to population performance. Both species are currently common arable weeds in Central Europe and potential future weeds in Scandinavia. The population dynamics of these species were modelled with the aid of stochastic matrix population models that incorporated temporal variation in survival, growth and fecundity. All models were parameterised based on greenhouse and field experiments conducted under different temperature and competition regimes in Southern Finland 2008-2009. A comparison of population maintenance was conducted between the temperatures of the current climate and the predicted future climate (difference 3 °C) in ideal conditions without competition, and also in more realistic conditions where weeds were competing with a crop species (either maize or barley).
We found that A. retroflexus and E. crus-galli were able to form viable populations in both the current and predicted warmer climate when growing without competitors. However, competition with a crop plant considerable reduced population growth rate also in the warmer climate, with seed survival and survival to a reproductive stage being the most critical traits to population performance. Barley, which is a dominating crop species in Finland, appeared to be too strong competitor for both weed species. Maize, which is a future crop in Finland, was weaker competitor allowing A. retroflexus to establish viable populations in the current climate conditions. Increase in the temperature would enable also E. crus-galli to establish viable populations in maize. The results suggest that A. retroflexus and E. crus-galli may be able to establish and maintain viable populations in arable lands in Scandinavia in future but an increase in the temperature together with a suitable crop are required to enable population maintenance.