As a consequence of climate change, plant species are expected to shift their distribution ranges northwards. Plant migrations may be facilitated or restrained by the reproductive ecology of invaders but also by suitable habitat and competition from established vegetation. Unlike grasslands in more fertile regions, infertile temperate grasslands may respond slowly to climate warming due to slow growth and long lifespan of component species. The aim of this project is to predict the capacity of Bromus erectus and Brachypodium pinnatum, two dominant grasses of calcareous grasslands in southern England, to move northwards and invade infertile grasslands in northern England under different climate change scenarios. Since 1993, the Buxton Climate Change Impacts (BCCI) study simulates winter warming in combination with summer drought in a limestone grassland in Derbyshire, UK. Seeds of B. erectus and B. pinnatum were introduced into the BCCI grassland in autumn 2008. Germination, survival, and seedling growth were monitored along a soil depth gradient in the following year. Potential reproductive barriers were tested with pollination experiments in two common garden sites with differing climatic conditions.
Results/Conclusions
The reproduction of B. erectus was limited both by climate and self-incompatibility. Nevertheless, sowing the BCCI grassland with seeds of B. erectus and B. pinnatum resulted in the successful establishment of both species in the first year of the experiment. The germination rate of both invaders was only slightly affected by climate treatments with germination rates being higher in plots with winter warming than in plots with summer drought. However, winter warming altered the timing of germination, especially in B. pinnatum. In plots with winter warming, 68.5 % of the B. pinnatum seeds germinated during winter (‘winter seedlings’), whereas only 12.9 % of the seeds germinated in the same period in the controls. 97.8 % out of 8’070 invader seedlings survived the first year, irrespective of climate treatment. At the end of the first growing season, winter seedlings had considerably higher aboveground biomass than spring seedlings, which resulted in higher average aboveground biomass of B. pinnatum seedlings in warmed plots compared to controls. Differences in aboveground biomass between warmed and non-warmed treatments were even more pronounced in B. erectus, where winter seedlings grew faster in plots with winter warming than in controls. Seedling performance after the first growing season suggests that higher winter temperatures will be more important for species invasions in infertile temperate grasslands than extended drought periods during summer.