Atmospheric CO2 concentrations and temperatures are rising and precipitation regimes are changing at global scale. How ecosystem will be affected by global climatic change is dependent on the responses of plants and plant communities. This thesis focuses on how climate change affects heathland plant communities. Many heathlands have shifted from dwarf shrub dominance to grass dominance and climatic change might affect the competitive balance between dwarf shrubs and grasses. We looked at heathland vegetation dynamics and heathland plant responses to climatic change at different spatial and temporal scales by combining results from 1) a study on long term vegetation dynamics at an unmanaged Danish heathland; 2) a study on competitive interactions between, and coexistence of, two of the most common heathland species (the dwarf shrub Calluna vulgaris and the grass Deschampsia flexuosa); 3) a study on the effects of elevated atmospheric CO2-concentration, warming and drought on the photosynthetic capacity and phenology of C. vulgaris and D. flexuosa in an outdoor climate change experiment on a grassy heathland in Denmark; 4) a study on climate change impacts on the competitive interactions between C. vulgaris and D. flexuosa in the same climate change experiment and 5) a study where we compared the responses of shrubland plant communities to experimental warming and recurrent experimental droughts in seven climate change experiments across Europe.
Heathland vegetation dynamics are slow and both grass and dwarf shrub dominated plant communities can be relatively stable. C. vulgaris and D. flexuosa differ in their responses to experimental climate change. D. flexuosa down-regulates photosynthetic capacity under elevated CO2, can opportunistically green up earlier in warmer springs and flexibly reduces its green biomass under drought conditions. C. vulgaris is less flexible and hardly adjusts photosynthetic capacity or green biomass to drought or warming. Despite these differential responses, competitive interactions were robust. C. vulgaris, in the building phase, outcompetes D. flexuosa under ambient conditions and this remained so for all climatic treatments, although experimental drought tended to reduce C. vulgaris carrying capacity biomass by about 25% and slowed down the process of competitive exclusion. Although C. vulgaris in the building phase is fairly resistant to climatic change, its recovery after disturbance is severely hampered by drought and a combination of drought and disturbance can cause a shift from dwarf shrub to grass dominance. Comparison of multiple European climate change experiments show that plant communities are more vulnerable to climate change during early successional stages where climatic change can lead to shifts in plant community composition. The impacts of climate change on plant communities is likely to depend more on short periods where plants are relatively vulnerable (like regeneration phases after disturbance) than on the longer periods between disturbances where established vegetation is relatively robust. We suggest that future climate change experiments deliberately include disturbances and focus more on life phases where plants are most vulnerable.