Manipulations

The three factors CO₂, temperature and precipitation will be manipulated in the field in a full factorial design. Each study plot will be 4 m2 and all treatments will be replicated 6 times (48 experimental plots in total) and established randomly at the site. The experimental treatments will be started in year 2 of the project leaving a full growing season for pre-treatment measurements, installation and testing of experimental techniques. The treatments are:

• Reference (R) - Untreated reference plots for assessment of treatment effects. The reference plots will be established similar to all treatment plots including “dummies” of heaters, CO₂ dozers etc. in order to make sheltering and shadow effects similar.
• CO₂-enrichment (C) - Free Air CO₂ Enrichment (FACE) experiment. The study plots will be exposed to elevated CO₂ concentrations in the air through fumigation. The target CO₂ concentration will be 510 ppm. The CO₂ will be added to the plots via a mini/mid FACE (Free-Air CO₂ Enrichment) system (e.g. Miglietta 1996 and Miglietta 1997) Concentrated CO₂ will be released from tubes surrounding the plots. The release will be controlled via a data-logger/CO₂ monitor feedback system. The target CO₂ concentration of 510 ppm shall be kept in the centre of the plot. To minimize the CO₂ consumption only tubes situated in the up-wind direction will release CO₂. Fumigations will only be done during daytime and fumigation will be reduced or interrupted during episodes with high wind speeds.
• Temperature increase (T) – The study plots will be heated from above by IR lamps (e.g. Shaw et al. 2002) or a passive night time warming system (IR reflective curtains e.g. Beier et al 2000) to increase the nighttimes temperature in the air and topsoil layer by c. 2 °C. The IR lamps or reflective curtains will be chosen as they imply only minimal artefacts on light, water and wind conditions compared to most other systems (e.g. Beier et al., 2000). However, IR lamps may alter the energy balance and accelerate the evapotranspiration. Specific design of the heating system will be tested in the field and assessed in terms of heating efficiency, optimal lamp size and potential artefacts.
• Precipitation (W) - Study plots are exposed to changes in precipitation patterns. 10-20% of the precipitation will be removed during the summer (July-September) by automatically/manually operated plastic covers allowing prolonged summer droughts to occur. A similar amount of additional precipitation will be applied manually during the winter in 15 mm rain events (Dec-Feb). Precipitation for reapplication will be collected at the site shortly before application.
• CO₂-enrichment and Temperature increase (CT) – combination of the above C and T treatments
• CO₂-enrichment and Precipitation (CW) – combination of the above C and W treatments
• Temperature increase and Precipitation (TW)– combination of the above T and W treatments
• CO₂-enrichment, Temperature increase and Precipitation (CWT)– combination of the above C, T and W treatments.