1 Recent findings indicate that the interactions among CO₂, temperature and water can be substantial, and that the combined effects on the biological systems of several factors may not be predicted from experiments with one or a few factors. Therefore realistic multifactorial experiments involving a larger set of main factors are needed. 2 We describe a new Danish climate change‐related field scale experiment, CLIMAITE, in a heath/grassland ecosystem. CLIMAITE is a full factorial combination of elevated CO₂, elevated temperature and prolonged summer drought. The manipulations are intended to mimic anticipated major environmental changes at the site by year 2075 as closely as possible. The impacts on ecosystem processes and functioning (at ecophysiological levels, through responses by individuals and communities to ecosystem‐level responses) are investigated simultaneously. 3 The increase of [CO₂] closely corresponds with the scenarios for year 2075, while the warming treatment is at the lower end of the predictions and seems to be the most difficult treatment to increase without unwanted side effects on the other variables. The drought treatment follows predictions of increased frequency of drought periods in summer. The combination of the treatments does not create new unwanted side effects on the treatments relative to the treatments alone.

Global change factors affect plant carbon uptake in concert. In order to investigate the response directions and potential interactive effects, and to understand the underlying mechanisms, multifactor experiments are needed. The focus of this study was on the photosynthetic response to elevated CO₂ [CO2; free air CO₂ enrichment (FACE)], drought (D; water-excluding curtains), and night-time warming (T; infrared-reflective curtains) in a temperate heath. A/C_i curves were measured, allowing analysis of light-saturated net photosynthesis (P_n), light- and CO₂-saturated net photosynthesis (P_max), stomatal conductance (g_s), the maximal rate of Rubisco carboxylation (V_cmax), and the maximal rate of ribulose bisphosphate (RuBP) regeneration (J_max) along with leaf δ¹³C, and carbon and nitrogen concentration on a monthly basis in the grass Deschampsia flexuosa. Seasonal drought reduced P_n via g_s, but severe (experimental) drought decreased P_n via a reduction in photosynthetic capacity (P_max, J_max, and V_cmax). The effects were completely reversed by rewetting and stimulated P_n via photosynthetic capacity stimulation. Warming increased early and late season P_n via higher P_max and J_max. Elevated CO₂ did not decrease g_s, but stimulated P_n via increased C_i. The T×CO2 synergistically increased plant carbon uptake via photosynthetic capacity up-regulation in early season and by better access to water after rewetting. The effects of the combination of drought and elevated CO₂ depended on soil water availability, with additive effects when the soil water content was low and D×CO2 synergistic stimulation of P_n after rewetting. The photosynthetic responses appeared to be highly influenced by growth pattern. The grass has opportunistic water consumption, and a biphasic growth pattern allowing for leaf dieback at low soil water availability followed by rapid re-growth of active leaves when rewetted and possibly a large resource allocation capability mediated by the rhizome. This growth characteristic allowed for the photosynthetic capacity up-regulations that mediated the T×CO2 and D×CO2 synergistic effects on photosynthesis. These are clearly advantageous characteristics when exposed to climate changes. In conclusion, after 1 year of experimentation, the limitations by low soil water availability and stimulation in early and late season by warming clearly structure and interact with the photosynthetic response to elevated CO₂ in this grassland species.