Background
The authors write that "plant responses to predicted global warming, elevated CO2 and precipitation changes involve complex interactions of the factors," and, therefore, they say that "continued focus on the combined impact of factors is needed to understand the directional responses of ecosystem processes," citing, in this regard, the work of Beier et al. (2004), Rustad (2006), Heimann and Reichstein (2008) and Dieleman et al. (2012).

What was done
Working in a dry heathland ecosystem on sandy soil in North Zealand (Denmark), which was dominated by the evergreen dwarf shrub Common heather (Calluna vulgaris L.) and Wavy hairgrass (Deschampsia flexuoso L.), Albert et al. studied the effects of potential changes in three environmental factors and their combined impacts on photosystem II (PSII) performance during an autumn-to-winter period. These three factors were "elevated CO2 (free-air carbon enrichment; CO2), warming (passive nighttime warming; T) and summer drought (rain-excluding curtains; D)."

What was learned
The six Danish scientists report that "neither passive nighttime warming nor elevated CO2 as single factors reduced PSII performance via incomplete cold hardening," in contradiction of what might have been expected based on the results of several prior studies. In fact, they say that "the passive nighttime warming strongly increased PSII performance, especially after freezing events; and when combined with elevated CO2, a strongly skewed positive TxCO2 interactive effect was seen," indicating, as they put it, that "these plants take advantage of the longer growing season induced by the warming in elevated CO2 until a winter frost period becomes permanent." However, they also found that if previously exposed to summer drought (D), the positive effect of TxCO2 is immediately reduced after freezing events, "causing the full combination of DxTxCO2 not to differ from control."

What it means
In a CO2-enriched and warmer world of the future, the dry heathland ecosystem studied by Albert et al. should do much better than it does currently. And if significant summer droughts occur, it should do no worse than it does at the present time.

References
Beier, C. 2004. Climate change and ecosystem function - full-scale manipulations of CO2 and temperature. New Phytologist 162: 243-245.

Dieleman, W.I.J., Vicca, S., Dijkstra, F.A., Hagedorn, F., Hovenden, M.J., Larsen, K.S., Morgan, J.A., Volder, A., Beier, C., Dukes, J.S., King, J., Leuzinger, S., Linder, S., Luo, Y., Oren, R., De Angelis, P., Tingey, D., Hoosbeek, M.R. and Janssens, I.A. 2012. Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature. Global Change Biology 18: 2681-2693.

Heimann, M. and Reichstein, M. 2008. Terrestrial ecosystem carbon dynamics and climate feedbacks. Nature 451: 289-292.

Rustad, L.E. 2006. From transient to steady state response of ecosystems to atmospheric CO2-enrichment and global climate change: conceptual challenges and need for an integrated research. Plant Ecology 182: 43-62.