Background
The authors note that a warming of the climate may cause plants to come out of winter dormancy earlier in the spring, and that "because physiologically active plants are more vulnerable than dormant plants to a transient low-temperature event (Larcher, 1995), earlier acclimation to warmer temperatures and onset of photosynthetic activity ... may make plants more susceptible to damage [caused by temporary but extreme cold weather] that reduces overall functional leaf surface area and productivity."

What was done
Loik et al. tested this hypothesis for specimens of the evergreen shrub Artemisia tridentata and the sub-alpine herbaceous Erythronium grandiflorum that had been growing at 3050 meters in the Rocky Mountains of Colorado, USA, "where overhead infrared heaters have continually simulated atmospheric forcing of the future (Harte et al., 1995)," enhancing the downward infrared radiation flux by 22 Wm^-2.  This they did by comparing photosynthetic responses of the plants before, during and after an imposed in situ freezing event, where air and leaf temperatures of selected plants were "continually decreased, as would occur during a natural episodic freezing event."  This environmental manipulation started at an ambient temperature of about 10°C and adjusted to -30°C over a period of 10 hours, mimicking what occurs naturally at this site about once every ten years.

What was learned
The authors state that "the infrared warming treatment did not [our italics] reduce the ability of the subalpine herbaceous geophyte Erythronium grandiflorum to tolerate in situ freezing, in contrast to our hypothesis."  In addition, they report that the warming treatment actually "led to a significant increase [our italics] in photosynthetic tolerance of an experimentally imposed in situ freezing event for the Great Basin Desert evergreen shrub Artemisia tridentata ... in contrast to our hypothesis."

As for the reasons for these positive responses, Loik et al. suggest that "enhanced tolerance of episodic freezing is a result of enhanced physiological activity, in particular, higher plant water potentials and photosynthetic gas exchange caused by the infrared treatment effects on soil water content and soil temperature," leading to "an increased level of photosynthates available for allocation to cryoprotection or other acclimation mechanisms (Iba, 2002)."

What it means
The authors conclude, contrary to prior expectations, that some species of plants will not be negatively affected by episodic freezing events in a warmer world, while others "may exhibit enhanced tolerance of subzero air temperatures under a future warmer climate."

References
Harte, J., Torn, M.S., Chang, F., Feifarek, B., Kinzig, A.P., Shaw, R. and Shen, K.  1995.  Global warming and soil microclimate: results from a meadow-warming experiment.  Ecological Applications 5: 132-150.

Iba, K.  2002.  Acclimative response to temperature stress in higher plants: Approaches of gene engineering for temperature tolerance.  Annual Review of Plant Biology 53: 225-245.

Larcher, W.  1995.  Physiological Plant Ecology.  Springer-Verlag, New York, New York, USA.