What was done
The authors conducted a meta-analysis of the results of 45 area-based dark respiration (Rda) and 44 mass-based dark respiration (Rdm) assessments of the effects of an approximate doubling of the air's CO2 content on 33 species of plants derived from 37 scientific publications.

What was learned
With respect to area-based dark respiration, the mean leaf Rda of herbaceous plants was significantly higher (+29%, P < 0.01) at elevated CO2 than at ambient CO2, while the mean Rda of woody plants was unaffected by elevated CO2.  When the herbaceous plant studies were separated into groups that had experimental durations either shorter or longer than 60 days, however, it was found that the short-term studies exhibited a mean Rda increase of 51% (P < 0.05), while the long-term studies exhibited no effect.  Hence, for conditions of continuous atmospheric CO2 enrichment, both herbaceous and woody plants would likely experience no change in leaf Rda.

With respect to mass-based dark respiration, there was no difference between the mean leaf Rdm responses of herbaceous and woody plants to elevated CO2.  There was, however, a time effect.  Plants exposed to elevated CO2 for < 100 days "showed significantly less of a reduction in leaf Rdm due to CO2 enrichment (-12%) than did plants exposed for longer periods (-35%, P < 0.01)."  Hence, for conditions of continuous atmospheric CO2 enrichment, both herbaceous and woody plants would likely experience an approximate 35% decrease in leaf Rdm.

What it means
On a per-unit-mass basis, the results of this study suggest that most plants will likely expend about 35% less energy maintaining vital life functions during the night in a doubled-CO2 world of the future.  This phenomenon will likely prove a real benefit for the biosphere.