What was done
The authors grew four-year-old oak trees (Quercus robur) in pots placed within greenhouses receiving ambient and twice-ambient atmospheric CO2 concentrations in combination with ambient and elevated (ambient plus 3°C) air temperatures for about one year to study the interactive effects of elevated CO2 and temperature on leaf nutritional quality.

What was learned
Elevated air temperature significantly reduced leaf palatability.  Oak leaf toughness increased as a consequence of temperature-induced increases in condensed tannin concentrations; and the elevated air temperature significantly reduced leaf nitrogen content.  Elevated CO2, on the other hand, had only minor and contrasting effects on leaf palatability: a temporary increase in foliar phenolic concentrations and decreases in leaf nitrogen content and leaf toughness.

What it means
As the CO2 content of the air continues to increase, it will likely have less of an impact on the nutritional and palatable qualities of oak leaves than would an increase in air temperature.  As described by the authors, "a 3°C rise in temperature might be expected to result in prolonged larval development, increased food consumption, and reduced growth" for herbivores feeding upon oak leaves, while elevated CO2 "is not expected to affect nutritional quality of foliage for spring-feeding larvae."  Thus, increasing CO2 concentrations should have little, if any, impact on herbivores and their feeding patterns with regard to oak leaves.