What was done
The authors grew two-year-old saplings of Quercus myrsinaefolia, an evergreen broad-leaved oak species, in controlled environment chambers having various atmospheric CO2 concentrations and air temperatures for approximately one year to study the interactive effects of elevated CO2 and temperature on the development and growth of this important tree, which is widely distributed throughout Laos, Vietnam, China, Taiwan, South Korea and southwestern Japan.

What was learned
In ambient air, 3 and 5°C increases in air temperature boosted final sapling biomass by 53 and 47%, respectively.  At elevated CO2 concentrations that were either 1.5 or 2 times greater than the ambient CO2 concentration, however, the same 3 and 5°C increases in air temperature enhanced final biomass by 110 and 140%, respectively.

To partly explain this phenomenon, the authors noted that net assimilation rates in saplings exposed to the more-elevated air temperature treatment of 5°C above ambient were depressed to levels that were 70% below those exhibited by control trees exposed to ambient air.  However, when simultaneously exposed to twice-ambient concentrations of atmospheric CO2, the negative effects of warming on net assimilation rate were almost completely ameliorated.

What it means
As the air's CO2 content rises, this evergreen oak species will likely exhibit enhanced rates of photosynthesis and biomass production that will increase even more with a concomitant increase in air temperature.  Indeed, the beneficial effects of elevated CO2 and air temperature on biomass production in this species were more than additive.  Consequently, although the authors say that any global warming that might occur in the future will only "enhance the growth of Q. myrsinaefolia saplings in natural forests, and accelerate [their] succession and poleward migration," it is clear that their positive response to concomitant increases in air temperature and CO2 concentration obviates the need for any poleward migration.  Although this pleasant fact does not rule such out, neither does it mandate it.  Hence, it seems likely that poleward range boundaries may well expand, but that equatorward boundaries may be little changed.