What was done
The authors grew water horsetail (Equisetum fluviatile L.) plants at normal and doubled concentrations of atmospheric CO2 and two different air temperatures to study the interactive effects of these variables on the growth of this emergent aquatic species.  Although plants were grown for three years, they were only subjected to twice-ambient CO2 concentrations for approximately five months each year.

What was learned
The effects of elevated CO2 on plant growth were minor in comparison to the effects of elevated air temperature (+3°C).  Elevated air temperature, for example, significantly increased maximum shoot biomass by 60%, while elevated CO2 had no significant effects on this parameter.  However, elevated CO2 and temperature each significantly impacted root growth, as they did in combination.  Root biomass was enhanced by 10, 15 and 25% by elevated air temperature, elevated CO2, and the combination of these two experimental treatments, respectively.

What it means
As the air's CO2 concentration increases, water horsetail plants will likely respond by increasing their root biomass and possibly transferring additional carbon to the sediments in which they are rooted.  If air temperatures also rise in the future, this species will likely display increases in both root and shoot biomass.  Thus, this aquatic plant will likely grow faster and produce more total biomass under future atmospheric conditions of elevated CO2 and possibly higher air temperatures.