What was done
The team of eight Nordic scientists used a boreal version of a process-based simulation model (BIOMASS) to quantify the individual and combined effects of elevated air temperature (2 and 4°C above ambient) and CO2 concentration (350 ppm above ambient) on the net primary production (NPP) of both coniferous (Pinus sylvestris, Picea abies) and deciduous broad-leaf (Fagus sylvatica, Populus trichocarpa) forests growing in Denmark, Finland, Iceland, Norway and Sweden.

What was learned
For three of the four species (P. sylvestris, P. abies, P. trichocarpa), air temperature increases of 2 and 4°C led to mean NPP increases of 11 and 20%, respectively; while for the other species (F. sylvatica) there were corresponding 21 and 48% decreases in NPP.  However, when the air's CO2 concentration was simultaneously increased from 350 to 700 ppm, the corresponding mean NPP increases of the three species rose to 41 and 55%; while the NPP of F. sylvatica jumped from -21 and -48% to +37 and +10%.  Last of all, when the air's CO2 content was doubled at the prevailing ambient temperature, the mean value of the NPP of the three species rose by 27%, while that of F. sylvatica rose by 58%.

What it means
As the air's CO2 content continues to climb, the major tree species of Denmark, Finland, Iceland, Norway and Sweden should grow ever more productively; and if air temperature also rises, most of them will grow better still.