Main measure is what they call GPP:
>
> Gross primary productivity (GPP) quantifies the amount of carbon dioxide (CO2) fixed by plants through photosynthesis. Although as a key quantity of terrestrial ecosystems, there is a lack of high-spatial-and-temporal-resolution, real-time and observation-based GPP products.
>
> Looks like the northern latitude boreal forests, if they haven't burned down, 'could' increase their GPP by a full factor of 3X over pre-industrial levels. NASA's OCO has been observing this, and...
The 3x number isn't what's measured, however; at current (circa 1.5 C) warming, the measured plant uptake is about 1.1x over pre-industrial and if we don't go disastrously higher in warming we won't ever see the 3x or 2x change occur (in that single term of the full series of CO2 sources and sinks that adds up to a net effect).
It's not a game-changing effect being discussed.