Probably misinterpretation. They don't accumulate carbon, but this activity depletes carbon reserves, increasing capacity to accumulate carbon in the summer months.
Sadly, roots respire- meaning that depletion of the carbon reserves happens by releasing CO2 (in addition to building lignin). It's only when they photosynthesize that plants produce more O2 and absorb more CO2 than they release.
> It's only when they photosynthesize that plants produce more O2 and absorb more CO2 than they release.
And even narrower than that. I fuzzily recall seeing rainforest curves, where only for some hours around noon were trees net producers. Hmm... though perhaps that was for forest-not-just-tree?