There are a number of reasons for the significant variation in numbers. Some are down to misunderstanding and some are genuine variation based on climate, species, weather etc.
First the common misunderstandings:
- Carbon dioxide (CO2) weighs 3.67 times as much as pure carbon © because CO2 has a couple of oxygen atoms extra. https://thinkprogress.org/the-biggest-source-of-mistakes-c-vs-co2-c0b077313b/
- Trees take in carbon dioxide but then convert this to store the carbon in more complex chemical structures so mass of carbon is normally what is measured in scientific papers (such as the one you referred to).
- Some carbon offset schemes quote the total amount of carbon dioxide absorbed throughout the trees lifetime of several decades wheras a more important number is the amount absorbed per year.
Referring specifically to the study the value of 149 tonnes is the total mass of carbon currently present not the mass of CO2 absorbed over the 2 year period of the study.
As for the articles about tree planting not being the answer, what the study actually says is ”Although we find that this strategy of sequestering carbon is not a viable alternative to aggressive emission reductions, it could still support mitigation efforts if sustainably managed."
Basically, it’s not a magic bullet that would allow us to carry on with modern life as usual, but that’s not to say we shouldn’t do it for other reasons. Even if we stopped emitting all CO2 tomorrow there’s still an excess of CO2 in the atmosphere which would need to be absorbed to bring the ppm down to pre industrial levels to reverse the damage we’ve caused.
To answer the title question, 0.5kg CO2 /m2/year is similar to numbers I’ve found elsewhere. (I’ll post my findings and further references if I get time)