That twitter link talks about other complex effects, so let's focus on the PBS video. I'll have to find out more about the research, but just from the video things don't make sense.
If a young forest is a net carbon source, they question remains: where does that carbon come from? Was there carbon stored in the soil that is released? If not, the carbon balance doesn't make any sense. As with anything, change in mass over time equals flow rate in minus flow rate out. If biomass increases, stored carbon increases as well, so flow rate in is larger than flow rate out. That means we have a carbon sink.
I don't understand the research results for mature forests either. How can they be a carbon sink when the amount of biomass is constant?
Something doesn't add up. It could be my understanding of things. I'm going to try and find some time to delve deeper in the research.
Edit: it looks like the video mainly talks about disturbed forests, where trees are planted again to regrow the forest. It's not explicitly mentioned though, just silently assumed, and that creates lots of unnecessary confusion and misunderstanding. A disturbed forest likely still has relatively large amounts of biomass in the soil, that get released. That is not what I call a young forest though, which is what we were talking about here. A young forest doesn't have a lot of pre-existing biomass.
If a young forest is a net carbon source, they question remains: where does that carbon come from? Was there carbon stored in the soil that is released? If not, the carbon balance doesn't make any sense. As with anything, change in mass over time equals flow rate in minus flow rate out. If biomass increases, stored carbon increases as well, so flow rate in is larger than flow rate out. That means we have a carbon sink.
I don't understand the research results for mature forests either. How can they be a carbon sink when the amount of biomass is constant?
Something doesn't add up. It could be my understanding of things. I'm going to try and find some time to delve deeper in the research.
Edit: it looks like the video mainly talks about disturbed forests, where trees are planted again to regrow the forest. It's not explicitly mentioned though, just silently assumed, and that creates lots of unnecessary confusion and misunderstanding. A disturbed forest likely still has relatively large amounts of biomass in the soil, that get released. That is not what I call a young forest though, which is what we were talking about here. A young forest doesn't have a lot of pre-existing biomass.