Ah, overlooked they actually acknowledge the "cap" directly in the preceding paragraph, and even put it into "coming decades" time frame. Makes much more sense now, thanks for the pointer!
Still a bit confused about the emphasis in wood deposits in "streams" – reportedly way more effective, but I'd guess with very limited capacity to really "lock" the mass – compared to regular hummus – not that effective, but for forest with couple of centuries of growth ahead I'd guess way more capacious. Good news either way, though!
Reading between the lines in the article (which is of course always subject to incorrect interpretation) I think the reason for the focus on streams is just that nobody else has looked at that before and thus it is a factor not previously accounted for. Other sources have already been accounted for - they may be worth more than what is in streams, but it is already known so the article didn't mention them.
“Coming decades” is an understatement. It depends on local conditions but douglas fir pines in the PNW take 200-300 years to decay completely, so that’s centuries more of carbon capture as long as we let our forests rewild. Realistically a forest becomes old growth once there are at least three generations of trees in various states of decay. That may decades in warmer climates but much longer in the north.