Trees are nature's best solution after millions of years of evolution. The R&D cost is low as well as the complexity. The problem is that dead trees turn back into CO2 so there will need to be some kind of sequestration like burying lumber.
Trees did not evolve solely for the purposes of carbon sequestration. Similarly, it would be silly to say that mammals are nature's best solution at sequestering oxygen.
Even burying the lumber doesn't remove the coal from the cycle, just delays it by a few years. You'd need to get the carbon into a "not readily accessible to biological processes" form. Charcoal is probably an OK start, but something like calcium carbonate would be even more stable.
And you can (probably) do better than trees, in terms of carbon captured per time unit. But I guess you'd have to run comparative studies between birch and bamboo for biomass grown per time unit and area.
Hm, that means that a sensible unit for carbon capture would be "kg / (m * m * s)".
There's always some local maximum most efficient process. If you take into account process inputs trees would likely come out on top. A lot of carbon capture tech can capture lots of carbon but consumes a huge amount of power/energy intense resources. My unit would be kg / (W * m^2 * s)
IIRC, bamboo ends up as "less wasted" when you turn it into charcoal than most woods. And you'd need to do a lot more energy-intensive processing of the tree to get it into a for suitable for turning into coal. With bamboo, it's mostly "cut into suitable length". With wood, it's at least "cut into suitable length", but may also require splitting.
But, yes, my main reason for thinking bamboo is that it is pretty fast-growing under the right conditions.
And with a Watt actually being "kg * m^2 / s^3", your unit ends up being s^2 / m^4, which dimensionally looks less pleasant.
