https://www3.epa.gov/climatechange/Downloads/method-calculat... page 8
There is a maximum rate somewhere in between, in an age that varies by orders of magnitude for different species. It's pretty meaningless to talk about young or old trees without context, except on that general way in that trees get old at some point.
Allocate some large area of ground for planting fast-growing trees. Dig a large and deep hole in the middle; something like 20 meters in diameter x 500 meters in depth. Let the trees grow; when they're mature, cut them down, throw into the hole, and throw a bunch of dirt (from the mound you made by digging the hole) behind them. Plant new trees, rinse and repeat until the hole is full, then dig another one somewhere near. Would that even make sense as a carbon sequestration facility?
One scheme that actually would work is to convert the wood to coal, then burying it. That could be done without adding any energy by using old-fashioned methods for charcoal burning.
https://en.m.wikipedia.org/wiki/Charcoal_burner
Because charcoal is chemically inert, the buried charcoal should stay put for up to a few hundred years,rather than a couple of decades for the decomposing wood.
Only if there is water. Bury it deep enough, or protect it from water (i.e. like a landfill).
500m high is another level. If it took, say, 1000 years for them to decompose fully and release their CO2 in to the atmosphere, that's still potentially useful even if its technically net zero.
We really only need to get past the next 100 years.
That said, you aren't going to make a considerably dent in the CO2 emitted in a given time period doing this. Used in combination with dozens of other things though...
https://www.permaculture.co.uk/articles/many-benefits-hugelk...
This does not seem logical. Two cars emit more C02 than one car. Two leaves will absorb more CO2 than one leaf. A 2m young tree will never be able to consum so many CO2 as its older 40m high grandparent with a x10000 mass (mass that must be maintained).
Trees don't absorb CO2 just by existing- they absorb CO2 by growing. The carbon from the air is converted into structure, body of the tree. When the tree stops growing, the CO2 stops being absorbed.
Cutting down the tree and using the wood for long-term storage contains all that CO2. Planting new trees means growing trees that absorb more CO2 from the air.
Some hard nuts remains have been found after 3000 years. They will survive for as many time than wood or longer. A big walnut can produce 160 Kg of nuts each year. A sequoia can produce a number of pine cones that I can't quantify, but would be probably measurable in thousands of Kg/year. Easily
Even more, upper members of the Plant kingdom produce a very special substance. One of the most inert polymers found in the nature that is much more desirable than wood for our needs of stabilising climate. Its degradation time can be measured in millions of years and appears fosilized or semifosilized in all continents and all ecosystems, with or without trees. Is called sporopollenin, and is the extra hard and waterproof material that makes the outer layer of pollen grains.
You don't need a tree to produce really hard pollen. A humble daisy can sequester carbon in a powder that will easily remain structurally intact for the next 1000 years. Everybody that has a garden and has cultured a big conifer knows that males of this creatures release copious amounts of the substance each year. Also old plants. _Specially_ old decaying plants that are about to die and invest all its reserves on reproduction. Maybe its time to start quantifying it.
Therefore your statement that plants only sequester CO2 when they grow is clearly false. If your maths and calculus do not include nuts and pollen, and do not include soft herbs, flowers and annual weeds they are incomplete, and most probably wrong, IMHO.
Mature trees grow much, much, slower. The wood is (largely) the captured carbon so once a given species reaches a certain maturity, harvesting it for lumber to be used in construction and planting a several year old tree would be the most optimal.
Some actually want to genetically engineer trees that grow much faster to do this (although faster growth generally means softer woods, but simply drying the wood and storing it in mines would buy you many decades to centuries before the bulk of the carbon was re-released).
And two leaves will also release more CO2 than one leaf, once it's on the ground, decaying.
Trees don't make carbon magically vanish, trees store it in cellulose, aka (more or less), wood. A tree that doesn't grow more wood does not store more carbon, and there is nothing to be "maintained", mass does not just disappear.
Perhaps use it to build long-lasting structures, such as houses?
Do trees stop growing?