The problem is at some point the tree dies, decays, and releases the carbon it was holding.

Contrast to an oil deposit which, left undisturbed, might never release its carbon.

Thinking of it at the individual tree level is wrong.

An acre of forest sequesters somewhere in the region of 30,000 lbs of carbon dioxide. Individual trees die and are reborn, but the forest as a whole, if it does not shrink, retains that carbon.

"An acre of forest sequesters somewhere in the region of 30,000 lbs of carbon dioxide. Individual trees die and are reborn, but the forest as a whole, if it does not shrink, retains that carbon."

But it's even more complicated than that ...

We are reforesting ~20 acres of woodland just outside of San Francisco and we are not, as you say, thinking about individual trees - we are thinking of the woods as a whole ...

... which means clearing out a tremendous amount of sapling / deadwood / litter material that would quickly combust in a wildfire and destroy the entire forest.

So while we are on track for planting >200 redwood and douglas fir trees, we have also had to cut down >100 marginal trees to reduce fuel load. We're thinking on the scale of the entire forest and the entire forest needs to survive a burn ... including our relatively new trees that aren't as fire resistant as the old growth redwoods that were originally logged out of here.

Generally speaking:

The complexity of carbon mathematics is fraught and complex and I don't think it's reasonable to expect end users like consumers to navigate it.

I’m not an expert on the market, but I don’t think there are many carbon offsets sold as “we will plant and maintain an acre of forest in perpetuity.”

And frankly it’s not something any organization can promise. Governments change and clear forest that was previously protected. Brazil provides a current and unfortunate example of this.

Also I wonder what ratio of those schemes are trees planted that would have anyway been planted for industrial forestry... We grow trees for reasons. And if there is extra cash in it there is few who don't take it.

It doesn’t release all its carbon. Much of the biomass does not end up back into the atmosphere but ends up in the ground. Those oil deposits were once forests.

One of the issues is that planting a tree is a slow form of sequestration. An individual tree may sequester a lot of carbon over its lifetime (measured in decades) as it grows, but it doesn't (and can't) immediately offset things as a seed or a sapling. Some carbon offset schemes attempt to amortize for this with accounting tricks, but for the most part when people are buying things like carbon offsets they are offsetting a pound of CO2 (or equivalent) emitted today with the hope that maybe a tree planted today will maybe sequester that pound over say the next ten years. (If it survives, if it is in a forest that is well maintained and hopefully not in a region prone to wildfires, if that forest is protected as an entity and not clear cut, and so forth.)