As a follow up to my above, it's the weight of the forest that matters. Older forests typically fit bigger and denser and heavier trees, so the age of a forest still matters a lot. But a fairly established forest like the Amazon is not changing the amount of oxygen out carbon dioxide in the air unless it's expanding or contracting

There is no active sequestration for a static forest. Only if the forest is expanding in size will sequestration occur. Obviously reducing the size via burning has the opposite effect

But doesn't it regrow from the hashes?

From the ashes? Not if it's turned into a ranch!

And I'm not qualified to answer what happens when you burn down an entire millenia-old ecosystem. Seems plausible that it might not go straight back to its old self in a human-relevant timeframe.

“Not if it's turned into a ranch!”

That’s not obvious to me. Most plants are C2 plants, but grass is a C4 so it captures sunlight more efficiently.

Not to say burning the Amazon isn’t a terrible idea.

Plants pretty consistently are ~40-50% carbon by mass, so the amount of biomass on the ground is a fairly solid indicator of the amount of carbon sequestered. Unless the ranchers have a truly massive amount of hay at all times I don't see how a ranch can approach a forest.

Don’t forget the soil. Old prairie has immensely thick soil, with a high carbon content. Think deep plant roots. With the right management, ranchers could mimic the prairie ecosystem, restore and build soil, and sequester carbon into the soil. It’s not easy, but the most successful farmers have been astoundingly successful.

Not that any of this is a valid excuse for burning down the Amazon.

Well my question is rather if it becomes a growing forest again, ie capturing co2.

Eventually. See my other comment