It's far less than what a professional tree planter will manage in a single day in Canada.

It's definitely possible for any healthy adult.

Keep in mind that the 600-1000 number is for "tree age 0" which is a sapling about 6 feet (two meters) tall - the germination/survival rate for planting smaller saplings or seeds is much lower.

Yep, when my wife tree planted she'd carry bags of those on her hips. One member of her team planted a tree every seven seconds, while they were working in BC.

Whoops, my tired morning self read "6 inches" and not "6 feet".

That's a hell of an error. Sorry folks.

Most people don't have acres of land to plant trees in.

Which is why you outsource the tree planting to a place that has room for trees -- and it's more efficient because they can grow trees better than you can, anyway.

nationalforests.org, the Arbor Day Foundation... there are probably lots of good options.

nationalforests.org does reforesting of recently burned forests which are probably burning more frequently due to climate change. I don't think replacing forests that were alive when you were born should count as offsetting your personal CO2 emissions, if the idea is to have an actual effect it has to be new trees in new forests.

On a related note, today's New York Times has an article about the recent fires in California. Near the end the author states that the 2018 Camp Fire released

...an estimated 3.6 million metric tons of greenhouse gases that, as seems to happen at least once every fire season lately, was more than enough to obliterate the progress made by all of California’s climate-change policies in a typical year.

https://www.nytimes.com/interactive/2019/07/31/magazine/para...

I’d assume that the following spring and for the next year or 3, there would be a huge growth increase in the area due to ash, light and space.

If it's a tree/forest that wouldn't have otherwise existed but for your donation/their action, I think it should count.

Whether it's at a lat/long that previously held a tree or not is largely irrelevant when considering whether your action was helpful or neutral.

Nope! I'm growing Goat Willow, Cherry and a rescue-acorn that's now a 4 inch oak sapling, all in a small space.