Questions about tree planting have come up on StackExchange quite often, and in the news various countries have great drives to plant vast numbers of trees in a single day.
A common issue seems to be a misunderstanding of the relationship between planting 'a tree' and 'offsetting carbon'.
Whilst its true that a single mature twenty year old tree will both sequester around 3 tonnes of CO2 in its woody mass and also lock about 3 tonnes of CO2 into a cycle of growing leaves, dropping them and decomposition, so a total of 6 tonnes offset; in order to get a twenty year old tree its not as simple as planting one seedling.
This news article covers Turkey's attempt to plant 11 million trees in 2019, of which about 90% died. This article questions whether the fragile state of Ethiopia was able to plant a billion trees in one year.
From my own experience, in order to grow a healthy twenty year old tree, you need to plant about a dozen seedlings of various species, relatively close together, and over a twenty year period, you cultivate and care for them and occasionally cull them until one remains.
Can you simply say that to offset the 6 tonnes of CO2 with a mature tree you need to plant 12 seedlings, so each seedling is equivalent to offsetting half a tonne? Maybe, but that nuance is too much for most government policy headlines.
Another point to consider is that most trees drop a thousand or so seeds each year, many of them can germinate all by themselves without human interference. In the UK there are 3 billion trees, so that could be 3 trillion seeds dropped, even if only one seed per tree successfully germinated into a new seedling, that's still the tree population doubling every year. Planting seeds is easy.
The Damcon PL10 (with four row attachment) is a popular tree planting trailer for use on farms and plantations, and can plant around 20,000 seedlings in a single day. Planting seedlings is easy.
What's difficult, both practically and politically, is cultivating the new trees for twenty years. Its a lot of commitment. It takes land and funding and people, and at any moment a new government or local authority with different priorities could decide to rip up the new forest and build a freeway, or use the land for farming and all the effort of planting trees is gone in a puff of smoke (CO2).