What could happen if Italy planted one billion trees?

Sometimes I like to put an imaginary scenario in my head. And one thing that always comes to my mind is trees, I love trees, they give me peace and tranquility. Since my parents came to Italy from a North African country where there are few trees except in the mountains and forests, and where the trees planted in the streets and cities are often inadequate and don't provide enough shade (think of palm trees or eucalyptus, which needs a lot of water, so planting it there doesn't make much sense to me).

And honestly, I hate it when some people don't like trees and cut them down just because they want more space, or because they don't like the fact that in fall their leaves fall and, according to them, make the road dirty. I would like to take those people to live in a desert for a couple of weeks so they'd understand how it feels (to give an example, I know an acquaintance who has a villa, and in front of the entrance there's a public road; on the other side of that road, so not on his property, there was a tree. When it was windy, its leaves would blow into his garden, so he secretly poisoned that tree. The tree died, the municipality came and cut it down, and now where there used to be a tree there's just a cut down trunk, where before there was a tree that gave shade and shelter to many birds and animals).

I was wondering what would happen, climate wise, if a country like Italy, which is where I was born and raised, decided to plant a billion trees.

Italy is a country with plenty of trees and forests, and it has recently been confirmed that around a third of the country (over 100,000 km², roughly 11 million hectares) is now covered in forest, but that coverage is very uneven: it's concentrated in mountainous areas and in regions like Liguria, Trentino-Alto Adige, and Tuscany, while regions like Puglia and Sicily have the lowest forest cover in the country. Since I love countries with a lot of trees (dense, green ones that actually cool things down) like Slovenia or some Nordic countries, I was wondering what would happen to Italy if it did the same.

For example, I drove in France, and on some highways there are trees on both sides, trees to your left and to your right. So that's maybe a starting point: plant trees along highways, then maybe also in the middle of cities, along streets, but also planning some big parks in the middle of cities. And ideally focus on the areas with the least forest cover.

Of course it's not easy, because I imagine Italian nurseries wouldn't be able to supply enough saplings, and maybe not even European ones, especially if the goal is to do it quickly. And ideally the trees planted should be native and diverse, to avoid creating monocultures (like some Nordic countries have done), which often turn out to be not very beneficial.

Evapotranspiration, greater shade, and all those tree-related processes: how would they alter the Italian climate? Would it actually be possible to lower the average temperature, perhaps to fight the effects of global warming a bit? Would there be more rain? Or is there a risk of pushing the rain away instead, given that Italy is long, and increased evaporation feeds clouds, but those clouds could easily drop their content slightly further east over the Adriatic, or slightly further west over the Ligurian-Tyrrhenian sea?

Thanks to whoever answers.