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I don't have any idea about the number of books in the saga. I read it three or four years ago, but I'm pretty sure it was a re-edition, whereas the tone of the story doesn't sounds to me like a very old book (more like post 1980's, or younger).

I just remember that:

• There were big bad trees that appear (coming from another world?) and kind of curse the territory, making some sort of demons appear as well.


• There also were humans realms, including a king (or something) that was not that good, and lived on a giant tree (a good tree?).


• At the end of the first book (I guess), there was a battle in a stronghold, or a fortified city.


• I don't even remember who the hero was, I guess it was a young man.


• As far as I remember, there were epic/geopolitical overtones in this book.

This sounds like a segment from the 4th book of James Clemmens' The Banned and The Baished series.

The series is a five book epic centred around the journey of the Wit'ch from childhood innocent to fantasy epic hero with her surrounding band of support heroes freeing the land from the Tyranny of the big bad.

One of the original members of the party is a Tree Spirit called Nee'lahn. Her story through the five books is a redemption arc both personally and for her people.

In Nee'lahn's case, her race are the spirits of a particular type of tree that used to cover most of the continent. In ages past the trees became affected by a Blight and started to go bad. The spirits of such contaminated trees become Wraiths and are both terrible and deadly. Nee'lahn has to accept that the original situation where they covered the continent was as bad as the current state of them dying out is, before she's able to find a solution to the problem with the help of her new friends.

In book one Nee'lahn finds the remains of the forests "King" Tree which though a series of co-incidences convinces her to join the Heroic group.

At the end of the 3rd book there's a running battle in a fortified city

Where the heroes aided by a band of Noble pirates, their former slave master mermaids and a Dragon defeat the local overlords/bad wizards. Just in time to realise how much bigger their problems are.

Geopolitical/Epic overtone and mostly book 5 spoilers.

The whole series is framed as "historical documents" by a future civilisation who are keen to restrict the information in them to only a trusted few to ensure the stability of the society. The saga itself ultimately is all about stopping the big bad before he gains control of all magic and total control of the whole world.

Turning someone else's comment into an answer.

Uprooted is a 2015 novel by Naomi Novik

Agnieszka loves her valley home, her quiet village, the forests and the bright shining river. But the corrupted Wood stands on the border, full of malevolent power, and its shadow lies over her life.

Her people rely on the cold, driven wizard known only as the Dragon to keep its powers at bay. But he demands a terrible price for his help: one young woman handed over to serve him for ten years, a fate almost as terrible as falling to the Wood.

But Agnieszka fears the wrong things. For when the Dragon comes, it is not Kasia he will choose.

The wood is a demon forest that spawns terrifying creatures as minions to kill/infect villagers to gain more territory.

The Dragon is a wizard who protects the area living in a nearby giant tower, which could be confused with a tree. All mages in the setting are ultimately subjects of the king, who is ineffective and stuck in a war with a neighbouring country.

The hero is a young woman, one of the villagers who turns out to have magic. There is a young man as a secondary character, the king's younger son who is obsessed with finding his mother who was abducted/ran away with the Prince of the rival country and was then 'taken' by the wood on the journey.

Conflict between the magicians jockeying for status at court makes up around 1/3 Of the book. While the war between the two kingdoms is a constant background.

The final battle takes place in the wood rather than a city.