Short answer. The person who succeeds in recovering possession — the winning owner, not the possessor who is being ousted. Improvements that came about by nature or simply through time belong to the property, so whoever ends up entitled to the property gets them, with nothing to reimburse.
What the law says
Improvements caused by nature or time shall always inure to the benefit of the person who has succeeded in recovering possession.
Civil Code, Article 551 — Improvements From Nature or Time. Read the full provision →
What counts as an improvement from nature or time
The rule covers gains the property produced on its own, with no human effort behind them. Soil gradually deposited by a river along a riverbank, trees that grew up untended, a herd that increased naturally, alluvial build-up along a shoreline, and the plain appreciation of value that comes from years passing are the classic examples.
What it does not cover is anything the possessor built, planted or spent money on. A fence, a house, a drainage system, a planted orchard — those are the possessor's own works, and they are dealt with by the separate rules on useful and necessary expenses, which depend on whether the possessor was in good faith or bad faith.
"Always" means good faith does not matter
Article 551 of the Civil Code uses an absolute word: improvements of this kind always inure to the person who recovered possession. Most of the Civil Code's possession rules split sharply between the possessor in good faith and the one in bad faith. This one does not.
The reason is that neither party earned the improvement. Nobody laboured for it and nobody paid for it, so there is no expense to reimburse and no equity to balance. It simply attached to the thing. Since ownership of the thing has been vindicated in favour of the person recovering it, the accretion follows the thing to him.
Where it fits in a recovery case
The article speaks of the person who has succeeded in recovering possession, so it presupposes a concluded dispute — an ejectment case, an action to recover ownership, or a return following a judgment. Until that point the question is theoretical.
When a court restores possession, the accounting between the parties usually covers three things: the fruits, the possessor's expenses, and the improvements. Article 551 disposes of one of the three cleanly. It does not affect the possessor's separate right to be reimbursed for necessary expenses, nor a good-faith possessor's right of retention until he is paid what is due to him.
What to do if this touches your property
The practical trouble is proof. Land disputes turn on whether a gain came from nature or from someone's work, and years later that is hard to reconstruct. Old survey plans, tax declarations, aerial or satellite images, photographs and receipts are what separate a natural accretion from a filled and levelled lot.
Be careful too with river and shoreline land: accretion has its own requirements, and land bordering the sea or forming part of the public domain follows different rules that this article does not override. If you are in a boundary or recovery dispute where the property has visibly changed, have a surveyor and a lawyer look at the documents together before you commit to a position.