Short answer. Not automatically. Article 566 of the Civil Code gives the usufructuary all the natural, industrial and civil fruits of the property, but expressly excludes hidden treasure from that entitlement: for treasure found on the land, the usufructuary is considered a stranger, not the person entitled to the fruits.
What the law says
The usufructuary shall be entitled to all the natural, industrial and civil fruits of the property in usufruct. With respect to hidden treasure which may be found on the land or tenement, he shall be considered a stranger.
Civil Code, Article 566 — Right to All the Fruits. Read the full provision →
The general rule: fruits belong to the usufructuary
Article 566 opens broadly: the usufructuary shall be entitled to all the natural, industrial and civil fruits of the property in usufruct. That is a generous entitlement covering everything the property produces on its own, through cultivation, and through legal relationships like rents. It is the baseline that makes a usufruct valuable — the usufructuary, not the owner, collects what the property yields while the usufruct lasts.
Hidden treasure is carved out of that entitlement
The same article draws a specific line around treasure: with respect to hidden treasure which may be found on the land or tenement, he shall be considered a stranger. Treasure is not treated as a "fruit" of the property for this purpose, and the usufructuary's status regarding it is deliberately different from their status regarding ordinary fruits — they stand outside the usufruct relationship on this specific point, as though they had no special connection to the land at all.
What being 'a stranger' does not, by itself, tell you
Article 566 says what the usufructuary is not — not the person automatically entitled to treasure the way they are to fruits — but it does not itself spell out what a stranger's rights to found treasure actually are. That question is governed by rules outside this article, and this provision's job is only to place the usufructuary into that separate category rather than to answer, on its own, how much treasure a stranger in general is entitled to. It also does not address whether the usufructuary at least benefits from the treasure indirectly, say through the general fruits the land continues to produce — that separate question is not something this article resolves.
What this means if treasure turns up
If treasure is discovered on land you hold under usufruct, do not assume the same automatic entitlement that applies to the property's ordinary fruits carries over to it. Article 566 treats this as a genuinely separate question, so working out what you might be owed for the treasure specifically requires looking beyond the usufruct arrangement itself, rather than treating it as just another fruit of the land.