Short answer. The owner. Article 597 of the Civil Code makes taxes imposed directly on the capital of property, during the usufruct, the owner's expense. If the owner paid them, the usufructuary owes interest on those sums; if the usufructuary advanced them instead, he recovers the amount when the usufruct ends.
What the law says
The taxes which, during the usufruct, may be imposed directly on the capital, shall be at the expense of the owner. If the latter has paid them, the usufructuary shall pay him the proper interest on the sums which may have been paid in that character; and, if the said sums have been advanced by the usufructuary, he shall recover the amount thereof at the termination of the usufruct.
Civil Code, Article 597 — Taxes on the Capital. Read the full provision →
Why capital taxes fall on the owner
Article 597 draws a line between taxes on the capital, meaning the underlying value of the property itself, and other charges connected to the property's use or income. The taxes which, during the usufruct, may be imposed directly on the capital, shall be at the expense of the owner. This reflects that the owner, not the usufructuary, holds the underlying value the capital tax is assessed against, so the burden of that tax follows the ownership interest rather than the temporary right to use and enjoy the property.
What happens if the owner actually pays the tax
Article 597 does not leave the usufructuary entirely untouched even though the tax is the owner's expense: if the latter has paid them, the usufructuary shall pay him the proper interest on the sums which may have been paid in that character. This means that while the usufructuary does not have to reimburse the principal amount of the capital tax the owner paid, the usufructuary does owe interest on that amount, reflecting continued benefit from the property that the owner's payment helped preserve.
What happens if the usufructuary advances the payment instead
Article 597 also addresses the reverse situation: if the said sums have been advanced by the usufructuary, he shall recover the amount thereof at the termination of the usufruct. If the usufructuary ends up being the one who actually pays the capital tax, perhaps because it was more practical to do so at the time, that amount is not lost; it becomes recoverable from the owner once the usufruct ends, since the tax was always meant to be the owner's ultimate expense.
What this means practically for owner and usufructuary
Understanding this division helps both sides know what to expect financially during the usufruct: the owner ultimately bears the actual cost of any tax on the capital, while the usufructuary's obligation is limited to paying interest on that amount if the owner covered it directly. If the usufructuary ends up advancing the payment instead, Article 597 protects that outlay by making it recoverable from the owner once the usufruct terminates, rather than leaving the usufructuary to simply absorb a cost that was never really his to bear.