Is it permissible to sell the fruit of trees, such as lemons and guavas, in advance for a period of one or two years, with the buyer bearing the costs of care, given that the trees may only be fruiting or flowering at the time of the contract?

This transaction is incorrect, whether it is for selling the fruit for years or for a single year before its ripeness becomes apparent, or for the buyer to bear unknown costs of tree care. It falls under the category of gharar (uncertainty) sales, which are prohibited. The sale can be corrected through a salam contract, by paying a known price for a known weight of fruit after a known period, whether it is from this orchard or another.