I did not find that Rashi says that the trees were fruit trees. The simple meaning of the Gemara is that even if they were not fruit trees, Rabah bar Rav Huna would still suffer a huge loss if his trees would be cut, for they could be grown to cultivate wood for building material, and if they would be cut they would not grow back.

Nevertheless, the question remains: Why did Rabah bar Rav Huna curse whoever cut down his trees? He should have brought the perpetrator to court and not cursed him, as the Gemara says in Moed Katan (17a), that one should not curse or excommunicate a person whenever it is possible to bring him to court. This questions is asked by Rav Elyashev as cited in He'oros l'Maseches Bava Metzia. He gives two approaches to answer this question.

1. Rabah bar Rav Huna did not make his statement as a curse, but rather as a statement of fact. He was saying that one who cuts trees indiscriminately like that will certainly meet the fate of having no living sons, as the Gemara says in Bava Kama (91b), that one who cuts trees before their time to be cut will suffer the death of his children.

2. He cites the Me'iri who explains (in contrast to the explanation of Rashi and Tosfos) that Rabah bar Rav Huna's forest that was cut down was not the same forest that was adjacent to the forest of the Persian viceroy. Accordingly, he indeed had no legal claim against the one who cut it down. Therefore, Rabah bar Rav Huna did not bring Rabah bar Rav Nachman to court, but rather he cursed the person who had the temerity to cut down the forest of a Talmid Chacham without informing him.

(We may add that this explains why Rabah bar Rav Nachman said, "Cut it down, cut it down," repeating his command two times. He was telling his agents to cut down this forest, and to cut down the other forest, the one adjacent to the Persian viceroy. The fact that there was an additional forest that Rabah bar Rav Huna owned which he did not want to be cut down shows that the real reason why he did not want his first forest, that next to the Persian viceroy's forest, to be cut down was not because his neighbor (the Persian viceroy) would not cut his own forest, but because he wanted to spare his trees for his personal benefit. Therefore, Rabah bar Rav Nachman cut down both forests, and Rabah bar Rav Huna would have had no recourse against him in court. The reason why Rabah bar Rav Huna cursed him for cutting down the first is because he maintained that he certainly had a claim for that one (i.e. that the Persian neighbor would not agree to cut down his forest).)

M. Kornfeld