Avoda Zara page 53. Rosh Hashana page 13.The trees that were planted and worshipped after the land of Canaan was given to Avraham were idols of gentiles which only need nullification, not burning. So, when the Torah says to burn the asherot, it has to refer to trees that were planted before the land was given to Avraham. But even those were still asherot of gentiles. So tosphot explains the trees that Israel were commanded to burn must have been planted and not worshiped, and then the land was given to Avraham along with the trees. Then Israel served the golden calf, and by that showed that serving idols was ok from their point of view. So, when after that, the gentiles worshipped those trees, those trees became ashorot of Israel which need to be burnt, not just nullified. But then the question comes up that a tree that was planted normally, but later was worshipped does not become an idol. Answer Tosphot holds like R. Jose HaGalili who holds that trees that were planted and later were worshipped become idols. The only question is then how would the sages that disagree with R. Jose explain the problem of why certain trees had to be burnt. [An idol that is owned by a gentile needs only nullification. But an idol owned by a Israel needs to be burnt, Therefore why were Israel commanded to burn the asherot (trees that were worshipped) when they entered into the land of Canaan. This above paragraph comes to answer this question. Another important factor here is that a gentile that planted a tree (after the land of Canaan was given to Avraham) owned the tree. So, the only tree that were owned by Israel when they entered the land of Israel were those that had been planted before the land was given to Avraham. There the only possible answers for the sages to answer are that they held that even trees that were planted to be worship before the land was given to Avraham were in fact owned by Avraham after the whole land was given to him. and this I think is the best answer for the sages. However, the gemara has to be referring to trees that were planted in order to be worshipped before the land was given to Avraham, but not worshipped until afterwards. Those trees did not become idols even after they were worshiped because a person can not cause to be forbidden that which does not belong to him until Israel served the golden calf. Then, if those trees were worshiped after that, they became forbidden and became idols of Israel and had to be burnt. ] -----------------------------------------עבודה זרה דף נ''ג וראש השנה דף י''ג. The trees that were planted and worshipped after the land of Canaan was given to אברהם were idols of gentiles which only need ביטול, not burning. So, when the תורה says to burn the אשרות, it has to refer to trees that were planted before the land was given to אברהם. But even those were still אשרות of gentiles. So תוספות explains the trees that Israel were commanded to burn must have been planted and not worshiped, and then the land was given to אברהם along with the trees. Then Israel served the golden calf, and by that showed that serving idols was ok from their point of view. So, when after that, the gentiles worshipped those trees, those trees became אשרות of Israel which need to be burnt, not just ביטול. But then the question comes up that a tree that was planted normally, but later was worshipped does not become an idol. Answer תוספות holds like ר' יוסי הגלילי בעבודה זרה דף מ''הwho holds that trees that were planted and later were worshipped become idols. The only question is then how would the sages that disagree with ר' יוסי explain the problem of why certain trees had to be burnt.[[ [An idol that is owned by a gentile needs only nullification. But an idol owned by a Israel needs to be burnt, Therefore why were Israel commanded to burn the אשרות (trees that were worshipped) when they entered into the land of Canaan. This above paragraph comes to answer this question. Another important factor here is that a gentile that planted a tree (after the land of Canaan was given to אברהם) owned the tree. So, the only trees that were owned by Israel when they entered the land of Israel were those that had been planted before the land was given to אברהם. There the only possible answers for the חכמים to answer are that they held that even trees that were planted to be worship before the land was given to אברהם were in fact owned by אברהם after the whole land was given to him. and this I think is the best answer for the חכמים. However, the גמרא has to be referring to trees that were planted in order to be worshipped before the land was given to אברהם, but not worshipped until afterwards. Those trees did not become idols even after they were worshiped because a person can not cause to be forbidden that which does not belong to him until Israel served the golden calf. Then, if those trees were worshiped after that, they became forbidden and became idols of ישראל and had to be burnt. ]]