Avodah Zarah chapter 3, mishnah 10 - the last mishnah in the chapter and the last one dealing with the asherah. The question of the mishnah is "Keitzad mevatlah?" - how does one strip the tree of its status as a tree of avodah zarah, so that it becomes an ordinary tree? We will deal further with the methods of nullification in the next chapter, in mishnayos 4 and 5.
The basic principles of nullifying avodah zarah:
A Jew can never nullify avodah zarah, even if it belongs to him and he himself worshipped it or made it.
A gentile can nullify it, as long as it belongs to him.
In principle, the manner of nullification is an act that involves breaking, or expressing contempt toward it in some other way.
The methods of nullification with an asherah:
How does its worshipper remove the asherah from the category of avodah zarah and nullify its status?
"Kirsem" - he pruned it and removed the dry twigs.
"Zeirad" - he cut off the moist twigs.
"Natal mimenah makel o sharvit" - he took from it a thick stick, like a walking stick, or a rod that is thinner and more flexible than that.
"Afilu aleh" - even if he picked from it only a single leaf, "harei zo betelah" - it is nullified.
The reason for this: from that moment on he no longer relates to it as avodah zarah or as a deity, but rather uses it merely as a source of wood.
The next word in the mishnah is "shipah". The Bartenura, and the Rambam like him, explain that he carved and shaved it - that is, he scraped off its bark rather than taking leaves or branches. Although the action is different, the principle remains the same.
"L'tzorkah asurah, shelo l'tzorkah mutar":
L'tzorkah (for its own needs) - if he removed that part for the sake of the tree itself, so that it would grow better or look better, the tree remains forbidden.
Shelo l'tzorkah (not for its own needs) - if he cut off that part for his own needs and not in order to improve the tree, then he is using it as a monetary resource and not as avodah zarah, and the entire tree becomes permitted.
It is worth noting that it appears the Tur holds that the "shipah" of the mishnah is not a new type of action, such as peeling off bark, but rather refers to all the methods listed in the earlier part of the mishnah - any manner of pruning or removing things from the tree. And the principle is one: from the moment a person uses the tree as a resource for his own needs, he no longer relates to it as avodah zarah, and therefore it becomes permitted.
The practical halachah: If the idol worshipper pruned the tree for the sake of the tree itself, the tree remains forbidden and is an asherah just as it was. However, the parts that he cut off - the branches and the bark - are not an asherah, since they are not part of the avodah zarah, but rather he relates to them as ordinary wood. And a gentile has the right to do so, that is, to relate to them as his own monetary resource and to remove them from the category of prohibition.
A Jew, by contrast, does not have the power to nullify, as was explained at the outset of our discussion.