Which pronoun should be used for animals and for trees? Is there any change in pronoun according to the situation? If yes, then what is it?
"It" is the pronoun used (in general).
You may want to know more from this link, definition 1 copied below:
"that one —used as subject or direct object or indirect object of a verb or object of a preposition usually in reference to a lifeless thing (took a quick look at the house and noticed it was very old), a plant (there is a rosebush near the fence and it is now blooming), a person or animal whose sex is unknown or disregarded (don't know who it is), a group of individuals or things, or an abstract entity (beauty is everywhere and it is a source of joy)"
Situation? You mean this?
When we speak about animals or plants in the plural, we say they,them,their. We usually say it,its in the singular. Your pet is often regarded as he or she. When you read fairy-tales, you can also see he or she used to call animals.