To clarify: by murder, I did not mean the legal term, but simply killing (I'm sorry for any potential bad wording or misspelling, repetition etc. English isn't my 'main' language).
I don't think felling a tree is either killing or immoral; it was just an example to show that it is not ending 'life' that makes abortion morally wrong to you.
I assume your reasoning for why abortion is immoral is that (valuable) human life is being ended. I, too, believe that human life is inherently valuable, though not at every stage.
For example, I don’t believe that a sperm cell is just as valuable as an adult human being, and I’m pretty sure you’d agree with me on that.
On that basis, I would argue that an abortion (up to a certain point) is not very different from felling a tree.