Ok this seems like a relatively new concept to me. I'm now hearing that you join 2 independent clauses with a comma as well as a conjunction after. I can understand the conjunction as it expresses a relationship between the 2 clauses (and without it, there would be nothing joining up the clauses) but why the comma?
Is it really necessary and why put it if it's not necessary?
Take for example:
I don't see why there is a comma but is it there because the part 'but they are not related to the American chestnut' an appositive or nonrestrictive clause?Today, along many streets in Canada, there are healthy chestnut trees, but they are not related to the American chestnut