I've never grafted but want to try. What tree species can be grafted? I know fruit trees can be grafted but what about starting out with something there's lots of? For example, could an aspen be grafted onto a cottonwood? I've heard evergreens can be grafted too but it's rare. Could a spruce be grafted onto a fir? These are just some ideas because I've only ever heard of same species being grafted, not similar species. There's a Norway Maple about a block away with several grafts of different cultivars on it. I'm just curious. Thanks.