This question has its origins in the word "dendraphile" used in a YouTube comment. It was used in an insult used by dwarves towards elves. My chain of thought then was that the dwarves would use plainer language like "tree hugger" and after drinking enough beer the "hugger" would be replaced by cruder word implying sexual intercourse. At that point my inner dwarf was like: "It would be just like Elves! Humpf!"
But the thing is having sex with trees really would fit Elves. They totally would do it if it was practical as part of some weird ritual about being more in sync with the nature. And with magic very little is impractical. Especially for a race that typically uses millennia on some pretty pointless projects because they have some serious personality issues.
Then I got to thinking about the specifics. Sex is basically exchange of DNA. It can happen between species and in some cases even between very different species. Bacteria like to exchange DNA with anything and viruses can ferry DNA between cells.
But all examples I know are fairly random and irregular. Despite this starting from and being based on fantasy I'd like a science based explanation of how regular and predictable genetic exchange would work. Specifically the consequences. We can leave the mechanics of how it happens to "Elves are weird and have magic". I am just interested on how the plant material would fit into the animal cells and what consequences it might have. And same the other way around. What could the plants give animals and what could animals give plants.
Note that despite the fantasy elements the actual question is about the science based part. I'd prefer hard science but honestly the background probably takes this too far outside of what might have been studied so requiring citations might be too much. Still it is the hard science part of plant and animal genetics I am interested in. Speculation I can do myself.