Firstly, welcome to meta. You're right in thinking there's no way to find out who downvoted your reply if the downvoter chooses to stay silent. Not even moderators can access this data.
You can add a polite comment to your answer asking something like "can anyone explain why this answer has been downvoted?" or ask in meta - as of course you have done. Of course, doing so only really makes sense if you undelete your answer. I would suggest that undeleting your answer and adding a comment would be worthwhile. It's certainly not seen by most people here as getting into an argument. I'm happy to ask/comment and see if we can elicit the downvoter's reasons, if you're unsure about it (but you have to undelete the answer).
I don't know much about trees so I don't know especially whether your answer is a good one or not but it looks OK to me.
I know when I've been downvoted I have felt a little disheartened and even singled out for attack. But you get used to it and don't need to feel it personally. Downvotes are quite a healthy part of the system. They are definitely not an attack on you personally or your advice generally. But someone, perhaps a very knowledgeable gardener or perhaps a total ignoramus decided this particular answer was inaccurate or unhelpful. In isolation it says nothing about the community's high regard for your contributions (your many other upvoted answers testify to that).