Tags are good and useful for sure: in flickr I use tags: 5-10-fav: https://www.flickr.com/photos/jeco/tags/510fav/
Then flickr tried to introduce machine tags. Now Google will bear AI to tag photos. But mistakes get made and tags suffer as a result. Even humans will falsely tag a photo using a popular tag rather than an appropriate tag, and the spamming begins.
This leads to coining unique terms and tags: https://www.flickr.com/photos/tags/cloudscape and groups and communities associated to those.
Freeform tags or controlled hierarchy both have there place. But the community will balk at 'you aren't tagging the right way' when it is a personal decision to them.
For the most part, a tree is a tree. https://www.flickr.com/search/?sort=interestingness-desc&safe_search=1&text=trees&view_all=1