Hello.
In my view, arbre singular (Q811534) means any notable tree, that is any tree that is covered individually by reputable sources. Some of them are included in official natural heritage catalogues or have some kind of legal protection, like arbre d'interès local (Q115867635). However, arbre singular (Q811534) is a value of instància de (P31), but arbre d'interès local (Q115867635) is a value of estatus patrimonial (P1435), as we do for buildings.
It would be possible to use arbre (Q10884) as instància de (P31) instead of using arbre singular (Q811534) and it would be fairly reasonable, but I see a couple of problems with that:
- It's different from what we do with animals. For example, we have Orca Ulisses (Q7879048)instància de (P31)animal individual (Q26401003), not animal (Q729). In fact, "individual tree" is an enAlias for arbre singular (Q811534).
- Notable trees should have arbre singular (Q811534) somewhere, but if they aren't officially protected (for example, trees covered in a book of notable trees but without legal protection) we can't use arbre singular (Q811534) with estatus patrimonial (P1435) nor premi rebut (P166), because it's not an status nor an award, it just means that some reputable source decided that the tree is worth mentioning or describing. Then, if arbre singular (Q811534) is neither instància de (P31), nor estatus patrimonial (P1435) nor premi rebut (P166), what is it?
Of course, there is an inconsistency in Wikidata between how we treat trees, buildings and people, specially in instància de (P31). For buildings we take a quite concrete instància de (P31) (like church or cathedral), for people we stick to human and all individual characteristics go to other properties and for living beings we take the middle ground of animal individual (Q26401003) and arbre singular (Q811534). I suppose we could take a different and unified approach and try to reduce the number of values of instància de (P31) (or expand them) across Wikidata, but that would go far beyond trees.
Where I'm usually doubtful is about what to do with small sets of trees, but also small sets of anything else (two buildings, two people, two hills...). To make things more complex, as far as I know, such sets of a few trees are usually protected in Barcelona as arbre d'interès local (Q115867635) and not as the equivalent protection for groves ("arbreda d'interès local", still not present in Wikidata). Therefore, I tend to use for them the same properties as for a single tree, which doesn't feel like a very satisfactory solution - although I think I've encountered only a few of such cases.