Use cases would be trees within cities, urban trees as part of green infrastructure. Natural forests or plantations are mapped as polygons, it does not make sense to individualize each tree into a node. It is in these areas where the 3 billion trees are found, in the cities there is a lack of trees by the way. In my city (Valdivia- Chile) we have 19,000 trees mapped in OSM, it is estimated about 25-30 thousand, to cover the whole city. 
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