I am not sure that documenting competition between neighbouring plants is a good use of interaction fields, because it takes us into a grey area. I think it’s fine to document parasites, hemiparasites, or epiphytes, because the interaction is very direct. A bromeliad growing on a branch is clearly depending directly on the tree it’s on for support. A strangler fig growing around another tree – fine. But a plant growing next to another and (probably) competing with it for light and water? While there is probably an interaction of some sort going on (everything in ecology is connected) I would draw the line at documenting that as an interaction.
The issue this does raise is whether we can come up with a standardised way of categorising interactions. Clearly one species feeding on another is an interaction, whether it’s predator-prey, parasite-host, herbivore-plant or pathogen-host. We could also document antagonistic interactions, such as songbirds mobbing a raptor; mutualistic interactions, such as ants living in Acacia thorns; and commensal relationships, such as remoras on a shark. In many cases it’s not obvious without detailed study whether an interaction is mutualistic or commensal, so we could have categories that relate more to what was observed, e.g., using another species as support or substrate – whether an epiphyte on a tree, an epiphyll on a leaf, a bat roosting in a palm or an anemone on the back of a crab. In some cases we might not know what the species is doing, but can still document an interaction, as with flower visitors (they could be visiting for pollen, for nectar, or to eat other flower visitors).
Charlie’s case of plants associating with others is more of a fuzzy area I think: are they in the same place because they like the same conditions, or is there actually any direct competition or mutualism happening? It’s often not clear, and I would suggest not to categorise these as interactions unless there is clear evidence of this.