Tree tag. I'm not a tree tree doctor, but that kinda looks like an ash. Around where I live too ash trees are commonly tagged and checked up on to monitor invasive emerald ash borer populations

Pretty sure they're id tags for trees. Basically a way for the maintenance people to track and log data for each tree.

The botanists in the biology dept have the answers you seek. I suspect there is a map of your campus trees, and this one is on it. It's common for universities to have an arboretum on campus. Find the map and look for trees you've never seen before! I bet you can find a ginkgo or a cork tree.

The tree is going through a rebellious phase.

I think that’s what note they’re in. 440 is A, so that tree is tuned to A.

That's the license plate for insurance. If the tree drives like the axe in the wood.

Now look up for tree 420

Adding to what others have said: It’s part of the tree inventory. It may be used by maintenance/physical plant, but can also be part of an arboretum status for the campus with a publicly facing GIS map and attributes.

Nice work. For every 100 you collect turn them in at campus security for a free tree!

they do it at our school to identify maintain the tree. unfortunately they are putting it on the invasive, fire-prone eucalyptus blue gums at our university, plus also bottle brush too which are all from australia and are invasive outside its range. although we also put these on our iconic native pine and cypress, in the wild they are not doing so well, because of disease, and pests, and thier range contraction. I believe the domesticated trees outnumber the wild ones.

Someone is claiming them as pets.

It's an old MOPAR engine

Trees have rings which they use as mating displays much like H. sapien.

That's the TF2 tree