As others have noted, single iNat observations really aren’t designed to represent multiple observations of one individual over different times. Each observation should generally be of a specific interaction with an organism (ie, not separated by years). Observation fields are editable by other users, but otherwise, you’d have to give your account login credentials to others to have them logon and change any info (never a great practice).
I would argue that the way you’re proposing to use iNat really isn’t a great choice for what you’re trying to do (long term monitoring), but good be a piece of that. A simple other solution could just be to have a shared Google Sheets (or any other shared spreadsheet you like) that all users can edit. You could create an ID field (column) with codes for each individual ash tree and then add rows for each observation of each tree. These rows could include links to relevant iNat observations as well as other info (date, time, diameter, etc). This would let you work with the data and calculate relevant measurements easily. I’m sure there are other solutions as well.