Hi,
From personal experience of importing things into OSM (recycling containers and city green) I can probably give some advise on how to do this.
First you need a way to process/split the source data. The dataset contains almost 200k entries, which is way too much to import in one go (20 changesets is the technical minimum you would need for this). So you need to split the source data into smaller batches, preferably split based on coordinates. The next thing is to convert the tags of the source data to the corresponding tags used within OSM. Make sure you check that you use the correct tags! Wrong tags and typos have happened in the past. I consider all of this to be the easy part and should not really be a problem considering your background.
The hard part of the import process is actually to merge the data you want to import with the already existing data. With standalone objects and features like trees and lampposts this usually is not that big of a deal. But you still need to check for already existing trees in Openstreetmap. And if they exist, only delete those which are contained in the import dataset.
In some places a user might have mapped large trees in backyards and other private grounds. These are almost never present in datasets supplied by the city (the only exception being monumental trees). So you cannot just delete all existing trees in the area which you import. You should manually check these conflicts. It is not like tools to easily do this for you exist right now anyway.
There is (in my opinion) definitely a place for datasets like the Utrecht tree dataset in Openstreetmap. They do have to be of sufficient quality though, as an incomplete or outdated dataset has almost no value. With the Utrecht one you mention this obviously is not a concern.
What I believe to be one of the most important things. Is to have a way to easily filter the imported data from the rest of the OSM database. This greatly helps with keeping the data in OSM up to date. But more importantly, it gives the commutity an easy way to (in case of problems) undo the import any time in the future.
The best way of doing this is to always add the source= tag to all objects you import. This is (unfortunately) not always mentioned in the wiki guides.
As an example: In Lelystad there appears to be an import of many thousands of trees. But there is no source attached to those objects. So no easy way of finding out were they come from. They could of course have been mapped manually, but i doubt that’s what happened.
It is great you reached out to the forum for doing the tree import. It is always appreciated when things get discussed with the community before actually doing it.
I do advise, before doing an import like this, you get some familiarity/experience with the tools used to edit Openstreetmap. Imports are generally done using JOSM. It is a pretty powerful editor with lots of functionality, plugins and options, but that also means that the program does have a slight learning curve. Simply do some local edits first just to get some experience with OSM. It really helps if you know exactly what you are doing when doing an import. 