xarray.Dataset.to_netcdf() allows you to save a dataset into a netcdf file as a group, but to make a DataTree.to_netcdf() we need to save many groups into the same file. I'm not sure how to do this with xarray.Dataset.to_netcdf(), and xarray's backends code is pretty complicated. It could probably be done with the netCDF4 python library at a lower level but I havent' really tried that yet, and it would be nice to not have to use that.

The hard bit is saving multiple groups to the same file, actually iterating over the groups is easy, just something like

def to_netcdf(dt, filepath):
    for node in dt.subtree_nodes:
        group_name = node.pathstr
        
        node.ds.to_netcdf(filepath, group=group_name)