I'm not sure about this but I think all you need is

    tree | sed 's/├/\+/g; s/─/-/g; s/└/\\/g'

For example:

    $ tree
    .
    ├── file0
    └── foo
        ├── bar
        │   └── file2
        └── file1
    
    2 directories, 3 files
    $ tree | sed 's/├/\+/g; s/─/-/g; s/└/\\/g'
    .
    +-- file0
    \-- foo
        +-- bar
        │   \-- file2
        \-- file1
    
    2 directories, 3 files

Alternatively, you can use the `--charset` option (might be a GNU extension, I'm not sure):

    $ tree --charset=ascii
    .
    |-- file0
    `-- foo
        |-- bar
        |   `-- file2
        `-- file1
    
    2 directories, 3 files