Cannot call .chunk('auto') on DataTree.

What happened?

Got an error when calling data_tree.chunk('auto')

This is pretty frustrating, when xr.open_datatree supports it. I would expect both to be equivalent.

What did you expect to happen?

I expect it to succeed, and be equivalent to chunking the dataset, or opening the datatree with the chunks="auto" argument.

Both the type of the argument andthe docs imply this should work.

Minimal Complete Verifiable Example

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!

import xarray as xr
xr.show_versions()
# your reproducer code ...

import numpy as np
import xarray as xr

ds = xr.Dataset({"a": (("x", "y"), np.zeros((10, 5)))})
dt = xr.DataTree(ds)

ds.chunk("auto")  # works
dt.chunk("auto")  # TypeError

Steps to reproduce

uv run the above script.

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Traceback (most recent call last):
       File "/root/timaeus/.claude/worktrees/auto-chunking/repro_script.py", line 23, in <module>
         dt.chunk("auto")  # TypeError
         ^^^^^^^^^^^^^^^^
       File "/.uv/cache/environments-v2/repro-script-8f1b2638dcd6a5aa/lib/python3.11/site-packages/xarray/core/datatree.py", line 2655, in
     chunk
         raise TypeError(
     TypeError: invalid type for chunks: <class 'str'>. Only mappings are supported.

Anything else we need to know?

I've worked aorund with
dt.map_over_datasets(lambda ds: ds.chunk("auto"))
which I think is equivalent?

Environment

Start at xarray/core/datatree.py:2655 and compare DataTree.chunk with the Dataset.chunk behavior shown in the MVCE. Run the supplied script with uv, then add coverage for dt.chunk("auto") and confirm it succeeds with behavior equivalent to the documented dataset or open_datatree usage.