Is there a standardized tree hash? SHA-1, SHA-2, and the standardized version of SHA-3 are all sequential.  This is impractical for hashing very large files distributed across machines.  Any sequential hash can be straightforwardly converted into an efficiently parallelized hash using Merkle trees, but then I lose the standardization, and this is undesirable if the hash is to be used for long term authenticated storage.

Question: Is there an officially standardized tree-based (parallelizable) hash?

The most important property of the tree-based hash for my purposes is that for any partition of the input string, each portion can be reduced down to $O(\log n)$ intermediate space in parallel such that the intermediate values can be combined into the final hash, with at most a constant factor overhead over a sequential hash.
With SHA-3 Derived Functions (SP 800-185, pdf) there is now a standardized parallel hash based on SHA-3, called ParallelHash, appropriately.

However, it is not a tree hash, but more of a hash-list-based mode. The string to be hashed is divided into equal-sized blocks, which are hashed, concatenated and then hashed again.

While it is not a tree hash it should cover the use case of hashing very large files across many machines. The arbitrarily small blocking size allows you to get to your $O(\log n)$ requirement if you base it on an appropriate function of the input size.