Are C++11 thread_local variables automatically static?

According to the C++ Standard

When thread_local is applied to a variable of block scope the storage-class-specifier static is implied if it does not appear explicitly

So it means that this definition

void f() {
    thread_local vector<int> V;
    V.clear();
    ... // use V as a temporary variable
}

is equivalent to

void f() {
    static thread_local vector<int> V;
    V.clear();
    ... // use V as a temporary variable
}

However, a static variable is not the same as a thread_local variable.

1 All variables declared with the thread_local keyword have thread storage duration. The storage for these entities shall last for the duration of the thread in which they are created. There is a distinct object or reference per thread, and use of the declared name refers to the entity associated with the current thread

To distinguish these variables the standard introduces a new term thread storage duration along with static storage duration.

Yes, "thread-local storage" is very similar to "global" (or "static storage"), only that instead of "duration of the entire program" you have "duration of the entire thread". So a block-local thread-local variable is initialized the first time control passes through its declaration, but separately within each thread, and it's destroyed when the thread ends.

When used with thread_local, static is implied in block-scope (see @Vlad's answer), requied for a class member; I guess, means linkage for namespace scope.

Per 9.2/6:

Within a class definition, a member shall not be declared with the thread_local storage-class-specifier unless also declared static

To answer the original question:

Are C++11 thread_local variables automatically static?

There is no choice, except for namespace-scope variables.

Is there a difference between these two code segments:

No.