when calculate a^b why parallel not work but parallelStream could

I want to calculate a^b , e.g. 2^30,

public long pow(final int a, final int  b)

first I used this manner

return LongStream.range(0, b).reduce(1, (acc, x) -> a * acc); // 1073741824

Got right result. Then I want to calculate it parallelly, so naturally I changed it to

return LongStream.range(0, b).parallel().reduce(1, (acc, x) -> a * acc); // 32

but in this case the result is just 32. Why?

So for supporting parallel I changed it again

return Collections.nCopies(b,a).parallelStream().reduce(1, (acc, x) -> acc * x); // 1073741824

in this case it works.

So what's wrong with parallel manner?