Forest, how to specify distance between levels

Question

I have this code, but want to reduce the distance between the levels. Any suggestions?

\begin{forest}
    for tree={
        circle,
        draw,
        fill,
        minimum width=2pt, % size
        inner sep=0pt,
        parent anchor=center,
        child anchor=center,
        s sep+=25pt, % distance between children
        grow = north
    }
[ 
[
[]
]
]
\end{forest}

Answer 1

The level distance can be adjusted using keys l and l sep. For details, see the sections 3.7.2 (reference) and 2.4 (tutorial) of the manual.