Kaarna means tree bark in Finnish. The rough and dark surface of Arabia Kaarna’s brown version is not unlike bark.
The series was designed by Goran Bäck and was in production from 1961 to 1971. The shape of the series is called NV. Another design based on this shape is Mahonki. Another design by Bäck is Korpi.
The series is often mistaken for Arabia Ruska. I made a blogpost clarifying the differences here: link.