There are at least three families with name Tuohiniemi, not being related to each other.
One of the Tuohiniemi families are descendants of Juha Mäntysaari, former Tuohiniemi, née Iso-Komero (1847–1908), who was a tenant farmer (torppari) in Tuohiniemi torppa in Meskala village, Kiikoinen, South-Western Finland, and his wife, Hilma (1857–1927). For more information about this family, please see Juha Iso-Komeron ja Hilma Yli-Ketolan sukukirja by Hely Lenni-Taattola ([Vammala]: [Hely Lenni-Taattola], [2000]. ISBN 952-91-1786-8).
Another Tuohiniemi family are the descendants of Juho Tuohiniemi (1883–1951), who was a farmer (talollinen) in Maula, Kaasmarkku village, Ulvila, Western Finland, and his wife, Ida Katarina (1882–1941). For more information about this family, please see tables 9201 onwards in Sukukirja Rågell, volume 2, edited by Ansa Huhtala ([Pori]: Rogel-sukuseura ry, [2016]. ISBN 978-952-93-7467-0); a few non-related Tuohiniemis are also listed in tables 10586 onwards (a Pomarkku-based family) and in table 13450 (a Kullaa-based family).
Another Tuohiniemi torppa, in Kiikoinen, has a family associated with this name, all being descendants of Antti Juhonpoika (1791–1867).