Section 21. The selectmen in the town and the mayor and the city council in a city shall appoint a special town forest committee to manage and care for that portion of the public domain known as the city or town forest; provided, however, that a town may vote to designate the conservation commission of said town to manage and care for such town forest; provided, further, that a town may vote to authorize said conservation commission to appoint such forest committee; and provided, further, that in a city, the mayor and the city council may appoint a forester or other board or officer to provide such management and care of such forest. Such forest shall exclude lands held within the watershed of any pond, stream, reservoir, well or other water used by a city of town as a source of water supply. The special town forest committee shall be composed of three members who shall choose their own chairman, and shall serve without compensation. One member of said committee shall be appointed each year for a term of three years, except that, when first appointed, one shall be appointed for a term of three years, one for a term of two years and one for a term of one year.