Section 12. Fixtures annexed to the freehold by a life tenant or by his assigns may be removed during the continuance of the life estate or within a reasonable time thereafter; and in determining what are fixtures, the common law rules prevailing between a landlord and a tenant for years shall govern. This section shall not affect the right of the owner of land to make a different provision by will or otherwise as to the removal of fixtures, nor impair or affect the provisions of any will or other instrument by which an estate for life in land is created or limited.