Where a mobilehome park or trailer camp abuts an R-A, R-1, R-2 or R-AS Residential District there shall be a buffer area along the district boundary line within the R-4 Mobilehome District, the depth of which shall be at least 50 feet, measured from the district boundary line. A screen of trees, shrubbery or hedges shall be planted and maintained within the buffer area sufficient in density to constitute an effective screen and give maximum protection and visual screening to abutting properties. The buffer area may be included in any yard areas required by the provisions of the section, but the buffer area shall not be used for any purpose other than planting and screening.