Thomas L. Lee will step down as chief executive and chairman of Newhall Land & Farming Co. on March 31, the Valencia-based company said. Gary M. Cusumano, currently president and chief operating officer, will succeed Lee as chief executive. Lee, 58, said he had recommended to the company's board that he retire to "provide new opportunities and responsibilities to other members of management." Lee, who joined the giant land developer in 1970, has served as chief executive since 1987 and was named chairman two years later. Cusumano, 57, is also a long-term member of Newhall Land, where he has worked in the company's real estate and agricultural operations since 1969. He was appointed president and chief operating officer of the company, which developed the planned community of Valencia, in 1989. Cusumano will assume control of Newhall Land as it attempts to undertake development of its newest planned community, Newhall Ranch.