Fifteen thousand acres of citrus groves once blanketed the San Fernando Valley, but now just a single commercial citrus grove remains, a half-mile from the 101 Freeway on the border of Tarzana and Woodland Hills. At 14 acres, Bothwell Ranch represents less than one-thousandth of what once was, before the orchards and ranches of the Valley gave way to tract housing, cul-de-sacs and two-car garages. Citrus production amid the multimillion-dollar homes is far from a viable occupation, and the Bothwell family put the property on the market earlier this summer. The $13.9-million real estate listing boasted of the potential for constructing 26 single-family homes on half-acre lots.