Just off Palm Canyon Drive in Borrego Springs, a dead honey mesquite tree remains rooted in the hot sand. It’s lifeless but not yet useless — not to the creatures that find shade under its branches or the plants that count on its nutrients. Over the last year, mesquite has been at the heart of a growing water war in Borrego Springs, a tiny but scenic town deep in the San Diego County desert that for years seemed blessed with a rare combination of blazing sun and a font of available groundwater. A century ago, abundant green mesquite blanketed the landscape. But in the decades since, the forest the trees form has deteriorated — just as the town has pumped too much water out of its underground subbasin to sustain its farms, resorts, golf courses and some 3,000 residents. Now, controversy has broken out over whether that mesquite forest relies on the same water as the town.