Drought-resistant pistachio orchards at Gilkey Ranch near Five Points, California. (Photo: Jonno Rattman/Bay Nature) Inside a climate-controlled laboratory at the Duarte Nursery outside Modesto, an experiment is taking place that could help determine what food we will eat for decades to come. Rows of steel racks contain numerous tiny almond, apple, walnut, pomegranate, pecan, avocado, fig, and pistachio trees in small, translucent plastic cylinders. The saplings, planted in a high-nutrient agar mix that accelerates growth, are no more than two inches high and a few weeks old. Each is being subjected to versions of the stresses experienced just outside these walls in fields across the Central Valley: declining levels of water, escalating levels of salt. The big overarching, if unmentionable, force driving these experiments is climate change, which is beginning to roil the Central Valley. Duarte, one of the largest commercial nurseries in the world, specializes in tree nuts and fruits, which have boomed across the valley in recent decades. Founded four decades ago, the nursery grew rapidly as water piped into the valley from the Sierras gave birth to the most bounteous center for...