EL YUNQUE RAINFOREST, Puerto Rico -- Palm trees flower here in flashes of magenta and orange. Luscious green fronds drag on the floor, and ropelike lianas hang off trees. Ignoring the brilliance, 22 scientists trudged through their forest with their eyes planted on the floor. At times, they picked up roots. This subterranean stuff will decide El Yunque's fate as the planet warms in the Anthropocene, an age when humans have overcome nature as the prime shapers of the planet. And this forest, and other tropical ecosystems like the Amazon, will, in turn, dictate Earth's future.