A long-running experiment is testing tree mixes to develop the healthiest forests. Mickey Pullen/Smithsonian Environmental Research CenterAround the world, people plan to plant more than 1 trillion trees this decade in an ambitious effort to slow climate change and reduce biodiversity loss. But if the past is prologue, many of those planted trees won’t survive. And if they do, they could end up as biological deserts that lack the richness and re…