WASHINGTON—Promising to assist nations that lack the bargaining power to negotiate with drug manufacturers, President Joe Biden issued assurances Monday that the United States would donate Covid vaccinations to impoverished countries as soon as officials had finished inoculating all of America’s trees. “We want our friends in developing nations to know that, once we’ve reached herd immunity among our trees, you will be the beneficiaries of any doses that remain,” Biden said during a press conference, admitting that a slow rollout meant vaccines had arrived too late to prevent plant deaths in many ailing U.S. forest preserves and national parks. “Some of our redwoods and giant sequoias are hundreds, even thousands, of years old, making them incredibly vulnerable to a virus that targets the elderly. We also of course want to vaccinate any shrubs, mosses, woodland creatures, and insect pollinators vital to our forest ecosystems. But after that, we will definitely, definitely commit to an equitable distribution of vaccine supplies throughout the world.” At press time, administration officials had walked back Biden’s statement, a concession to suburban voters clamoring for their lawns and flower beds to be vaccinated immediately.