SAN FRANCISCO — As hotter and bigger fires blaze through the West and yet another year passes with a disastrous toll, America’s wildlands are having a harder time bouncing back. …The fiercer fires are killing more of the vegetation needed to provide seeds for regrowth, and scientists are learning that even when new trees sprout, many are struggling with the warmer and more extreme weather wrought by climate change. …The latest research on forest regeneration, published last month in the journal Ecology Letters, found that as much as one third of the areas burned since 2000 saw no trees return, about double the treeless land that resulted after fires before 2000. In 54 percent of the areas burned this century, the research suggests, too few trees grew back to ensure a full forest recovery.