GUNNARSHOLT, Iceland — With his flats of saplings and a red planting tool, Jon Asgeir Jonsson is a foot soldier in the fight to reforest Iceland, working to bring new life to largely barren landscapes. The country lost most of its trees more than a thousand years ago, when Viking settlers took their axes to the forests that covered one-quarter of the countryside. Now Icelanders would like to get some of those forests back, to improve and stabilize the country’s harsh soils, help agriculture and fight climate change. But restoring even a portion of Iceland’s once-vast forests is a slow and seemingly endless task.  “It’s definitely a struggle,” said Mr. Jonsson, a forester who works for the private Icelandic Forestry Association. Even in a small country like Iceland, a few million trees a year is just a drop in the bucket.