Ice and glaciers covered much of North America 26,000 years ago. As they advanced, they pushed many tree species south. When they receded, some species did not move back north. The Torrey pine is an example. The rarest pine in North America, it grows wild only on a few strips of the southern California coastline. And yet, it can survive through Boston’s cold winters, withstanding harsher temperatures than those in its current native range.