As greenhouse gas emissions nudge temperatures higher, trees’ growing ranges are shifting northward, projections from the U.S. Forest Service show. Trees near the southern edge of their geographic ranges — what scientists refer to as “plant hardiness zones” — will be left behind, while northern latitudes will welcome new species from the south. Plant hardiness zones represent the 30-year average of the coldest temperature at each location across the country. Zones increase with each 10-degree Fahrenheit (5.6-degree Celsius) interval. Factors besides extreme cold — such as soil, rainfall and humidity — influence which trees grow. “The hardiness zones have always been a useful tool — not necessarily a Bible — but a tool to help people think about what they’re planting and why and where,” said Dan Lambe, the Arbor Day Foundation’s chief executive.
