In 1927 Dr. Alfred Rehder of Harvard’s Arnold Arboretum in Boston created a plant hardiness map for the United States. He divided the 48 states into 8 zones based on a survey of plant species and their survivability across the nation. In 1938, Dr. Donald Wyman, also at the Arnold Arboretum, updated Rehder’s map with 45 years of US Weather Service information, basing the map not only on plant surveys but also on climatic conditions. Wyman’s map became the standard and was updated in 1951, 1967 and 1971.
The US Department of Agriculture (USDA) published its own map in 1960, which though similar, was not identical to the Arnold Arboretum map, which remained the standard until the USDA’s 1990 update which included information from more than 5,000 weather stations across the country. After several more revisions since then, the USDA map remains the standard, though its usefulness is sometimes questioned. Zones are based on minimum average winter temperature, without regard to the amount of time spent in that temperature range, which means that, in the 1990 plot, Raleigh, NC shares the same climatic zone with parts of Vancouver Island. This is not at all to say that the map is useless—it is not—but like statistics, its information must be used with perspective and intelligence.
However, the changing hardiness zones themselves, as plotted by the USDA, can also serve as an indicator of changing climate patterns across the continent. The Arbor Day Foundation created a difference map using the 1990 and 2006 USDA publications, showing the northward shift of hardiness zones over the intervening 16 years. It is now 16 years since 2006, so another such difference map might show similar or even more northward migration of the zones.
Tomorrow: Oceanic species migration.
Be brave, and be well.
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