Warming trend visible in the trees

Posted 1/15/2007 10:57 PM ET

By Patrick O'Driscoll, USA TODAY

Rising temperatures are allowing Southern trees to thrive farther north and stressing trees used to colder weather, according to new national guidelines issued by planting experts.

The National Arbor Day Foundation last month updated the Agriculture Department's "hardiness zones" map, which was last issued in 1990. The group acted after noticing that some tree species were thriving where they had not before, while others were doing poorly in what had been a suitable region on previous maps.

The shift in zones may allow people in northern areas to experiment with flowering Southern trees such as apple and cherry where they used to plant only fir, spruce and pine, says group spokesman Woodrow Nelson.

The map divides the nation into 11 planting zones tied to average low temperatures. It shows significant boundary changes as the continent has warmed. For example, in southern Texas, the edge of one zone moved more than 200 miles north to the Panhandle. A few locations jumped two zones.

The map is based on 15 years of minimum temperatures from 5,000 observers used by the National Weather Service.

Last year was the warmest on record for the USA. Twelve of the top 25 warmest years have been since 1990.

The foundation cautions not to dig up gardens or cut down trees just because zones have shifted. The map is a guide for new plantings, especially for seedlings, which are more vulnerable to temperature change than mature trees.

The Agriculture Department has been reworking its map for several years using 30 years of data instead of 15 but has set no release date. The department's Kim Kaplan says the 1990s "were a very warm decade, but is that a weather cycle or is that a climate cycle? We think 30 years represents the best compromise, smoothing out the natural fluctuations of weather."

Nelson says the foundation will distribute 10 million seedlings this year. "Those need to be planted under the most up-to-date information."

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