Spring comes earlier to cities

Spring has been arriving earlier in recent years, but it's especially early for urban dwellers, new research shows.

According to a study published in the February issue of the journal Global Change Biology, the growing season is two to three weeks longer in cities in the Chesapeake Bay region than it is in the exurbs. 

After analyzing 25 years' worth of high-resolution satellite images of the mid-Atlantic region, researchers with the University of Maryland Center for Environmental Science, the U.S. Geological Survey and Harvard University determined that leaves on trees start budding out about five days sooner in city centers than they do about 20 miles out, and they hang around about 10 days or so longer at the end of summer.

"The trees are keeping greenness in their leaves and keeping leaves longer in the fall in the city centers in the mid-Atlantic, and in the spring they're greening up earlier, too," says Andrew Elmore, the study's lead author and a geologist with the university's Appalachian Laboratory in Frostburg.

The reason most likely is what's known as the "urban heat island effect," with cities' extensive pavement and rooftops soaking up heat from the sun and then radiating it back into the urban air.

While that might mean urban gardeners can cheat a bit on when to start planting, Elmore says it doesn't guarantee that vegetation or trees grow any bigger or better in cities than they do in the countryside.  Moisture, nutrients and even air quality also influence plant growth.

The researchers also found that the growing season tends to be longer close to the bay.  Elevation, however, seems to be a particularly strong factor, but not necessarily as one might expect. Trees keep their leaves longer along the Blue Ridge Mountains, Elmore said, up to an elevation of about 275 meters, or 900 feet.  Above that, the growing season shortens.

For more on the research, go here.



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