Global changes fool Mother Nature

Arbor Day Foundation's new hardiness map reflects Earth's warming

April 22, 2007|By Nancy Taylor Robson | Nancy Taylor Robson,Special to The Sun

Does it seem as though your lilacs are opening earlier than they did in your childhood? Have you noticed the dogwood, wild columbine and Virginia bluebells blooming earlier?

It's not your imagination. Though there are certainly seasonal fluctuations from year to year, as the recent cool spell can attest, studies are showing global warming is having an effect on our gardens. "Many plants are blooming weeks earlier than they used to," says David Inouye, professor of biology at the University of Maryland, College Park.

A 30-year Smithsonian Institution study of first-flowering dates from Jan. 1 to June 1 in the Baltimore-Washington area shows distinct changes in bloom times. Scientists tracked 2,500 species, then selected 100 for which they had the most years of record.

"Of 100 species, 89 were flowering earlier," says Stanwyn Shet- ler, emeritus curator of botany at Smithsonian Institution's National Museum of Natural History. "From a fraction of a day to up to 46 days earlier."

The earlier warmth has prompted a recent revision in the hardiness map. Using the temperature data from the National Oceanic and Atmospheric Administration, the National Arbor Day Foundation revised the hardiness map to reflect the fact that the South is creeping north.

The map has been adjusted twice before - once in 1965 to add data missing from the original, and then again in 1990 to incorporate changes in weather patterns.

This time, the changes are striking. Chunks of many states have warmed one full hardiness zone. Some have even jumped two zones.

While Baltimore has stayed the same, Western Maryland has shifted from Zone 6 to Zone 7 and the Easton/Oxford/Cambridge area of the Eastern Shore has moved from Zone 7 to Zone 8.

So what if spring comes earlier? Couldn't we just shift the cherry blossom festival? Plant more Southern cultivars? What's the big deal? For one thing: pollination. Just as with catching a plane, in the intricate dance of plant-and-pollinator: Timing is everything.

"There is a rather tight schedule to meet, especially for forest-floor flowers," Shetler says. "They can't come out too early or there aren't pollinators there [to enable them to reproduce]. If they bloom too late, the canopy has closed over and they don't get enough sun to go through their full cycle."

The pollinators have their own circadian clocks that determine emergence, feeding and reproduction times that are not purely reliant on warmth. If bloom time and pollen production don't coincide with pollinator emergence, insects (and insect-eating birds) starve. Without pollinators, plants don't produce seed or fruit.

Despite this critical connection, most gardeners rarely give pollination a thought. "It's a service that's provided by ecosystems," Inouye says. "But there has been growing concern over the past few years about the status of pollinators in North America."

While home gardeners may blithely assume pollination will just happen, commercial growers, keenly aware of their pollinator-dependence, often pay commercial beekeepers to transport bees into orchards and farms to pollinate their crops. But honeybees are suffering from colony collapse disorder, which kills off as much as 60 percent of a hive in a winter.

Yet it isn't just a change in pollinators that will affect gardeners. Warmer temperatures also have paved the way for a new crop of invasives coming up from the south.

"When you shift minimum winter temperatures up, that acts as less of a brake on where these invasives can grow," says Lewis Ziska, plant physiologist at the U.S. Department of Agriculture's Crop Systems and Global Change Laboratory in Beltsville.

Another spur to invasive weed growth is increased carbon dioxide (CO2) levels, a key component of global warming. Since 1960, there has been a 20 percent increase in CO2 levels outside of urban areas and a 24 percent increase in Baltimore, Ziska says.

While plants require CO2 in addition to light, nutrients and water, when one of the legs in that four-legged stool changes, it unbalances everything that rests on them. In this case, plants that do well with higher CO2 concentrations multiply, while those that don't, die.

"Unfortunately, what we've seen is selection for invasives - Tree of Heaven, Norway maple, [white] mulberry and invasive vines like [Japanese] honeysuckle, morning glory, English ivy and kudzu, which we didn't have here 20 years ago," Ziska says.

In addition to pollinator decline and invasive onslaught, plant colonies are challenged by the weather extremes we're seeing now.

"When we have rain now, we have a lot of it," says Paul Babikow, president of Babikow Greenhouses in Baltimore. "When we have drought, it lasts longer. When we have a cold spell, it's really, really cold and then two weeks later, it might be really, really warm. It's a lot more random than it used to be only 20 years ago."

How can a gardener help fight global warming?

Mitigate the impact of weather extremes on your plants with row cover and other buffers.

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