Wednesday, March 19, 2014

Stormy Weather: What Temperature Fluctuations Mean for Duke Gardens

by Erika Zambello

We have had a crazy spring in Durham this year, with temperatures fluctuating from the warm seventies to below freezing in mere days.  I sat down with horticulture director Robert Mottern and Stefan Bloodworth, curator of the Blomquist Garden of Native Plants, to learn how these extreme fluctuations are affecting the plants throughout Duke Gardens.

Ice storms are caused by freezing rain, which solidifies when it hits a leaf, twig, driveway, home, powerline or any other surface. Though it may be pretty to look at, the resulting ice is very heavy and can cause structural damage to plants, in addition to wreaking havoc on roads and power grids. Though Durham has experienced multiple ice storms this winter, we have been lucky, says Mottern. At Duke Gardens, "the ice is not heavy enough to do a lot of damage," he tells me, "but we were really close."

What has caused the most damage this winter, he says, is the cold.

There are two main ways that plants survive cold weather. "Typically speaking, they break along the line of plants that are adapted to extremely cold weather or adapted to moderately cold weather," explains Bloodworth. The former use "cell dessication" and the latter "supercooling" in order to survive. While cell dessication is rare in the Southeast (as those plants normally grow in the colder Northeast and Midwest and in Canada), plants found in North Carolina employ supercooling to combat the cold.  

What is supercooling? As temperatures drop, plants "change the chemical content of liquid inside their cells by changing the solute levels, or levels of dissolved minerals and salt within their tissue," says Bloodworth. Like anti-freeze in a car, the increase in solute levels allows the plant to lower the freezing temperature within its cells.

While this strategy works well when temperatures remain in the double digits, plants can no longer prevent freezing when the air drops below 10 degrees Fahrenheit. Because ice expands, frozen cells will rupture, causing damage to the plant. This winter Durham has experienced a few nights of bitterly cold weather, with temperatures dropping as far as 3 °F.

Photo by Robert Mottern
The damage to plants around the gardens is visible. Many of the palm species have browned out, says Mottern, and the Gardens staff must wait and see if the plants can recover. The Terrace Gardens have lost many of their bedding plants, which then had to be removed before replanting can occur. The early blooming trees, including magnolias, camellias and cherries, have lost their flowers to the freezing temperatures. While this will not kill the trees, it has disrupted their annual life cycle and will prevent certain species, like the Prunus mume cherry, from fruiting this year.

In the Blomquist Garden of Native plants, the weather has caused damage to the margins of plant leaves, which -- like human fingers, toes, noses, and ears -- are more suspeptible to the cold. In addition, the single digit temperatures have frozen plant buds.  "All of that year's leaves are hidden inside that bud," Bloodworth tells me. When the buds freeze, "the plant now has to reinvest energy to redo the work that has already been done, which takes a lot of energy and depletes a lot of stored carbohydrates." While the plants will be able to produce new buds and leaves for the upcoming growing season, if such cold winters occur multiple years in a row, plant energy reserves will be severely depleted.

While this year's cold weather and ice storms have caused damage to the plants in Duke Gardens and the greater Triangle, it is not unprecedented. "This is not abnormally cold," Mottern says, "this is really the more normal conditions that we should be having. Having these nice mild winters for the past five to 10 years has really spoiled us." But while the frigid temperatures and snow may be normal, the single digit cold remains outside North Carolina's "safety zone," says Bloodworth, as native plants have not adapted to temperatures below 10 degrees.

Duke Gardens plants are resilient, and most will recover and be as beautiful this year as they were in previous years. Still, it is important to remember that while the cold and ice storms this winter have disrupted our daily lives, they have also impacted the plant ecosystems that surround us.

Blogger and photographer Erika Zambello is a graduate student studying Ecosystem Science and Conservation at Duke's Nicholas School of the Environment.

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