SMRs and AMRs

Sunday, July 06, 2008

Rising temperatures mean we're headed for a 'weedy' world

Field of Canadian thistleBy Don Gordon

How plants will respond to global warming is of utmost interest. Which ones will survive? Which will perish? How will elevated temperature affect crop plants? Unfortunately the answers to those questions are far from definitive. Last week, Tom Christopher, writing in the New York Times, reported on a recent research project that is attempting to provide some answers on how the climate crisis might affect a group of plants we commonly call weeds.

What is a weed? Well, there are a myriad of definitions. One of my former professors said they were just a resource out of place. Emerson described a weed as “Any plant whose virtues have not been discovered.” Lowell said they were “no more than a flower in disguise.” No matter how they are defined, weeds have remarkable survival characteristics. For example, curly dock and mullein seeds will germinate after 70 years. Dandelions have managed not only to survive the best chemical arsenal known to man, but they have also been able to geometrically increase each year.

Some information on how plants will respond to global warming may be obtained by putting plants in a growth chamber and manipulating the temperature and carbon dioxide, but the results from a closet sized experiment may not be representative of what will happen in nature.

Lewis Ziska, a weed scientist with the U.S. Department of Agriculture, decided it was time to conduct research in the real world. He looked for three locations that might mimic not only present conditions, but also the mid-century future as predicted by the Intergovernmental Panel on Climate Change (IPCC) and something in between. He selected three sites. An organic farm in western Maryland was chosen to represent present conditions, a park on the edge of Baltimore for the intermediate site and one in downtown Baltimore for 2050 conditions. He took soil known to contain 35 common weeds from the organic site and established uniform plots in each of the rural, suburban and urban sites.

After five years, the plots were analyzed and the results even surprised the author. Weeds grew much larger in the urban areas where temperatures were hotter and carbon dioxide levels were elevated. In Baltimore, temperatures are three to four degrees hotter than the surrounding countryside. Carbon dioxide levels in the 440-450 parts per million in the urban areas were well above the national average. To put that in perspective the carbon dioxide level 50 years ago was 310 p.p.m.

Weeds loved this urban habitat. For example, the weed, Chenopodium alba, lamb’s quarters, was 6-8 feet tall on the organic farm, but a gigantic 10-12 feet in the urban area. In the urban area, succession was also greatly accelerated. In a normal situation annuals are the first invaders of bare sites, then perennials come in and are followed by trees and shrubs. The time period to woodland normally takes decades, but in the hotter carbon dioxide enriched area, trees covered the plots in just five years. This urban area looked as though it had been on steroids. For example, fast growing “weed” trees such as Ailanthus (tree of heaven), Norway maples and mulberries dominated the plots. The suburban park area with slightly lower temperatures and carbon dioxide levels followed the lead of the city environment, but it was a few years behind.

Ziska found that common weeds like Canadian thistle and quack grass are harder to control when carbon dioxide levels are increased. They not only grow faster, but are more resistant to herbicides. Fast growing weeds like ragweed may be bad news for allergy suffers. When carbon dioxide levels reach 600 parts per million (the level predicted at the end of this century) they produce twice as much pollen as plants did in 1957. It will also be bad news for individuals who come in contact with poison ivy because the plants will have more of the chemical that causes skin rash.

Are weeds growing faster now? Ziska believes there is good evidence that some are. For example, Cheat grass, Bromus tectorum, has been increasing since the Industrial Revolution. This plant has spread over 100 million acres in the West and it has destroyed many species of native grasses along the way. It is also very combustible, but it is able to survive fire while native species cannot. At carbon dioxide levels of 420 p.p.m, the growth rate of cheat grass increases significantly. That means the West can expect even more grassland fires in the future.

Another weed that is growing rapidly is kudzu, which is commonly known as "the weed that ate the South." This species has already spread to Illinois and by 2015 some researchers are predicting it may be as far north as the Upper Peninsula of Michigan. This vine would be a disaster in Minnesota.

Before man’s arrival there were probably no such things as a weed. Weeds appeared when agriculture began about 10,000 years ago. Any unwanted plant that showed up in an agricultural field was declared a weed. The more we attempted to eliminate these weeds the more resilient some became. Along the way, we bred the diversity out of our crop plants and made them more genetically uniform. The weeds that survive today are much better adapted to a changing climate than our crop plants because of their diverse gene pool. According to Ziska, “When you change a resource in the environment, you are going to, in effect, favor the weed over the crop. There is always going to a weed poised genetically to benefit from almost any change.”

The clear message here is that we are headed for a “weedy” world. That may not be all bad because these weeds may help solve the climate crisis. How, because they contain the genes for adaptability. Transferring genes from weeds to crop plants may be our way out of a bleak future.
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Don Gordon is professor emeritus of botany at Minnesota State University-Mankato.

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