By Guy Gugliotta, New York Times
New York, NY (November 26, 2012) – How do trees growing in urban areas stack up against the same trees growing in more rural areas? Now add global warming to that mix. Scientists at Columbia University have been studying essentially that—the effect of urban heat islands on trees and what that can tell us about how rising levels of heat and emissions might provide at least some benefits for the planet. This includes the role of carbon dioxide and some trees growing faster and with bigger leaves in cities than rural areas.
Heat, carbon dioxide and air pollution are already having significant effects on trees, plants and crops, and for most plant scientists, the debate over climate change ended long before the arrival of extreme weather like Hurricane Sandy.
Now, some of those scientists have moved beyond political questions to explore how rising levels of heat and emissions might provide at least some benefits for the planet.
“There is a lot of emphasis on the mitigation of global warming, and we need that,” said Lewis H. Ziska, a plant physiologist for the Department of Agriculture, who is one of a growing number of scientists studying how plants react to elevated levels of greenhouse gases and other pollutants. At the same time, he added, “we need to think about the tools we have at hand, and how we can use them to make climate change work for us.”
Among the tools are cities, which have conditions that can mimic what life may be like in the temperate zone of a heated planet.
“The city is our baseline for what might happen in future decades, and with all the negative effects global warming may have, there may be a bit of a silver lining,” said Stephanie Searle, a plant physiologist who led a Columbia University research project on tree growth, and now works as a biofuels researcher at the nonprofit International Council on Clean Transportation. “Higher nighttime temperatures, at least, may boost plant growth.” Robust growth takes more carbon dioxide out of the atmosphere.
Still, some emissions are not helpful to plants. There are also plenty of modern pollutants, like ozone and heavy metals, which are toxic to plants, to humans or to both. And so far, the long-term effects on plant life on a heated planet are unclear. “I try to avoid words like ‘good,’ ‘bad,’ ‘detrimental’ or ‘beneficial,’ ” said Kevin L. Griffin, an ecophysiologist at Columbia University who participated in a study about the “heat island effect” on the red oak trees in New York.
The effects of higher, mostly urban emissions are what prompted Dr. Ziska to reappraise global warming as a potential benefit to humanity. In an essay last summer in the journal Proceedings of the Royal Society B, Dr. Ziska and a group of colleagues from across the world argued that an expected increase in world population to 9 billion people from 7 billion by 2050 necessitated a “green revolution” to enhance yields of basic grains. Carbon dioxide, the group suggested, could be the answer.
Since 1960, world atmospheric carbon dioxide concentrations have risen by 24 percent to 392 parts per million and could reach 1,000 parts per million by the end of this century.
While plants need carbon dioxide and generally perform better with higher concentrations of it, “not every species responds,” Dr. Ziska said. This may be especially true of domesticated species, which have little genetic diversity.
“Breeders are not actively selecting for CO2 response,” Dr. Ziska said. “They are more interested in drought resistance and pest control.”
In the wild, however, “nature selects for whatever works,” he said. “Our hypothesis is that nature in the wild will select much faster for CO2.”
Dr. Ziska said that his research focuses on rice, but that scientists should also be able to find the wild progenitors of “soybeans, wheat, oats and on down the line.” If they are successful, “we get a double value,” he said.“What we want is to absorb more CO2 and exploit the CO2 as a means to increase yield. That’s the goal.”
In New York, the Columbia researchers studied for eight years the growth of red oak seedlings at four locations, including an “urban” site near the northeastern edge of Central Park at 105th Street and a “remote” site in the Catskills 100 miles north of Manhattan near the Ashokan Reservoir.
Dr. Griffin, who supervised the project in conjunction with the Black Rock Forest Consortium of upstate New York, chose red oaks because they are a native New York species. He said he wanted his students to see if they could figure out whether city oaks grew differently from country oaks, and, if so, why.
Cities produce high levels of atmospheric carbon dioxide, nitrogen oxides and ozone, all of which influence plant performance.
The heat island effect arises because buildings, pavement and asphalt are better at absorbing and retaining solar energy than the fields and forests of the countryside. During the hot months, the city’s stored energy radiates back into the atmosphere after dark, keeping nighttime urban temperatures markedly higher than rural temperatures.
The Columbia team’s first red oak experiments ended in 2006, and average minimum temperatures in August were 71.6 degrees at the city site, but 63.5 degrees in the Catskills. Researchers also noticed that the city oaks had elevated levels of leaf nitrogen, a plant nutrient.
The team did two more rounds of experiments, then in 2008 made a final outdoor test using fertilized rural soil everywhere so all the seedlings got plenty of nitrogen. The urban oaks, harvested in August 2008, weighed eight times as much as their rural cousins, mostly because of increased foliage.
“On warm nights, the tree respires more,” Dr. Griffin said. “It invests its carbon sugars to build tissue.” By morning, the tree’s sugars are depleted, and it has to photosynthesize more during the day, he continued. The tree grows more leaves and gets bigger.
Still, it is clear that there are some emissions that are not helpful to plants, even in the north. The inspiration for the red oak experiments was a 2003 study in the journal Nature describing how cottonwoods grew twice as fast in New York City as they did in the country. But in that case, the difference in growth was not a matter of benefits from city emissions. Rather, cottonwoods in the country faced higher concentrations of atmospheric ozone, which stunted their growth.
Jillian Gregg, an ecologist who led the study, said that while cottonwoods were sensitive to ozone, many plants are susceptible to its effects. Ozone, or O3, a three-atom molecule of oxygen, can severely damage plant pores, causing them to grow more slowly.
The elevated ozone comes from the city, where nitric oxide from automobile exhaust and factories becomes a catalyst enabling free oxygen atoms to combine with atmospheric oxygen, O2, to create ozone molecules. But much of the urban ozone eventually reverts back to O2. The ozone that does not change back blows out to the country.
Scientists caution that while the studies of New York City in August may be a way to preview what the temperate zone might be like in the future, lush parks during northern summers could mean trouble in hotter latitudes. Also, if trees grow faster for a couple of years, that says nothing about how their root systems might handle drought or windstorms after 100 years.
Old growth city oaks, Dr. Griffin noted, are no bigger than old growth country oaks.
Original article source: New York Times, Looking to Cities, in Search of Global Warming’s Silver Lining