Assumptions of the response of the ecosystem to rise in CO2 and pollution levels are incorrect
By Michael Smith (Veshengro)
North American forests appear to have a greater capacity to soak up heat-trapping carbon dioxide gas than researchers had previously anticipated.
The results of a 12-year study at an experimental forest in northeastern Wisconsin challenge several long-held assumptions about how future forests will respond to the rising levels of atmospheric carbon dioxide blamed for human-caused climate change, said University of Michigan microbial ecologist Donald Zak, lead author of a paper published online this week in Ecology Letters.
“Some of the initial assumptions about ecosystem response are not correct and will have to be revised,” said Zak, a professor at the U-M School of Natural Resources and Environment and the Department of Ecology and Evolutionary Biology in the College of Literature, Science, and the Arts.
To simulate atmospheric conditions expected in the latter half of this century, Zak and his colleagues continuously pumped extra carbon dioxide into the canopies of trembling aspen, paper birch and sugar maple trees at a 38-acre experimental forest in Rhinelander, Wis., from 1997 to 2008.
Some of the trees were also bathed in elevated levels of ground-level ozone, the primary constituent in smog, to simulate the increasingly polluted air of the future. Both parts of the federally funded experiment - the carbon dioxide and the ozone treatments – produced unexpected results.
In addition to trapping heat, carbon dioxide is known to have a fertilizing effect on trees and other plants, making them grow faster than they normally would. Climate researchers and ecosystem modelers assume that in coming decades, carbon dioxide's fertilizing effect will temporarily boost the growth rate of northern temperate forests.
Previous studies have concluded that this growth spurt would be short-lived, grinding to a halt when the trees can no longer extract the essential nutrient nitrogen from the soil. But in the Rhinelander study, the trees bathed in elevated carbon dioxide continued to grow at an accelerated rate throughout the 12-year experiment. In the final three years of the study, the CO2-soaked trees grew 26 percent more than those exposed to normal levels of carbon dioxide.
It appears that the extra carbon dioxide allowed trees to grow more small roots and “forage” more successfully for nitrogen in the soil, Zak said. At the same time, the rate at which microorganisms released nitrogen back to the soil, as fallen leaves and branches decayed, increased.
“The greater growth has been sustained by an acceleration, rather than a slowing down, of soil nitrogen cycling,” Zak said. “Under elevated carbon dioxide, the trees did a better job of getting nitrogen out of the soil, and there was more of it for plants to use.”
Zak stressed that growth-enhancing effects of CO2 in forests will eventually “hit the wall” and come to a halt. The trees' roots will eventually “fully exploit” the soil's nitrogen resources. No one knows how long it will take to reach that limit, he said.
This is, however, once again but an assumption of the scientists and does not meant that that will actually happen, in the same way that they now have to rethink their previously held claims and beliefs that forests of the future would not be able to cope with increased CO2 (if that is happening even) and would become sources of CO2 themselves.
The ozone portion of the 12-year experiment also held surprises. Ground-level ozone is known to damage plant tissues and interfere with photosynthesis. Conventional wisdom has held that in the future, increasing levels of ozone would constrain the degree to which rising levels of carbon dioxide would promote tree growth, canceling out some of a forest's ability to buffer projected climate warming.
In the first few years of the Rhinelander experiment, that's exactly what was observed. Trees exposed to elevated levels of ozone did not grow as fast as other trees. But by the end of study, ozone had no effect at all on forest productivity.
“What happened is that ozone-tolerant species and genotypes in our experiment more or less took up the slack left behind by those who were negatively affected, and that's called compensatory growth,” Zak said. “The same thing happened with growth under elevated carbon dioxide, under which some genotypes and species fared better than others.”
“The interesting take home point with this is that aspects of biological diversity – like genetic diversity and plant species compositions – are important components of an ecosystem's response to climate change,” he said. “Biodiversity matters, in this regard.”
We do have to remember one thing, however, and that is that if those trees are “allowed” to stand over the maximum span they, themselves, in fact, will release CO2 rather than absorb it and it is here were forestry comes in.
I would say that, you might say, being a professional commercial forester by original trade, but it is the truth. There is a certain lifespan when a tree is actually “healthy”, so to speak, and is absorbing CO2 and then come the phase when the tree will actually exhale carbon.
Therefore it is important that forests are managed in a proper system and are not just allowed to stand there while we pretend that everything is hunky dory and fine. This is important especially when we want forests and woodlands to act as serious carbon sinks. At the same time we also benefit from the wood, though a strict regime of sustainable management and replanting is required.
Sustainable wood is more important today than ever before and we must replace a lot of the stuff that we use plastics for today with wood and wood products again, as used to be the case.
Wood for heating, whether in the form of split logs, chips or briquettes or pellets, is carbon neutral, as wood, when burned, only releases the amount of CO2 that the tree absorbed during its productive life.
Leaving wood to go into decay, as with habitat piles and such like, is harmful, as not only the absorbed CO2 is being released but also methane gas during the process of decay. The latter being a GHG that is said to be 40 times more dangerous than CO2 as regards to climate change.
In addition to that said habitat piles also harbor diseases and are a fire hazard in forests and woodlands, despite what misguided environmentalists may claim. During the old times when there was, basically, a clean forest floor policy with anything that could not be utilized burned on site, though enough litter was always there, there was more wildlife, including fungi and invertebrates, than there are today. But, there were fewer tree diseases.
© 2011
