Future Forests May Soak Up More Carbon Dioxide Than Previously Believed

[SVT450R]

Found this article interesting just shows we still have things to learn.

Source

North American forests appear to have a greater capacity to soak up heat-trapping carbon dioxide gas than researchers had previously anticipated.

As a result, they could help slow the pace of human-caused climate warming more than most scientists had thought, a U-M ecologist and his colleagues have concluded.

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.

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."

[OHSnow]

Unfortunately, it looks like this only looked at the effects of elevated carbon dioxide on tree growth. One would imagine that changes in weather and climate could also effect the rate of growth in time. And that these effects could vary from region to region and from species to species.

[skierinvermont]

Well if carbon fertilization does lead to somewhat faster tree growth than expected, that is good news, but it will only have a marginal impact on slowing the rate of CO2 accumulation in the atmosphere. It's the difference between CO2 rising at 2.9ppm/y instead of 3.0ppm/y. Every little bit counts but don't get carried away in exaggerating the significance of this finding.

It also needs to be reproduced across a range of forest communities in different climates and with different N availability. Forests with less N availability will respond much less to CO2 fertilization than forests with greater N availability.

[tamarack]

Well if carbon fertilization does lead to somewhat faster tree growth than expected, that is good news, but it will only have a marginal impact on slowing the rate of CO2 accumulation in the atmosphere. It's the difference between CO2 rising at 2.9ppm/y instead of 3.0ppm/y. Every little bit counts but don't get carried away in exaggerating the significance of this finding.

It also needs to be reproduced across a range of forest communities in different climates and with different N availability. Forests with less N availability will respond much less to CO2 fertilization than forests with greater N availability.

It would be interesting to know what other studies of this sort might be in progress. I'd expect these results will hold relatively true for Northern Hemisphere temperate hardwoods in both E and W Hemispheres, and that the "nitrogen wall" will be reached rather quickly in tropical rainforests, where so little of the essential nutrients reside in the soil. That would leave the northern coniferous forests as the huge unknown.

If these results were to be limited to N.Hem temperate HW, then your "marginal impact" comment is likely to hold true. Add the Taiga, and the effect would be much more significant. Even though growth there is relatively slow, area is immense. (It's also interesting that two of the three species used in the study, paper birch and aspen, are found pretty much all the way north to treeline.)

[tacoman25]

Well if carbon fertilization does lead to somewhat faster tree growth than expected, that is good news, but it will only have a marginal impact on slowing the rate of CO2 accumulation in the atmosphere. It's the difference between CO2 rising at 2.9ppm/y instead of 3.0ppm/y. Every little bit counts but don't get carried away in exaggerating the significance of this finding.

It also needs to be reproduced across a range of forest communities in different climates and with different N availability. Forests with less N availability will respond much less to CO2 fertilization than forests with greater N availability.

And hopefully such exhaustive and thorough research is also being applied to all aspects of future climate impacts.

[WeatherRusty]

Presumably this process has been ongoing during the rise in atmospheric CO2 from the long term background level of 280ppm to the 390pmm of today. Regardless, we are pumping CO2 into the atmosphere much more quickly than natural sinks have been able to keep up, thus the rise in atmospheric concentration has occurred and is by all accounts accelerating. Even if natural CO2 sinks are becoming more effective at scrubbing the atmosphere of CO2, it is a loosing battle we, the oceans and the trees can not win.

[skierinvermont]

It would be interesting to know what other studies of this sort might be in progress. I'd expect these results will hold relatively true for Northern Hemisphere temperate hardwoods in both E and W Hemispheres, and that the "nitrogen wall" will be reached rather quickly in tropical rainforests, where so little of the essential nutrients reside in the soil. That would leave the northern coniferous forests as the huge unknown.

If these results were to be limited to N.Hem temperate HW, then your "marginal impact" comment is likely to hold true. Add the Taiga, and the effect would be much more significant. Even though growth there is relatively slow, area is immense. (It's also interesting that two of the three species used in the study, paper birch and aspen, are found pretty much all the way north to treeline.)

Yes that's probably a good guess about greater N availability in temperate and taiga vs tropical.

All I'm working on is this brief summary of the paper.. we could probably learn much more by reading the actual paper. For instance, is this a mature old-growth forest they're working with or a younger one? If it's a younger one, then the results may be restricted to only other young forests because the mechanism of fostering more root growth wouldn't hold true in old growth forests. And there's probably less N availability in old growth forests.

IIRC much of the taiga is old growth isn't it? So it's unlikely these results would apply there.

The other thing this study leaves out is even though NPP is up, meaning carbon sequestration is up, they also mention that N-cycling was increased by an increase in leaf drop and decomposition and this would likely be accompanied by an acceleration in the carbon cycle as well. It doesn't do any good to have trees grow faster if that means they drop branches and leaves and get old and die faster. The CO2 is just released straight back.

The primary way to get long-term carbon sequestration isn't NPP I don't think because all that leads to is an acceleration in the carbon cycle... what you need to sequester carbon is increased standing biomass. So in the long-run the real question is does CO2 lead to an increase in standing biomass?

[tacoman25]

Yes that's probably a good guess about greater N availability in temperate and taiga vs tropical.

All I'm working on is this brief summary of the paper.. we could probably learn much more by reading the actual paper. For instance, is this a mature old-growth forest they're working with or a younger one? If it's a younger one, then the results may be restricted to only other young forests because the mechanism of fostering more root growth wouldn't hold true in old growth forests. And there's probably less N availability in old growth forests.

IIRC much of the taiga is old growth isn't it? So it's unlikely these results would apply there.

The other thing this study leaves out is even though NPP is up, meaning carbon sequestration is up, they also mention that N-cycling was increased by an increase in leaf drop and decomposition and this would likely be accompanied by an acceleration in the carbon cycle as well. It doesn't do any good to have trees grow faster if that means they drop branches and leaves and get old and die faster. The CO2 is just released straight back.

The primary way to get long-term carbon sequestration isn't NPP I don't think because all that leads to is an acceleration in the carbon cycle... what you need to sequester carbon is increased standing biomass. So in the long-run the real question is does CO2 lead to an increase in standing biomass?

There have also been studies that suggest increased CO2 doesn't just allow faster growth, it also allows more growth...meaning more standing biomass would be present.

[skierinvermont]

There have also been studies that suggest increased CO2 doesn't just allow faster growth, it also allows more growth...meaning more standing biomass would be present.

Yes I know that and that is why all climate models assume CO2 fertilization will lead to a net flux of carbon into terrestrial ecosystems.

The question is how much of a factor will this be given limited nitrogen. What I am pointing out is that one limiting aspect of this recent study is that it focused on NPP which isn't the same thing as CO2-fux. It's only one component of CO2 flux. If the carbon cycle accelerated, which is likely given the nitrogen cycle accelerated, that would offset much of the increase in NPP.

Their primary argument is that NPP remained elevated relative to the control even after 12 years. But that doesn't necessarily mean that the carbon flux remained elevated relative to the control. They could simply be witnessing an acceleration of nutrient cycling, both nitrogen and carbon.

[SVT450R]

If these results were to be limited to N.Hem temperate HW, then your "marginal impact" comment is likely to hold true. Add the Taiga, and the effect would be much more significant. Even though growth there is relatively slow, area is immense. (It's also interesting that two of the three species used in the study, paper birch and aspen, are found pretty much all the way north to treeline.)

Very good point about the Taiga since it makes up for 29% of the worlds forest cover hopefully there is more research being done.

[Hambone]

Here's a new technology. Develop rapidly growing plants that have high CO2 scouring capacity that can be frequently harvested and used to build biomass energy systems.

Winner for AGW, alternative energy, farming and Biotech.

[skierinvermont]

Also the fact that they measured a 23% increase in NPP even in year 12 tells me they were almost certainly working with a fairly young forest... probably much younger than most of the world's forests.

[tacoman25]

Also the fact that they measured a 23% increase in NPP even in year 12 tells me they were almost certainly working with a fairly young forest... probably much younger than most of the world's forests.

Old growth forest is rarer than young forest.

[skierinvermont]

Old growth forest is rarer than young forest.

It's not black and white... old growth is 150-300+ years... but there is a big difference between a 50 year and 100 year forest.

[tacoman25]

It's not black and white... old growth is 150-300+ years... but there is a big difference between a 50 year and 100 year forest.

True. Well, you'd think the researchers would have considered some of these things. Guess we just need more information about the study.

[tamarack]

Yes that's probably a good guess about greater N availability in temperate and taiga vs tropical.

All I'm working on is this brief summary of the paper.. we could probably learn much more by reading the actual paper. For instance, is this a mature old-growth forest they're working with or a younger one? If it's a younger one, then the results may be restricted to only other young forests because the mechanism of fostering more root growth wouldn't hold true in old growth forests. And there's probably less N availability in old growth forests.

IIRC much of the taiga is old growth isn't it? So it's unlikely these results would apply there.

The other thing this study leaves out is even though NPP is up, meaning carbon sequestration is up, they also mention that N-cycling was increased by an increase in leaf drop and decomposition and this would likely be accompanied by an acceleration in the carbon cycle as well. It doesn't do any good to have trees grow faster if that means they drop branches and leaves and get old and die faster. The CO2 is just released straight back.

The primary way to get long-term carbon sequestration isn't NPP I don't think because all that leads to is an acceleration in the carbon cycle... what you need to sequester carbon is increased standing biomass. So in the long-run the real question is does CO2 lead to an increase in standing biomass?

Old growth is a relative term when looking at taiga, as it's mainly a fire-origin forest and trees are often less than 100 yr old. The south half of taiga areas, which have the better growth and usually much the greater standing biomass at maturity, have received considerable harvesting in Asia and extensive cutting in Canada. Farther north the economics (low volume, high harvest costs) have limited timber harvest.

Though we're talking about only a minor factor in the overall carbon picture, timber products can be a good strategy, especially when used for long-lived products in place of non-renewable and/or energy-intensive resources. Even biomass for electricity adds only that carbon used for harvest and utilization, while fossil fuels add all that plus the "geologic" carbon from underground. It has been a mystery to me (and to most foresters) why LEED green building standards had been so hesitant to embrance wood-based construction materials ("They kill trees and cause deforestation disaster!!!!!") while approving of things like steel and concrete which require much more energy to produce.