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PRI's Environmental News Magazine

Forest Canopy & Climate

Air Date: Week of January 16, 1998

Forests play an important role in regulating the earth's climate by filtering heat-trapping carbon dioxide out of the atmosphere. They eliminate so much CO-2 that some governments are counting on better forest management to help slow down climate change that's been set in motion by the use of carbon based fuels. But, little is known about how specific types of forests, in specific areas, interact with the atmosphere. How much carbon do they absorb and hold? How much carbon is released when they are cut or destroyed by fire? How do the forests themselves respond to changes in the climate? Orlando De Guzman (de- gooz-MAAN) of member station KUOW in Seattle traveled to the old growth forests and research labs of the Pacific Northwest to talk with scientists looking for answers to these crucial questions on the fragile relationship between trees, the atmosphere, and the world's climate.


KNOY: Forests play an important role in regulating the Earth's climate. They suck heat-trapping carbon dioxide out of the atmosphere, so much CO2 that governments are counting on better forest management to help slow down climate change that's been set in motion by the use of carbon-based fuels. But little is known about how specific types of forests in specific areas interact with the atmosphere. Exactly how much carbon they absorb and hold. How much carbon is released when they are cut or destroyed by fire. Or how the forests themselves respond to changes in the climate. Orlando De Guzman of member station KUOW in Seattle traveled to the old growth forests and research labs of the Pacific Northwest to talk with scientists looking for answers to these crucial questions.

(Clanking sounds)

SHAW: Hey, Bill. It's Dave. Ready to come up on the load. So we'd like to come up to about as high as we can go and ...

DE GUZMAN: Deep in the shadows of an old growth forest in Washington's Cascade Mountains, a team of scientists boards the yellow steel cage of a construction crane, which rises from a concrete base and disappears into an umbrella of deep green needles and heavy branches.

(More clanking sounds)

DE GUZMAN: The cage shakes and then slowly rises up a narrow canyon of massive Douglas firs, passing silvery snags and ancient Gothic spires that jut into the crisp autumn sky. At 250 feet, the canopy is a delicate web of lichen-draped branches.

SHAW: You'll notice that the forest here is a very rough upper surface topography. there's a lot of dead wood in the upper tree crowns here. And you'll also notice that the foliage on the trees is very much vertically oriented; it's not like the flat tropical forests where you tend to get what's called a canopy roof. Here, the roof has an awful lot of holes in it. (Laughs)

DE GUZMAN: David Shaw is the site manager for the canopy crane, which is run by the University of Washington. there's been a lot of research on the ecosystems of forest canopies in recent years. But Dr. Shaw says this is the first to study the exchange of gases between the atmosphere and the forest. In this suspended world of emerald needles, photosynthesis converts vast amounts of carbon dioxide from the air into carbohydrates, which are stored in the trees' wood and foliage; and oxygen, which is released back into the atmosphere. It's a basic biochemical process that every green plant performs, and that has a profound impact on local and global climate. But in many ways, scientists are only beginning to understand it.

SHAW: What they'd like to do, Bill, is essentially have the jib pointed right over the top of the tree...

DE GUZMAN: The cage inches closer to the top of the tallest Douglas fir on the site, where forest ecologist Bill Winner has been conducting a Department of Energy study on leaf photosynthesis.

WINNER: We have basically 2 tripods here. One tripod has a chamber attached to it that allows us to measure photosynthesis and other physiological properties of needles. So we're going to put the branches into -- a branch tip into the chamber.

DE GUZMAN: Dr. Winner's instruments measure exactly how much carbon dioxide the needles are absorbing every second, providing a detailed transcript of how this branch is interacting with the atmosphere. He can then extrapolate how much CO2 this whole tree, with perhaps 60 million needles, might be drawing out of the air.

WINNER: If we understand the carbon use of an individual tree, we can begin to think about how trees aggregated together could represent a stand and how that stand or system could handle carbon. And as such, begin to think about how the forest as a whole plays into the global carbon cycle.

DE GUZMAN: Scientists need to know how much carbon is being cycled through the Earth's vegetation, because carbon dioxide in the atmosphere traps heat form the sun, and increased levels of the gas from fossil fuel burning are causing the Earth to heat up slightly and disrupting global weather systems. We know roughly how much of the gas is being put into the atmosphere, but have only the vaguest estimate of how much is being drawn out. Dr. Winner hopes this project will help to fill in at least a small part of the picture.

WINNER: One of the questions, then, for this old growth forest stand is, is it a sink for carbon dioxide, which would mean that it's helping prevent further increases in atmospheric CO2? Or is this forest stand a source of CO2? That is, is it aggravating the problem of increasing CO2 in the Earth's atmosphere?

DE GUZMAN: This forest could be a source of CO2, because while trees absorb carbon while they're growing, they release it when they die and start to decay. Dr. Winner's initial measurements suggest that this particular old growth forest may be a sink for carbon dioxide, locking carbon away in logs, branches, roots, and soil.

(Clanking noises; footfalls and ambient voices)

DE GUZMAN: While Dr. Winner is looking up at Northwest forest canopies, other scientists are looking down at the forest floor.

HARMON: Well, the thing about forests is, of all the types of ecosystems there are on Earth, they store the most carbon.

DE GUZMAN: About 200 miles southeast in the Cascades, ecologist Mark Harmon clambers over the rotting remnants of gigantic moss-covered trees at Oregon's H.G. Andrews Experimental Forest. Dr. Harmon's research on dead wood has helped establish just how significant a sink for carbon these old growth forests are.

HARMON: they store it both in their living parts and in the soil, and in this detrital litter layer. That's what's bizarre. Not only is the living tissue of these forests gigantic, I mean, superlative on a world scale, but also the dead parts are just incredible.

DE GUZMAN: In fact, Dr. Harmon has found, the ancient Douglas fir forest of the Pacific Northwest can store 5 times as much carbon per hectare as a tropical forest. But because they hold so much carbon, these forests also pose a danger. When they're removed or altered, a massive amount of carbon dioxide is released back into the atmosphere. Dr. Harmon says intensive clear-cutting in the Pacific Northwest has turned the region from a carbon sink into a carbon source.

HARMON: Yes, it has been a source of carbon probably for well over 50 years. And a major source. Because the forests are so large and they store so much carbon, when you disturb them the consequences are quite large in terms of carbon flux.

DE GUZMAN: Dr. Harmon says that if you want to keep the largest possible amount of carbon out of the atmosphere, the best thing to do with at least these old growth forests is to leave them alone. He says his research contradicts the argument of some timber industry supporters that cutting and then replanting these forests could suck up more carbon dioxide.

HARMON: While it is true that young trees generally do grow faster than older trees, it isn't necessarily relevant to the issue. The issue really depends on which system stores more carbon on average. Then you look at a younger forest, some of them can accumulate carbon at fairly high rates, but the store is much lower.

DE GUZMAN: But the ability of these forests to hold onto carbon and help regulate the climate is complicated by the fact that forests also respond to changes in the climate. That's where our third researcher comes in.

(Splattering water)

DE GUZMAN: In her laboratory at the University of Oregon, paleoecologist Cathy Whitloc is hoping to get a handle on how forests might respond to the warming temperatures of the present by looking at the past. Specifically, at 21,000-year-old mud.

WHITLOC: We bring the cores here and we slice them open, and we take samples of them...

DE GUZMAN: At the end of the last Ice Age, Pacific Northwest forests moved north with the warming climate. Along the way, each tree species left distinct grains of pollen, which fossilized in lake bed sediments. Following this trail of pollen, Dr. Whitloc has calculated the ability of forests to move with the changing climate.

WHITLOC: When you look at any of these records, they suggest that species are able to move across the landscape at a rate of 300 to 900 meters per year. So something just under a kilometer per year to go from glacial conditions to the warm conditions of the Holocene.

DE GUZMAN: But temperatures are predicted to rise far faster in the near future than they did then. And Dr. Whitloc fears forests may not be able to keep up with the changes.

WHITLOC: You're asking species to move at rates that are maybe 50 times that fast to be in equilibrium with climate. We really have no evidence to expect that species will be in equilibrium with the sorts of climate changes that are predicted in the future.

DE GUZMAN: Finally, while the past is providing some clues about the fate of forests in the changing climate, computer projections of the future are providing others.

(Humming sounds)

DE GUZMAN: At the US Forest Service Research Station in Corvallis, Oregon, bioclimatologist Ron Neilson is using a sophisticated computer model as a sort of crystal ball to see the future of forests. And the picture isn't entirely bleak. While some forests might shrink, others might grow.

NEILSON: the world could get greener, and we could see more vegetation growth and forests potentially expanding into savannahs and grasslands.

DE GUZMAN: That's because along with temperature, trees also respond to carbon dioxide itself, which is essentially a sort of fertilizer. One possible result of more CO2 in the atmosphere could be more and bigger trees. But it's not that simple, because along with higher temperatures, more CO2 in the atmosphere will bring more volatility to the climate, more precipitation and droughts, hotter heat waves, and colder freezes. Dr. Neilson's computer model shows that warmer temperatures will put forests under stress.

NEILSON: When the trees get stressed, then bugs come in and you get infestations that produce an overkill of vegetation. So perhaps the drought would only have killed a fraction of the trees in there. The bugs will kill much more than that. And than you have a setup potentially for large catastrophic fires, because you've produced a huge fuel load.

DE GUZMAN: And more dying forests and forest fires, Dr. Neilson's models predict, will only release more carbon dioxide into the atmosphere and increase the volatility of the climate still more, in what's called a positive feedback loop.

NEILSON: The hotter the average simulated temperature increase for planet Earth is, the larger is the area of simulated forest die-back. So we're in a bit of a concern with regard to how hot we actually let the planet get. The hotter it gets, the greater is the likelihood for these positive feedback mechanisms to kick in and make it even hotter.

DE GUZMAN: And Dr. Neilson worries that there's a frightening imbalance between the rate at which forests can help suck up more carbon dioxide on the one hand, and the rate at which they will be damaged by the effects of rising CO2 on the other.

NEILSON: You can kill off forests and burn them up and have what remains decompose and emit carbon into the atmosphere much faster than you can pull that carbon back out of the atmosphere by regrowing new forests somewhere else.

DE GUZMAN: Dr. Neilson's computer models, together with the research being done on the forests themselves, present a vexing dilemma. the more we learn about the importance of forests in regulating the global climate, the more we also learn about the forests' vulnerability to a changing climate. Governments are hoping that preserving and planting forests may help us regulate CO2 levels without having to drastically reduce fossil fuel use. But scientists seem to be suggesting that the only way to ensure that forests are vibrant and healthy may be to reduce fossil fuel use first. For Living on Earth, I'm Orlando De Guzman.



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