Interview: Margaret Davis

Margaret Davis is a forest ecologist, but her specialty is the study of forests as they existed thousands of years ago. Dr. Davis travels back in time by analyzing fossil pollen left by the trees that made up the ancient forests. She has used this approach to track tree species as they migrated over the North American landscape when the climate changed during the Ice Ages. This investigation of the past is now helping ecologists think about the future, as they debate how global warming caused by human activities will alter forests and other ecosystems.

Dr. Davis is a member of the National Academy of Sciences and served as president of the Ecological Society of America in 1988-1989. The Ecological Society also recognized the importance of Dr. Davis's contributions when it selected her for its Eminent Ecologist Award in 1993. In this interview, Dr. Davis, who is Regents Professor of Ecology at the University of Minnesota, tells us about her science and its implications.

Dr. Davis, how did you become interested in science?

I got interested in science as a child because my father was a scientist, a geology professor at Harvard, and he talked a lot about science. My father seemed to really enjoy what he was doing, and that made an impression upon me. It seemed enviable that he had a job that he just really loved and really wanted to do. He never said he liked what he was doing; he just acted as if he did.

So did your family encourage you to become a scientist?

No, they didn't. They didn't approve of women being scientists, or even having careers of any sort, actually. However, my father encouraged it in a way, because he was most comfortable talking to his children about things he was interested in himself. So I think that he did foster my interest in science without meaning to.

Once your interest in science took form, how did it become focused on paleobotany, and more specifically on the study of fossilized pollen?

I suppose I'd always been interested in geology, and I also liked plants and field biology. Paleobotany was a combination of these interests, and I became very intrigued with the fossil record of plants. These weird plants from the past were just as intriguing to me as dinosaurs are now to children. I got interested in ecology as well, and particularly in field botany. I like to go out hiking, especially in the forest. The study of fossil pollen was a way of doing paleobotany and relating it very closely to ecology. That was what the attraction really was. It was ecology with a time dimension.

And what can the study of fossil pollen tell us about the past?

With most kinds of paleobotany, you have real plant parts, so you can tell a lot about the evolution of plants. But you can't tell anything about the quantities of plants. With pollen grains, you have less taxonomic resolution, but there's actually a quantitative relationship between the pollen grains in the sediment and the quantities of trees that were in the surrounding landscape. We're actively investigating this right now in my lab. We don't have all the answers to how you translate the quantities of pollen into quantities of plants. It's really still an active field. Pollen analysis has enabled us to get an idea about paleoclimate and to make a start at reconstructing whole plant communities and placing them on the landscape.

How do you obtain such a pollen history of a particular location?

Sometimes you can see sediment in open section. Actually, this is how the field started out. People were excavating peat bogs in Europe for fuel, and there were many peat bog profiles exposed. People took samples out of them. We don't have very many peat bogs in this country, so we usually work with cores taken through the sediments in a lake. We either go out in the winter and core through a hole in the ice, with the ice serving as a big platform, or we go in the summer with a couple of canoes or inflatable rafts with a platform over them. Through a hole in the middle of the platform, we core down into the sediments. The cores we collect now are big ones, about four inches in diameter. We can use them for radiometric dating and analysis of larger plant parts, and then we also study the pollen fossils up and down the core.

Using pollen as markers for the past distribution and abundance of trees, you've been able to track plant species as they migrated with the advance and retreat of ice sheets during the Ice Ages. What is the mechanism and speed of such tree migration?

Trees spread seeds very effectively. If you think of the leaves around a tree in the fall, there's a little shadow of leaves around the tree. Seeds are disbursed much the same way. When people put out seed traps, they see that the numbers are largest in the vicinity of the tree, and then they fall away quite rapidly. But a few seeds are transported much greater distances. We know this is true because in the fossil record you can find evidence for the existence of a tree on one side of Lake Michigan, and then you begin picking it up on the other side. So obviously seeds have gone all the way across this big lake, which is 100 kilometers across. For example, in the case of hemlock, which sheds its seeds in the winter, it's quite easy to see how this can happen. The cones open when the air is dry, and the seeds then fall out onto the surface of the snow. And if there's a high wind, they're scattered along the surface. So if you walk across a lake, you can see hemlock seeds and birch seeds way out in the middle of the lake on top of the ice. And if the climate is changing at the same time that seeds are dispersed into a new habitat, that habitat might, for the first time, be a place where this tree can become established. And that new colony disperses seeds in all directions, and so the migration continues. Trees can actually migrate this way, at remarkable rates—up to 40 or 50 kilometers in a century. That's pretty fast when you consider that the tree has to shed its seed, and then the seed has to grow up into a new tree and be old enough to produce seed.

Do animals also contribute to tree migration by dispensing seeds?

Some recent studies have been done on the dispersal of seeds by blue jays. They've been observed flying from a forest where they're collecting acorns across a bog to their nests. And then they bury the seeds. That's just the perfect situation for a seed, to actually be planted in the right habitat. Usually, the birds are pretty smart about finding their seed again, but on a big seed year, they bury more than they actually need, so some of those germinate the next spring.

One common image of the Ice Ages is of whole plant communities, such as hardwood forests, migrating southward with the advancing sheets of ice and then back northward with the retreat of the ice. How is your research changing this view?

When we show on a map how different trees have changed their ranges in the past, the maps show clearly that not all trees are changing in synchrony. They are following different migratory paths. At any moment in time, the coincidence of tree ranges is not the same as it is today. By studying fossil pollen, you can find out that different combinations of tree species made up forest communities in the past. During the height of the last glaciation, the forest that existed south of the ice sheet was not similar to the modern boreal forest, it had a different combination of species. People often relate the distribution of modern biomes to different climates. When ecologists think about global climate change in the future, they tend to push these modern biomes into new places. But the fossil record shows clearly that as the climate changes, new plant communities arise. Different combinations of species occur, because the distribution of each species is related to a particular set of climate parameters.

The Ice Ages are examples of past environmental change on a global scale. We now live in a period of global change caused by the human population. Among the concerns are an increasing concentration of atmospheric carbon dioxide and global warming. What is the evidence that global changes are under way?

We don't know for sure whether global warming is under way yet or whether what we're experiencing is just a climate anomaly. We do know that there's a strong relationship between carbon dioxide and climate, because in Antarctica and Greenland there are bubbles of atmosphere preserved in ice sheets, which clearly show that the carbon dioxide concentration was very low when the temperatures were low during the last glacial period. Carbon dioxide levels change from time to time, very much in concert with climate changes. That's some of the best empirical evidence that carbon dioxide levels are correlated with temperature.

What's causing the atmospheric carbon dioxide concentration to increase?

Our use of fossil fuel. There was a lot of carbon dioxide in the atmosphere of Precambrian Earth. After plants began to photosynthesize, they began taking up the carbon dioxide that was in the atmosphere and turning it into organic matter, much of which got buried in rocks. We are now digging up this carbon, which has been buried for a long time, burning it as fossil fuel and turning it into CO₂, which is then entering the atmosphere. There's also a lot of carbon held in living biomass, especially in the tropical forests. And we're very busy cutting down these forests and turning them into carbon dioxide and producing grasslands, which have very low biomass. This is all human participation in what were once natural biogeochemical cycles on the globe.

Why are ecologists and others concerned about increasing atmospheric CO₂ and global warming?

The rate at which we are changing the atmosphere will produce a much more rapid climate change than most of the climate changes in the past. When there's been a very rapid climate change in the past—for example, at the end of the last glacial period—plant species had a hard time keeping up with the temperature change. Now we may be causing climate changes of several degrees in less than a century. That's an order of magnitude faster than most of the climate changes in the past. In the worst-case scenario, you could have a climate that changed so much during the lifetime of a single tree that the tree was only briefly in the best environment for its growth. This would be tremendously disruptive, even in managed ecosystems; in natural ecosystems it would be devastating. There would be widespread extinction of species.

Can you speculate about how this climate change would affect forests?

That's a big question. I think that commercially valuable trees are going to be planted where they can grow, and I think foresters will be very alert to look at long-range predictions and try to plant trees that will grow. So I would expect that commercial forestry will continue. What I'm really worried about is the unmanaged forests, which are the source of a lot of commercially harvested timber. Some of the slow-growing trees that we're used to seeing, like sugar maple and hemlock, are going to lose out, and the landscape will be covered with successional species like aspen and paper birch. We already have a lot of aspen and birch here in Minnesota as a result of logging. I see a great loss in the diversity of forest species.

We have many preserves designed to save certain rare species. Those preserves are going to be hard hit. People will have to try to create communities artificially at more northern latitudes in order to save species. That kind of intervention is often not very successful. Aside from the ethical questions that arise when humans try to manipulate nature at that level of detail, I'm not sure we really know how to create an ecosystem similar to an old-growth forest, which takes many centuries to develop. I'm very concerned about this.

How is logging changing the North American landscape?

It's already had a huge effect. Essentially all of eastern North America was clear-cut. In the Great Lakes region, there's only a half of one percent left of the original northern hardwood forest. Actually, some of the cutting was done as recently as 1950. Small remnants of old-growth forest are still being cut here and there because nobody is keeping track of where they are in a systematic way. Individual property owners cut them, and there's no way to protect them. There are just a few preserves under management by the U.S. Forest Service or by the state that are kept as parks. It seems shocking to me that this ecosystem was essentially eliminated as late as the 1950s, when we knew better. People like Aldo Leopold were pointing out that we had an opportunity to save a big enough corridor for species to move north and south, but the corridor wasn't preserved when it could have been saved. And now in the Pacific Northwest, at an earlier stage, there's debate about whether we should save 10% of the forest, and whether it should be in big patches or little patches. I hope people will feel that ecosystems that take a very, very long time to develop should be preserved, and they recognize that you can't recreate them once you've cut them down. Unfortunately, it's clear that logging of old growth will continue because it's economically profitable. But we have a lot of wood that we can cut from plantations and second-growth forests, and that's what we should be concentrating on, rather than going after the old growth.

Logging is built into the economy of some regions. How can you make your point about conserving old-growth forests to people who make their living from timber?

They should learn a lesson from the northern Great Lakes region, which had a big logging boom and then the trees were all gone and there was a depression. I think the Pacific Northwest has to consider what's going to happen when the trees are all gone. How are people going to support themselves? I don't think they're thinking in those terms.

A sustained cutting strategy at a much lower rate would keep a smaller group of people employed forever. The Menominee Indians in Wisconsin, for instance, have resisted contracts that would have allowed logging companies to come in and clear-cut their forests. They're cutting trees themselves, but at a very slow rate that keeps people employed locally. That slow rate of cutting is such that it's very hard to tell the Menominee Reservation forests from old growth.

Beyond the pragmatic arguments for trying to conserve biodiversity in forests and other ecosystems, do you see the human-caused extinction of other species as a moral issue?

People learn in elementary ecology that populations grow at a rapid rate, and then they begin to level off as they reach their environmental limitation. The human population has never behaved this way. It keeps growing as we find more and more habitat where we can manage to support more people. We don't behave like a natural population at all. But there will come a point at which we reach a limitation. We're a dominant species in terms of our numbers and our huge impact on the global ecosystem. But that doesn't give us the right to take away space from other species and to cause their extinction. I feel that human beings, as a very successful and dominant species on Earth, should take responsibility for more vulnerable species. For us to take advantage of our power and walk over other species, causing their extinction, is arrogant, and I believe, immoral.

Do we also have a moral responsibility not to ruin Earth for future generations of our own species?

That's really true. The consequences of destruction can be so slow that one generation causes extinction of a species, and it's a later generation that regrets it. There's no going back to create that species again. Most people aren't thinking about the kind of world their grandchildren are going to inherit. When I was born there were only about two billion people on Earth, and now there are three times that many, increasing at an even more rapid rate. For some reason, concerns about population growth, destruction of habitat, and extinction of species don't seem to be in the general public's eye. I don't quite see why not, but maybe scientists haven't been doing a very good job of making these issues seem important to people.

Do you think biologists have a special responsibility to try to influence the business and political communities on environmental issues?

As an ecologist, you can't escape this responsibility even if you want to. Once you start doing research in an area, before you know it, somebody appears who wants to destroy it. For example, I spent quite a bit of time last year arguing about whether or not snowmobile routes ought to come through an area of old-growth forest. It seemed obvious to me that they shouldn't, but it was not at all obvious to the people who wanted the routes. Most ecologists I know who do long-term studies spend a lot of time just defending the territory where their study is taking place. Even if they initially felt no responsibility at all, they'd end up having to take some. There are certainly a lot of demands on ecologists' time to testify here and there and present these issues in a public forum. Ecologists have a special responsibility to give testimony in any way they can to try to head off environmental disasters.

©2005 Pearson Education, Inc., publishing as Benjamin Cummings