For residents of the Rocky Mountain region of the United States and Canada, global warming is not an abstract concept. It is a reality they confront every day in the form of tens of millions of acres of pine and spruce trees that have died as a result of beetle infestations spawned by a hotter, drier climate.
Few people know as much about this forest plague as University of Montana entomologist Diana Six, who is working with colleagues to understand why the genetics of some individual trees enable them to survive even as whole forests around them are turning brown and perishing. Six says the infestations are now happening worldwide, with major outbreaks in Africa and Europe. But the worst impacts to date are in the North American West, where bark beetles are spreading quickly into new territories and attacking high-altitude tree species that were previously unaffected.
In an interview with Yale Environment 360, Six explains the root causes of the beetle infestations, discusses why U.S. Forest Service policies may be making the problem worse, and describes why the best hope for Western forests will come from the trees’ capacity to genetically adapt to a new climate regime. Science, she says, can help support this natural process of evolution by mapping genetic markers in trees that survive the insects, so forest managers can breed trees that are better able to cope with future beetle onslaughts. “The only way that we can help our forests in the long term is to strengthen their genetic abilities for adaptation,” says Six. “We have to consider some radical new ways of thinking.”
Yale Environment 360: What is the focus of your research?
Diana Six: Basically I look at the ecology and evolution of bark beetles. And now, of course, I look at how climate change is affecting them, because it’s impossible these days to work on bark beetles or anything else in the forest without looking at climate change.
e360: People who have not been to the Rockies lately may not grasp the extent of the tree die-off due to the bark beetle infestation. Could you paint a picture of what is going on there?
Six: It’s pretty amazing. There have been tens of millions of acres of trees killed. If you go north to British Columbia, a really big province, something like 80 percent of the trees are dead. You can get in a plane and fly for literally hours over dead forest. So this is massive. Beetle outbreaks are normal, they have been happening for thousands of years. But this one is estimated to be more than ten times bigger than any event we know of in the past. And there is no end in sight.
e360: What factors have contributed to the current outbreak?
Six: Drought stresses trees, and when trees are stressed, they don’t fight back as much, so it takes a lot fewer beetles to kill them. So you’ve got this perfect storm of more beetles because it’s warmer, and it takes a lot fewer beetles to kill the trees due to recent drought, and that triggers an outbreak.
“Some areas have lost 90 percent of their trees within three years of the start of infestations.”
Typically when you return to cooler, wetter conditions, these outbreaks would poop out. They didn’t just continue for years and years where they basically killed everything — the infestations would sort of self-contain. However, the problem now is we aren’t returning to cooler weather conditions. So over the past 25 years this beetle has just continued to advance: It just goes on and on until the beetles run out of trees.
e360: What surprises you the most about what you have been seeing?
Six: I think it is how quickly it has moved. In lodgepole pine, the beetle usually takes seven to ten years to work through an area. Now it is taking two or maybe three years. Because it is warmer, the beetle has been able to move up in elevation. That means it has gotten into white bark pine, a tree the beetle typically didn’t do much to before because it was too cold for it up there. Some areas that I have studied have lost 90 percent of these trees within three years of the start of infestations. The white bark pine is now recommended for listing as an endangered species.
e360: Are we talking about one insect species that is doing all the damage here?
Six: There are a lot of different pine beetles out there. In the Southwest, it is mainly the genus ips. A species in that genus, the pine engraver, is killing ponderosa, and a related species has wiped out big areas of piñon pines, some of which will probably never recover. With drought in some areas, spruce is becoming stressed, and the spruce beetle is blowing up — it is really active now in Colorado. With increasingly warmer and drier conditions predicted in the years to come, we can expect the Douglas fir beetle to become active. There are a number of other species that are waiting in the wings for their tree hosts to become more and more stressed and then they’ll start to kick into action.
e360: Insects in general are barometers of climate, aren’t they?
Six: That’s right. Insects are cold-blooded, they respond very rapidly to temperature. Colder weather dampens their reproductive rate, their feeding rate. On the other hand, everything speeds up with rising temperatures. So even with something like a degree or a degree-and-a-half of warming, you can double their reproduction and development rates, leading to rapid population explosions.
e360: We read a lot about the effect of climate change on polar bears but not so much about insects.
Six: And they are going to have one of the biggest impacts on our forests and our ecosystems planet-wide. You look at what mountain pine beetle is doing that is so far outside the norm. Smaller outbreaks in the past helped to regenerate the forest, which evolved in tandem with the beetle. But when you enhance a disturbance to this degree, you start to see not regeneration but destruction.
e360: Where are the beetles currently headed?
Six: Right now the infestations are moving north. In the past, mountain pine beetles had only reached central British Columbia. It has moved in just four or five years all the way to the Yukon, due to warming temperatures that allow the beetle to live where it used to be too cold for them.
So many beetles were flying at one point that you could actually track their clouds on radar. There was one particular wind event that moved billions of them over the mountains and they became established on the eastern side of the Rockies in Canada, where they never existed before. They’ve adapted well to their new territory and have now moved clear across Alberta. I understand they are even in parts of Saskatchewan.
“So many beetles were flying at one point that you could actually track their clouds on radar.”
They have moved into Jack pine in the boreal forest, which extends clear across the continent and is such an important resource for carbon sequestration. Jack pine doesn’t have much of a defense against the insect, since it has not co-evolved with them. So projections are that it will make its way to the east and eventually enter our eastern pine forests in the U.S. Unlike some other beetle species, the mountain pine beetle is able to use most pines that it encounters. Pretty much every pine in its path is going to be susceptible.
e360: The U.S. Forest Service has been thinning forests as a response to this. Any evidence that this is helping with the beetle problem?
Six: The idea behind thinning is that if you have an over-dense forest where trees are so close together that they are competing for soil nutrients and water, if you thin that forest out, you will relieve that competition, the trees will become healthy, and that will strengthen their defenses against beetles. It was also thought that, if you thin forests, it disrupts the chemical communication of the beetles, which are the pheromones that they use to mass-attack trees. Neither of those ideas totally pans out.
Nobody has shown conclusively that pheromones get disrupted. Also, we know that thinned stands can go down as easily as un-thinned stands. In most of our forests, we have reached a situation where changes in temperature and drought have reached the point where thinning will no longer be effective. Under normal conditions it can help, perhaps. But the fact is we are just not there anymore. I visited one stand that had been thinned many years earlier as a demonstration of how thinning protects trees from bark beetles and enhances growth. Yet when the insects came through, that was the first stand to go down!
e360: So by trying to fix the problem, we sometimes only make it worse.
Six: As humans, we have this feeling that if something goes awry, we need to fix it, and that somehow we can. I don’t think that we necessarily always
You can’t force a forest to stay a certain way — eventually it will tip.”
know what needs to be done, or that when we do apply management that we are always actually doing the right thing. Sometimes we just need to realize that nature can sort itself out perhaps better than we can. So one of our biggest challenges as we approach our forests under the conditions of climate change is to recognize that things have shifted and that we have to consider some new approaches, some radical new ways of thinking.
e360: What new ways of thinking are you suggesting?
Six: Our past management practices do not incorporate genetics for the most part, and they certainly do not include adaptation. Typically, they focus on resistance, which is trying to force a forest to stay in a particular state. And that’s just not going to work — you can’t force a forest to stay a certain way. Eventually, it will tip.
Instead, we need to think about adaptation. When we talk about forests and natural ecosystems, adaptation comes down to the genetic potential of trees to deal with a new situation. So the only way that we can help our forests in the long term is to strengthen their genetic abilities for adaptation. We have to look at the way our practices either support or interfere with this capacity for adaptation.
e360: Are there human interventions involving genetics that can help trees survive?
Six: I think so. For the first time in history, we actually have the tools that would allow us to do this. If we go out in a forest and look at the trees of a given species, they may all look pretty much the same to us. But trees are actually some of the most genetically variable organisms on the planet. We are working on a project right now that is using genetics to detect trees that exhibit resistance to bark beetles and that have enhanced drought tolerance.
e360: Some trees are genetically predisposed to respond better than others?
Six: Absolutely. You see this massive devastation out there, and there are these single trees here and there that are doing just fine. If you look at their tree rings, they are growing much faster now under warmer, droughty conditions than they did when they first started to grow, say, 150 years ago. They don’t have a single beetle attack on them. So they are actually doing better under current conditions than they did in the past.
Those are the first trees that we are testing with our genetic methods, and we are comparing their genetics with that of the trees that were killed. We’re hoping that we can locate those genetic markers that will help us distinguish easily and cheaply which trees to cut and which to leave standing as part of forest management in the future.
e360: What do you see in the future? Is this outbreak just going to get worse and worse?
Six: Well, obviously we need to do everything we can to limit greenhouse gases and slow this train down. But a lot of it is going to be totally unfixable, because we can’t turn the thermostat down, we can’t turn the sprinklers on. In the central northern Rockies, where I live, it has been predicted that we will lose 90 percent of the lodgepole pines, 80 percent of ponderosas, and 66 percent of spruce by 2060. And that is independent of the mountain pine beetle — that’s just due to the loss of suitable habitat that will support these species. These are big, big changes.
But these models assume that the forest is genetically homogenous, that everything is the same. And they are not. I suspect that there is a lot more genetic variability out there that will allow for more adaptation and greater persistence than we currently anticipate.
e360: You are suggesting that evolution will kick in and help to a degree?
Six: If we let it. If we don’t go out and replant with stock that may not be genetically correct, if we don’t thin or cut down trees that may have been selected by beetles or drought to survive. We have to get smart about how we are treating our forest if we’re going to help nature’s process of adaptation to proceed.