Many of Colorado’s iconic aspen forests are declining as a result of climate change. Photo courtesy Jon Mullen/Ecostock
William Anderegg and ACES’ Adam McCurdy discuss climate change
William Anderegg, Ph.D., is the director of the Wilkes Center for Climate Science and Policy and an associate professor in the School of Biological Sciences at the University of Utah. His work explores the intersection of ecosystems and climate change and focuses on a central question: What is the future of Earth’s forests in a changing climate? Bill spoke with Adam McCurdy, ACES’ Forest and Climate Director, about what the future might look like for Colorado’s forests.
This topic is both professional and personal to Bill—so much so that it was the catalyst for his career. He traces his choice of work to his early experiences with Colorado’s forests. As he wrote in a 2017 article:
I grew up fishing and hunting in rural Colorado…We had our favorite campsites nestled in the quaking aspens that we returned to year after year…In 2008, I returned to western Colorado and went back to some of my favorite fishing holes and hunting campsites. And what I found astounded me. They were desolate. The aspen forests were dying. And it wasn’t just the aspens. You couldn’t drive a single mountain pass in Colorado without seeing dull gray mountainsides of dead trees. What the hell is happening, I wondered. So I started digging. Like following a windy game trail, the path ultimately led me to a career as a scientist.1
As Bill notes in this conversation, the past decade has witnessed massive mortality events of many tree species, which prompts concerns that the impacts of drought, insects, and wildfire may devastate forests in the coming years. He and Adam also explore what role forests might play in solving the climate crisis. The wide-ranging discussion took place in spring 2023 via video conference and has been edited for length.
Adam McCurdy: I want to start broad. What are some of the ways in which climate change is predicted to impact Colorado forests?
William Anderegg: On one hand, there’s a set of forces that could be advantageous to forests. A key benefit is that rising carbon dioxide concentrations in the atmosphere should mean that plants can conduct more photosynthesis and lose less water and therefore potentially grow faster and be somewhat less water stressed. That driver is fundamentally in tension with a lot of the main climate change impacts, or climate stresses, that we tend to think about. When we talk about western U.S. forests, we’re talking about drought stress, temperature stress, wildfires, and then biotic agents, which is the big category of pests and pathogens—such as bark beetles, defoliators, and fungal pathogens.
Climate change supercharges those stresses. It’s pulling forests toward higher mortality rates, regional scale die-offs, and it’s generally losing forests rather than gaining forests or seeing more growth. The big picture is that there are two opposing sets of forces, and understanding when and where these different drivers will win out is the crucial question for the future of Colorado forests in the next couple of decades and beyond.
Researchers with the Anderegg Lab, University of Utah.
A.M.: Different tree species and different forest types respond differently to climate change. In terms of forests in Colorado, which forests are more likely to be winners or losers? Which Colorado forests are most severely impacted by climate change, and are there forests that might benefit from climate change?
W.A.: We have quite a bit of data from the past 20 years about what Colorado forests have experienced and how they’ve responded. Lodgepole pine forests and aspen forests have been some of the most impacted in Colorado—lodgepole pine primarily by the mountain pine beetle and aspen primarily by drought and temperature stress. There also seem to be a couple of the fir and spruce species suffering extensively from other beetles.
There’s a nice paper by Hunter Stanke, a graduate research fellow at Michigan State University,2 with an analysis of forest inventory data that shows that five of the eight most dominant tree species in the western U.S. are declining. Pinyon pine and subalpine fir are losing, and quaking aspen, Engelmann spruce, and lodgepole pine are losing a lot. Douglas fir, ponderosa pine, and Utah juniper are three that are gaining. That totally scans with the work we’ve done and work from other people in Colorado.
A.M.: Looking more closely at aspen, it is well established that aspen benefits from frequent disturbance events. As conifer forests are increasingly impacted by drought, wildfire, and biotic agents, could aspen start to benefit from climate change? Or is aspen so drought- and temperature-stressed that it won’t be able to take advantage of increased disturbance?
W.A.: That’s an interesting question. There are studies indicating that in mountain pine beetle–driven die-offs, aspen and spruce are coming up in the understory.3 So, there are signs that aspen could be benefiting in certain places.
There are two primary questions. The first is: How widespread is that going to be? If disturbance occurs at hotter and drier places, you’re probably not going to see aspen come up, and if you have severe wildfire that bakes the soils, you’re probably not going to see aspen come back. Those are a lot of the cases where you tend to lose forests altogether.
The other question is: Does that outweigh the drought-driven losses from low elevations? In all the forest inventory data sets we have so far, the answer is no, that on the net, aspen is still losing ground. But in some places, increased disturbance may help aspen, and you may see either uphill movement of aspen or expansion into areas where conifers have been cleared out. This also touches on the question of management. Do you try to manage for aspen? In particular, do you clear out some north-facing slopes and higher elevation forests to try to create some climate refugia for aspen?
A.M.: In addition to managing for a specific species do you see a place for assisted migration? And if so, how could it be done to avoid some of the potential pitfalls?
W.A.: This is a tough question. I think it’s a possibility and something we need to be considering and thinking carefully about. There may be some forests and some communities where it makes sense. To some extent, you could structure it to minimize the chances of something going wrong, for example if you’re planting native species and just selecting specific genotypes. If you want to plant different seed sources, there are a lot of ways things could go wrong, but I think it could be minimized with careful planning. The big picture on assisted migration is the sheer amount of resources compared to the land area. It’s not going to be a widespread tool. I think it’s going to be more that this tool helps in some narrow cases.
A.M.: Could you talk a bit more about how disturbance and climate change combine to create a change in forest type or a loss of forests altogether?
W.A.: We always try to think of disturbance in terms of what we think the regime might have been for that forest pre-colonization. What climate change does is that it changes these regimes. You see more frequent and more severe wildfires, and there’s an additional layer in that our forests have historical legacies of management and fire suppression and clear cutting, etc. We understand the directions in which climate change is pushing systems, but we don’t really understand a lot of the magnitude or when and where different places will be more or less vulnerable.
A.M.: What key questions would you like to see resolved in order to help us understand what the next 50 to 100 years will look like for Colorado forests?
W.A.: I want to know what comes after disturbance in a lot of these forests, so what species come up next, or for the surviving trees, are they more or less resilient to the next disturbance or the next set of stresses? I’d like to get a sense of when and where disturbance and climate stress cause a change in the forest type, or an outright loss of forest. When do we see regrowth versus transition or loss? There’s some great work coming out on fires. We’re starting to see somewhat of a clear picture there, but it’s much less clear on drought and biotic agents.
With drought, for example, at what climate thresholds do you start to see an ecosystem crash? On the biotic agent side, can we predict the next thing that goes viral and starts taking out huge swaths of forests? Can you predict what’s going to be the next mountain pine beetle or the next spruce budworm or the next spruce beetle? These biotic agents are devastating forests. If you could predict the high-risk pathogens and pests, then you open up a lot of tools. If we’re worried about pest X, we’re going to monitor it or consider management. You can start to be proactive rather than reactive; currently we spend 90 percent of our time in the reactive zone.
A.M.: Some of the recent work coming out of your lab focuses on, or has been in conversation with, nature-based climate solutions. What role do you see for nature-based climate solutions in higher elevation, relatively dry forests such as Colorado?
W.A.: I have a lot of mixed thoughts toward nature-based climate solutions targeted at climate change mitigation, particularly in Colorado and the Rockies. On one hand, investing in nature brings so many benefits to us, and there are many reasons why we’d want to conserve, reforest, protect, or improve the management of forests in different regions in the Rockies. Those include biodiversity, fire resilience, tourism, clean air, clean water, and the list of ecosystem services and benefits goes on and on. Those are critical co-benefits and could also increase climate adaptation. But for climate mitigation specifically, focusing on carbon sequestered by nature-based solutions is tricky because, relative to much of the rest of the U.S., the Rockies are high-risk in terms of climate impacts. It doesn’t mean that you shouldn’t make these efforts, but there is a strong risk of losing some of that carbon to disturbance. Another huge challenge is in really dry regions and in snow-covered regions where there are many reasons that adding more forest carbon can actually heat the planet because of the effects on reflectivity of the surface. Chris Williams at Clark University’s School of Geography has a paper that finds most of the intermountain and Rocky Mountain forests can actually heat up the climate on net because of the albedo feedback and snowpack.4
What this means is that forest nature-based solutions probably only benefit the climate in a narrow set of species and environments in the Rockies. It’s probably only deciduous trees like aspens, and it’s probably only above other relatively dark understories. So, with something like low elevation pinyon-juniper or the high elevation spruce fir, adding more biomass probably is not a win for the climate. Unfortunately, due to these biophysical feedbacks, it’s a pretty narrow set of places where investing in forests for carbon only will actually benefit the climate in terms of mitigation. The key thing I try to emphasize is that when thinking about nature-based solutions from just a carbon-centric view, you can miss a whole bunch of these other important aspects.
A.M.: The western U.S. is in the midst of a 20-plus-year drought, but we had a big snow year in 2023. What impact does a year like that have on forests in terms of the backdrop of long-term drought?
W.A.: There’s no question that they’re helpful and that replenishing both the shallow soil, and especially the deeper soil moisture, really benefits forests. But one good year does not get us out of a 20-year drought, and the longer-term trends are quite clear that we’re headed toward a hotter and drier future. We can use a really good year to see, for one, how much do forests recover from this set of stresses? And two, it buys us a little time to try to plan more proactively to build more climate resilient forests.
Left: A Colorado State University student hangs a temperature sensor outside of Ashcroft.
Right: William Anderegg collects samples from a juniper tree.
A.M.: Let’s delve deeper into what the options are for proactive climate management in forested ecosystems.
W.A.: It varies from forest to forest and place to place, but generally speaking, having more diverse landscapes and more diverse forests in terms of the age structure, potentially the number of species that are present, those conditions tend to benefit forests. Trying to use management to shift some of these stand densities and fuels toward what we think the pre-European-colonization landscapes might have looked like, this is really trying to move toward more where the indigenous cultures managed a lot of these forests. And a final piece is thinking carefully about what we want from forests in different regions and what species and forest types we want to encourage to deliver those benefits. Planning for 30 years out or 50 years out is a challenge, but it’s the sort of thing we need to be thinking about with climate change and with how long it takes trees to grow and establish.
The shifting of densities is the trickiest part because it doesn’t make sense in all systems, but it can mean prescribed fires, it can mean some selective thinning in places. It has to be done carefully and with a lot of stakeholder input, but there are definitely some systems where it would increase resilience to fire and drought and passive pathogens.
A.M.: That’s a great point. One of the conversations I have with a lot of people is that not every forest should look like a ponderosa pine forest.
W.A.: I agree. This is one of the things that people don’t really realize, but the vast majority of our research, or the research that’s been done, has focused on these low-elevation pine forests and a lot of it has been done in wetter places than Colorado. A lot of it has been done in California and Oregon and wetter forests. Things like timber harvesting may not translate. You may end up with a very, very different answer with a dry place in Colorado or an aspen or spruce-fir forest. You can’t take a ponderosa pine management plan and then just copy-paste everywhere across the Rockies.
A.M.: That makes a lot of sense. It’s a complicated but very important concept that needs to be better understood by the general public. Is there anything else about Colorado forests that you want to bring up?
W.A.: I’m relatively skeptical that forest carbon offsets as mechanisms of nature-based climate solutions are actually going to deliver the climate benefits. We recently published a paper where we compared three different ways of looking at the future of forest carbon in the U.S.,5 and you get vastly different answers depending on the approach. In one set of demographic tools we considered, which includes models of fire, drought stress, and biotic agents, you tend to see carbon gains in the East and more carbon losses in most of the West, including Colorado, which, unfortunately, is probably the most likely scenario. It’s the set of models that I would put the most trust in.
A.M.: With regard to nature-based climate solutions, what are your thoughts about tree-planting versus forest conservation?
W.A.: That’s quite a bit clearer in the Rockies, but also in most places around the world. Conserving forests tends to have a much bigger and more immediate climate benefit than reforesting, than tree planting. Reforesting and tree planting have some advantages and there are definitely some places where it makes sense, but it’s slower. It takes decades to have much of a climate benefit, and the biodiversity and other ecosystem service benefits are so much smaller than actually conserving the mature forests that we have. So, there’s this huge push in the scientific community to really conserve first and foremost, and then as a second tier, think about reforesting and tree planting.
