A growing body of research suggests relocating some tree populations may help them adapt to future climates
Story by Shannon Kelleher
Douglas-fir trees of the Pacific Northwest are no shrinking violets. While quaint, domesticated variations make decorative Christmas trees that fit in the living room, wild Douglas-firs induce awe. Towering in the Cascades and coastal mountains at heights sometimes exceeding 300 feet, these are thick-barked giants capable of withstanding forest fires and surviving for a millennium. The species occupies an enormous range, growing alongside agave in Oaxaca, Mexico, and spanning northward to the temperate rainforest of southeast Alaska. The Douglas-fir’s biggest historical problem was claiming a name for itself—botanists waffled for years, calling it a pine, a spruce, a hemlock, and a true fir. In 1867, they finally threw in the towel and gave the tree its own genus, Psuedotsuga, or “false hemlock.”
But in recent years, the Pacific Northwest has begun heating up and drying out. It isn’t a big problem for Douglas-firs yet, with temperatures still hovering within their suitable climatic range. However, projections for coming decades suggest that native stands will soon be maladapted to local climates.
“By mid-century, local populations will be growing in a very different environment, and that environment will probably be outside the range [to which they are] adapted,” says Brad St. Clair, a research geneticist at the USDA Forest Service’s Pacific Northwest Station. “I’m very concerned they might have problems.”
And so, in an effort to learn how land managers can give Douglas-firs a nudge in the right direction, St. Clair and his team are taking up the torch of scientists who puzzled over these trees in centuries past, exploring not what to call them but where to put them.
Trees on the Run
As climates around the world grow warmer, rapid shifts in temperature and extreme weather events disrupt the planet’s equilibrium and exacerbate the impacts of unsustainable land use. For forests, climate change does not just mean fewer trees—instead, it threatens to reduce the biodiversity of species that make ecosystems productive and sustainable. An assessment of North, Central, and South America conducted by Forest Service research ecologist Eileen Helmer and her collaborators through the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services found that almost a quarter of 14,000 species studied in the region are at high risk of extinction. This biodiversity loss endangers human health and economies—but the disappearance of species is not always apparent to an untrained eye. “When most people go driving and see trees, they don’t recognize that we’re losing individual species, reducing the overall diversity of our forests,” says Kas Dumroese, a research plant physiologist for the Forest Service who studies adaptive management strategies to improve ecosystem resilience.
Dumroese believes that many tree species are up against more than they can handle, lacking the high reproductive turnover for speedy adaptation. As winters shorten, the growing season lengthens—a phenomenon that sounds like a blessing but functions more like a Trojan horse. The upshot is drier soil that makes forests vulnerable to worsening fires, earlier spring budding that leaves saplings susceptible to cold snaps, and more time for pests like mountain pine beetles to breed and wreak havoc. Forests just aren’t ready for such a rapidly changing world.
“It may be a little less [difficult] for plants that mature quickly and are able to disperse seeds long distances, like cottonwood trees,” says Dumroese. “They may have a better time keeping up with climate change through natural selection and migration. But long-lived trees that take a decade or more to reach maturity and then only disperse their seeds a few hundred feet are going to have a tough time.”
There are a number of techniques land managers can use to give struggling trees a boost. Planting diverse stands and taking care to maintain diversity during forest thinning can help sustain biodiversity and make forests hardier, at least for now. But Dumroese and other researchers believe a more forward-thinking style of intervention may benefit the plight of modern forests.
An option receiving heightened buzz in the forestry world is assisted migration—the intentional relocation of species and populations to new sites where they are predicted to be better suited to future climates. The idea is to characterize the adaptive capacity of a particular seedlot (a seed collection, usually from a limited geographic area), and, if necessary, to match a seedlot to a planting site where future climate projections are expected to fall within its optimal range.
Dumroese believes assisted migration can be a sensible choice for populations at high risk of poor survival and extirpation (local extinction), helping them keep up in the race to adjust their ranges to the shifting climate. “We want to help facilitate the natural movement of a forest ecosystem across the landscape,” he says. “As foresters, we’re pretty good at moving species. What we’re working with now, with climate change, is a more nuanced approach. But it’s very much a work in progress. It can be a complex, mind-boggling adventure.”
Beyond Seed Zones
Moving species and populations is actually nothing new. The Douglas-fir, for example, has been historically relocated far and wide within its native range. But these moves were not always a success. Moving Douglas-firs sourced in Montana all the way to coastal Washington, for example, could result in more change than a population could handle. When it became clear by the mid-20th century that some relocated populations weren’t as healthy as those that remained in their original locations, the Forest Service developed seed movement guidelines that restricted reforestation efforts facilitated by land managers based on geographic distance and elevation. In the Pacific Northwest, these requirements were further refined as “seed zones,”—areas within which land managers could collect seed and replant it without worrying that it might be maladapted.
Seed zones have so far worked well for maintaining adapted and productive forest stands, allowing land managers to undertake reforestation projects with peace of mind. But that may no longer be the case with climate change. Seed zones and seed movement guidelines assume that the climate is static, but changing climates mean that native forest ecosystems may soon no longer be adapted to local climates.
“Forest ecosystems can be quite resilient,” says St. Clair. “But we might be moving beyond the point of resilience so that the future does not look so bright.”
St. Clair is exploring how to move beyond existing seed zones, using assisted migration to help match ecosystems to future climates. It’s a hefty undertaking that requires maintaining a delicate balance between present and future. If land managers move trees too far from currently warm climates to cooler locations in order to match a future warmer climate, they might put trees at risk of cold damage in the near-term. But if they fail to move trees far enough from their present location, the relocation might not be enough to make a difference in the long-term. It’s a moving target.
“There’s a quote I use from Lewis Carroll’s book Through the Looking Glass,” says St. Clair. “‘Now, here, you see, it takes all the running you can do to keep in the same place.’”
Then there is the pure enormity of the task—try as they might, land managers cannot possibly migrate more than a small percentage of at-risk trees, dooming the rest to become maladapted as they struggle to keep up through natural selection.
Although it comes with overwhelming challenges, St. Clair still believes this land management technique can make a difference in the effort to stave off the impacts of climate change. “I think assisted migration is among some of the most important management options we can do,” he says.
Determined to give forests a leg up in the race against time, Forest Service scientists are lining up at the starting blocks. St. Clair and his team have developed the Seedlot Selection Tool, a GIS mapping program that allows scientists and land managers to peek decades into the future and provides a simple, organized, and accessible way to incorporate climate change into land management decisions. The tool uses existing data and climate projections to determine which seedlots will be best adapted to a given planting site and which planting sites will be most suitable for a given seedlot. By providing insight into how tree populations may need to shift, the tool takes the guesswork out of finding them new homes.
St. Clair is putting the Seedlot Selection Tool to the test in the Pacific Northwest, where his team has established a set of field tests called the Douglas-fir Seed-Source Movement Trial to evaluate assisted migration. In the winter of 2008-2009, St. Clair and his team planted Douglas-firs originating from 12 different regions across three latitudinal bands, with sites varying from high to low elevations. They installed weather stations at each site to carefully monitor the temperature and set up fences to keep the trees safe from roving herbivores in their new homes. By substituting space for time, the team can learn how well each population will likely fare in the future and predict suitable climates to relocate populations at risk, all the while testing their results against the Seedlot Selection Tool’s predictions.
After monitoring the plots for a decade, the team is beginning to understand just how much heat and cold Douglas-firs can withstand before they begin to struggle. Some populations that were initially adapting well to warmer locations are starting to develop problems, while others that were moved north to significantly cooler climates are showing signs of cold damage.
“The message is, we can be moving things around much farther than we currently are, but there are still limits to how far we can move things for assisted migration,” says St. Clair.
Navigating Uncertainty
British Columbia is currently leading the charge to put assisted migration into practice, revising their seed zones and seed movement guidelines to plan for climate change. But assisted migration is not yet broadly utilized in the United States. Some scientists fundamentally doubt its ability to mitigate the hazards of climate change, believing the only viable solution is to address the problem at its root. “An optimistic view of the world managing to maintain intact species and ecosystems into the future using strategies such as assisted migration may not be realistic, since ecosystems involve extremely complex interactions between many species that have evolved over millions of years,” says Dr. Sandy Boyce, the National Wildlife Ecologist. “Ultimately, carbon imbalances in both the atmosphere and the ocean must be reversed to prevent further environmental damage from greenhouse gas accumulations. Climate change effects are occurring faster than species can adjust, leading scientists to believe that many species will soon be driven to extinction.” Other ecologists object to assisted migration because they are uncomfortable about moving trees further than traditional designations allow. After all, local seed sources historically tended to be the best adapted. “If you start talking about mixing genotypes, it can seem like an affront to the old approach,” says Dumroese. The move to relocate populations in danger marks a major shift in how scientists and land managers think about conservation on a fundamental level. The focus for decades has been to protect species within their habitats. If habitats are no longer sacrosanct, some fear conservationists would be opening Pandora’s Box, making room for loopholes that could be exploited to negate existing protections. It is also uncertain just how assisted migration fits within existing regulatory frameworks. While the U.S. Endangered Species Act (ESA) states that people (and federal agencies) have an obligation to take action to prevent species loss, it also imposes restrictions on habitat degradation (which could unwittingly result when populations are moved). There are currently no regulations developed specifically for assisted migration.
Some scientists, including an interdisciplinary working group that convened in 2012, believe there is not yet enough information available to make informed decisions about how relocating organisms will affect ecosystems, leading to unintentional damage. Separately, Mary Mahalovich, a Forest Service geneticist based in the Inland West regions, emphasizes that other factors beyond climate, such as soil composition, must be taken into account when planting populations in new locations.
“I think it will help us in our work if we choose the right species for the future using climate models while also taking suitable substrate into account, as well as any other factors like elevation. Taken together, these will help determine what is a good seed source,” says Mahalovich. “There’s still some finessing that needs to be done.”
Other experts are concerned that unpredictable populations could act like invasive species, upending their new habitats. This has happened in the past, with nonnative species introduced to control pests sometimes targeting native species instead. To better understand the risk, a 2008 study built and analyzed a database of 468 current invasive species in the U.S. The researchers focused on those that originated in North America, since assisted migration would likewise take place over relatively small distances. Plants fared the best. They represented only 7.5 percent of the intracontinental invaders, while 11.8 percent were invertebrates, 31.3 percent were crustaceans, and 51.6 percent were fish.
Despite the risks and challenges involved with relocating populations, many scientists agree that a careful, well-regulated, and flexible approach to adaptive management strategies such as assisted migration is important to preserving biodiversity while the planet rapidly changes.
As a 2007 assessment on assisted migration concludes: “To an uncomfortable extent, this war [on climate change] will have to be fought with ‘the army we have, not the army we want."
Assisted Migration in Action
While few assisted migration projects are underway in the United States, there is growing interest in research that investigates its potential. In addition to St. Clair’s Douglas-fir Seed Source Movement Trial, the Forest Service is involved in the Assisted Migration Adaptation Trial (AMAT), a program evaluating how well 16 species of trees collected from seed sources across British Columbia, Washington, Oregon, and Idaho are adapted to a variety of sites in British Columbia. Researchers are using the trial to learn how well trees planted in northern latitudes survive and grow as the climate changes and how capable trees from the North are at withstanding warmer latitudes. The Forest Service is also using the Forest Tree Genetic Risk Assessment System (ForGRAS) to identify species at risk of genetic degradation (in which failure to reproduce leads to a shrinking gene pool) in the Pacific Northwest and Southeast.
In the Pacific Northwest, Forest Service geneticist Vicky Erickson is working to ease land managers into using the Seedlot Selection Tool to inform seed sourcing decisions. She notes that the region only needs to make relatively minor shifts in seed movement at this point, with sources adapted to planting site climates 20 years into the future striking a balance between risk management and long-term viability.
“We’re purposefully working to transition silviculturists and reforestation specialists to accommodate projected changes in climate,” says Erikson. “The Seedlot Selection Tool is very powerful but has a lot of complexity and inexperienced users need it simplified a bit. We are trying to move things forward and streamline the decision-making process.”
Paul Berrang, a research geneticist in the Forest Service’s Eastern Region, observes that land managers in some of the forests he works with have been proactively testing assisted migration. Superior National Forest managers in the far northern and eastern reaches of Minnesota, for example, have been working with the Nature Conservancy to experiment with planting seed sourced from central Minnesota in addition to locally-sourced seed. Berrang is currently re-writing a supplement to the Forest Service handbook, encouraging land managers to source seed from areas with climates similar to a forest’s projected future climate where appropriate.
As climates across the United States continue to change with unprecedented speed, Berrang hopes that research into assisted migration and other innovative conservation approaches will help pave a future for at-risk trees.
“Science-based applications of assisted migration have potential to help mitigate some effects of climate change on selected species,” says Berrang.