Trees on the Move? Debating Assisted Migration in Climate Change Mitigation

Tree Crossing. Borrowed from Clint Peters.

Tree Crossing. Borrowed from Clint Peters.

By Sarah Bisbing

Trees on the move?! I know you’re thinking, “Come on, Sarah. Trees can’t move.” And, generally, you would be correct in that statement. Tree species are now, however, in a position where movement may be necessary for survival under changing climatic conditions. How trees will move is under debate within the ecological community, but why trees will move is accepted as a survival strategy related to the adaptation of species.

Image borrowed from a lecture by Dr. Sally Aitken (University of British Columbia).

Image borrowed from a lecture by Dr. Sally Aitken (University of British Columbia).

Species are adapted to specific combinations of environmental and climatic conditions. These specific conditions produce suitable habitat and enable a species to grow, thrive, and reproduce. When these conditions are altered, as is currently occurring amidst our rapidly changing climate, species are often left exposed to novel, unsuitable habitat conditions. Species are then forced to either get used to the new conditions (adapt) or move (migrate) to areas where suitable habitat still exists. The alternative to these strategies is, at best, population loss or, at worst, extinction.

Examples of species' response to climatic change documented following the last glacial maximum. From Dawson et al., 2011.

Examples of species’ response to climatic change documented following the last glacial maximum. From Dawson et al., 2011.

Adaptation aside, migration and tracking of suitable habitat remain the next best option. But, as we discussed above, some species are virtually incapable of moving. Forest trees are long-lived and immobile, meaning they don’t move that far and when they do it isn’t done that quickly. Migration rates following the last ice age are estimated at a pace of around 100 meters per year. To keep pace with shifting climate and regenerate under suitable habitat conditions, trees will need to migrate at a rate upwards of 10 kilometers per year (that’s a migration rate 100 times faster than previously-documented!). Consequently, all tree species are expected to experience lags in their response to changing climate. If, as predicted, these migration rates cannot be met, how else can tree species move to keep up with the pace of shifting habitat?

One proposed strategy is the human-aided relocation of species (known as assisted migration), but this concept is one of the most controversial, divisive debates within the ecological community. A recent review on the topic (Hewitt et al., 2011) found that 60% of published articles are generally supportive of considering assisted migration, while 20% stand in opposition and 20% remain undecided. As with every story and every debate, this conversation has two sides to it. Each side stands firm in their position on human-aided assistance of species’ migration. Supporters (Hough-Guldberg et al., 2008; Gray et al., 2011, Camille Parmesan) argue that inaction will inevitably lead to extinction, while critics contend it is guaranteed to produce unintended, unpredictable consequences (Ricciardi and Simberloff, 2009).

Where do YOU stand on assisted migration?

Do you know both sides of the debate? Do you have the facts? Before you respond, let’s make sure you have all the information you need to make an informed decision. I do not intend to take a stance here but rather write this as a means of sharing available information.

Assisted Migration 101

Assisted migration is the movement of species within or beyond their historical range, implemented to conserve species and facilitate adaptation to predicted climate change. This term is often used interchangeably with assisted range expansion, assisted colonization, or managed relocation. The strict definitions of these terms are nuanced – although slightly different, each one describes the human-aided movement of species. Bottom line = we’re talking about physically moving a species from its current habitat to another habitat (presumably to areas that will encompass future suitable habitat conditions). Let’s leave it at that.

Here are some of the arguments for and against this proposed practice:

Benefits Risks
For ecosystems Averting the risk of inaction and loss of ecosystem function Potential for creation of new invasive species
Preserving ecosystems and communities in rapid decline Detrimental effects on local communities at transplant site
For species Protecting vulnerable species from maladaptation and possible extinction Potential loss of within-species local adaptation
  Maintaining species may outweigh the risks associated with inaction Uncertainty in species response and likely unintended consequences
For society Maintaining ecosystem health and economic viability At odds with goal of preserving natural ecosystems and processes

Conservation Perspective:

In the conservation world, assisted migration has, at times, been likened to ‘ecological roulette.’ The major critiques of this action are that it contends with well-established conservation practices and is not yet fully supported by rigorous scientific knowledge. Many argue that increasing landscape connectivity is instead the best option for the movement of species and one that has a much lower probability of unintended consequences (Krosby et al., 2010). Rather than working to manually move species, the ecological community could instead focus their efforts on working with land managers and government agencies to increase the area of protected landscapes and reduce barriers to species movement. Supporters assert that these alternative actions have the potential to increase the adaptive and migratory potential of both species and ecosystems.

Some conservation practitioners, conversely, argue that assisted migration may be the only way to save some species. One compelling argument for human-aided assistance is the mounting evidence suggesting that climate change will be a significant driver of extinction (Parmesan, 2006). Under these circumstances, both landscape connectivity AND direct mitigation actions may be key to species persistence under altered habitat conditions. McLachlan and colleagues contend that assisted migration must be considered if avoiding climate-driven extinction is a priority. Averting extinction may require such deliberate, human-aided range expansion.

The potential for extinction that is associated with inaction must be weighted against the uncertainty and potential consequences of action. Will assisted migration be the best option for the conservation of species, or will we ultimately regret our role in the interference of natural ecosystem functioning?

Management Perspective:

Foresters and land managers worry that the predicted increase in temperatures associated with ongoing climate change will leave tree species maladapted to local conditions. For managers, the permanent, irreversible loss of a species via extinction is just not an option. Species must be maintained to preserve both ecosystem health and economic viability.

Forest tree species (total = 15) used in British Columbia's AMAT trial. Photo courtesy of BC Ministry of Forests website.

Forest tree species (total = 15) used in British Columbia’s AMAT trial. Photo courtesy of BC Ministry of Forests website.

In areas strongly dependent upon the timber industry, as is the case in British Columbia (BC), persistence of dominant forest tree species is imperative. Nearly 200 million seedlings are planted in BC each year, with the province relying heavily on these future forests as a source of income. BC has already witnessed some of the detrimental effects associated with a rapidly changing climate, including declines in forest productivity and widespread mortality from insects and disease. As a result, BC has become a world leader in assisted migration research by setting up a large-scale, controlled experiment (known as the Assisted Migration Adaptation Trial) to test the tolerance of forest tree species to climate change. This long-term, province-wide study will quantify the lower and upper tolerances of species over the course of the next few decades. Additionally, researchers are pairing genomic analyses with experimental treatments to identify the genes associated with adaptation to local climatic conditions (AdapTree, UBC Centre for Forest Conservation Genetics).

It’s too early to say whether or not this experiment will support the implementation of assisted migration, but the ball is in motion. Only time and scientific rigor will really tell.

To assist or not to assist?

Regardless of what we decide, we will need to come up with a risk assessment, a set of best management practices, and some kind of decision process for the conservation of species (i.e. a way to decide which species get managed and which are left to adapt on their own).

Few papers lay out our options and their associated consequences, but I did find one that presents a nice, clean outline of action versus inaction. In their 2007 paper, McLachlan and colleagues suggest that we have three strategies to choose from:

  1. Aggressive Assisted Migration
    • Acting before we have all the facts.
    • Confidence that climate is the main driver of species distributions.
    • Trust in model projections of shifts in climate and suitable habitat.
    • Includes extensive translocation of species well beyond current ranges.
    • Maintaining species may outweigh the risks associated with transplantation.
    • May be the best way to minimize species loss.
    • But . . . may also lead to disruption of existing communities at translocation point.
    • Consequences may be irreversible.

     2.   Constrained Assisted Migration

    • Acting when necessary and supported by science.
    • Confidence that assisted migration is necessary despite risks.
    • Balance between the benefits and risks associated with assisted migration.
    • Action may require proof of imminent threat.
    • Full risk assessment and management plan in place.
    • Requiring rigorous scientific research, relying less on model predictions.
    • Could go either way – despite being more cautious, may still lead to disruption of ecosystems. On the other hand, lack of data may result in inaction and subsequent loss of species.

      3.    Avoidance of Assisted Migration

    • No action.
    • Confidence that species have persisted despite previous climatic shifts.
    • Uncertainty in ecological understanding of controls on species distributions.
    • Concern over the unintended consequences of well-intentioned human interference.
    • Concern over the potential for these species to become invasive.
    • Uncertainty in model predictions of future suitable and unsuitable habitat.
    • Instead, preserve isolated populations and increase landscape connectivity to facilitate dispersal.
    • But . . . inaction will increase the threat of extinction for species that are sensitive to any change in local conditions.
    • Extinction is irreversible.

NOW, what do you think?

Do we work to conserve spaces or conserve species? Will it be enough to conserve natural ecosystems and maintain connectivity between them? Or will we need to actively work to conserve species?

What role should assisted migration play in the conservation of forest tree species? Do we act now? Or do we wait and see?


In case you’re looking for more information, the body of literature on assisted migration is immense. To get you started, here are a few links:

Gray et al. 2011. Assisted migration to address climate change: recommendations for aspen reforestation in western Canada. Ecological Applications.

Hewitt et al. 2011. Taking stock of the assisted migration debate. Biological Conservation.

Krosby et al. 2010. Ecological connectivity for a changing climate. Conservation Biology.

Lawler and Older. 2011. Reframing the debate over assisted colonization. Frontiers in Ecology and the Environment.

Minteer and Collins. 2010. Move it or lose it? The ecological ethics of relocating species under climate change. Ecological Applications.

Ricciardi and Simberloff. 2009. Assisted migration is not a viable conservation strategy. Trends in Ecology and the Environment.

Sax et al. 2007. Managed relocation: a nuanced evaluation is needed. Response to: Assisted migration is not a viable conservation strategy. Trends in Ecology and the Environment.

8 thoughts on “Trees on the Move? Debating Assisted Migration in Climate Change Mitigation

  1. Pingback: Assisted migration- ecosystem protection versus unintended consequences | Energy and the Future

  2. Reblogged this on biogeocoenosis and commented:
    One of the realities of rapid climate change is that environmental conditions are changing so fast that many species cannot migrate or adapt quickly enough to keep up with them. This problem is especially present for some of my favorite organisms: trees. Trees, of course, can only move as seeds and their generally long generation times mean that evolution moves at the pace of, well, a tree.
    This problem has many ecologists and conservation biologists considering rather extreme measures: helping organisms move around to keep up with climate change. Of course, this sort of action has risks involved, so there is a robust debate going on in scientific and management circles. The following post, from the Early Career Ecologist blog, does a great job of laying out the basics of assisted migration.

  3. Great post! I was part of the team (along with folks like Steve Schneider) who advocated for the term “managed relocation” instead of “assisted migration,” in part because we felt that it’s a more accurate term. “Migration” and “colonization” aren’t really the processes at work in this case, and “managed” implies that it’s not just about moving species around, but also about active management to insure their survival.

    You might enjoy this paper, which came out of our working group: What I thought was really powerful about the group is that we had lawyers, ethicists, managers, policy folks, and ecologists all in the same room together.

    • Thanks, Jacquelyn! Glad you liked it! I agree. The term ‘managed relocation’ is much more appropriate, given the mechanism of movement and the role humans play in this movement of species.

      Thanks for sharing this paper with our readers (and me!). It’s now on my reading list for the weekend. I’m so captivated by this debate. I’d love to be involved in such conversations.

      Sarah B.

  4. This seems like a challenging management issue for a number of reasons, but one that stands out to me is the spatial scale of the issue. Species’ native ranges span jurisdictional boundaries and borders, so how will it be possible to manage the relocation of an entire species? It will take cooperation among states, countries and national agencies. It sounds like British Columbia is already working on this.

    Alternatively, relocation could be managed on a small scale where a single management agency or management unit (like a national park) takes things into their own hands and brings in species that will be well-suited to their land under climate change.

    It will be hard to make sure that all managers are on the same page about decisions.

    A parallel example is shown by the biological control of tamarisk in the American southwest. Biocontrol leaf beetles were introduced by a few land managers without the agreement of neighboring land managers and now the beetle has crossed many boundaries and expanded its range considerably. Whether the expansion of the tamarisk leaf beetle is good or bad for ecosystems is hotly debated and under study.

    Will it be important to have broad consensus before management decisions are made on a small scale that could have broad-scale implications?

    I think the risk of extinction and impoverished ecosystems are probably worth the risks, but this is an issue that managers should get ahead of before relocations occur where controversy over the issue persists.

  5. If you’re an ecologist- the idea of “trees moving” should be completely normal to you. Used to be the first thing you studied in ecology was “succession” – you know, were tree species move in to disturbed situations, gradually heading towards “climax” ecologies? If you’re an evolutionary ecologist- thinking about trees migrating is a given- of course they migrate- always have. 🙂

    A major consideration here should be – reality. I have two particular bits in mind-

    A) quite a few species; well adapted where they are, well integrated in their ecologies; will not be able to migrate fast enough to survive on their own. Case in point- whitebark pines in the N American west- they’re already existing as isolated mountaintop populations- disjunct- and disappearing fast. How can they jump from one peak to another- as reproductively viable populations? Very very slowly in the “natural world; now long gone.

    B) which brings us to this point- arguing about whether it “should be” done is utterly pointless. IT WILL BE DONE. By amateurs. Take a look at the history of game animals; like the recent helicopter transfer of asian deer from the island of Hawaii to Maui- entirely illegal, very destructive. Trout to- New Zealand; etc, etc, etc. AMATEURS WILL MOVE SPECIES. They move trees too- ever hear of ailanthus? Eucalyptus?

    Basically – humans are GOING to move species- I would contend it should be done with some modicum of forethought, and professional involvement – or it will be done without.

  6. Late to the debate but may be germane for you to know about bridal creeper (Asparagus asparagoides) control using rust in the wheatbelt areas of WA. It’s being used to clean up riparian areas, but the interesting/scary thing is that locals unconnected with the trial are taking infected material to put on their own outbreaks and while it’s a disease of the creeper, biological control can have unexpected consequences -on asparagus farms.Watch this space.

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