Assisted migration is the movement of populations or species by humans from one territory to another in response to climate change. This is the definition offered in a nontechnical document published by the United States Forest Service in 2023, suggesting that this form of climate adaptation "could be a proactive, pragmatic tool for building climate resilience in our landscapes."
Programs for assisted migration of forests in North America have been created by public and indigenous governmental bodies, private forest owners, and land trusts. They have been researching, testing, evaluating, and sometimes implementing forest assisted migration projects as a form of adaptation to climate change. Assisted migration in the forestry context differs from assisted migration as originally proposed in the context of conservation biology, where it is regarded as a management tool for helping endangered species cope with the need for climate adaptation. The focus in forestry is mitigating climate change's negative effects on the health and productivity of working forests.
Forestry assisted migration is already underway in North America because of the rapidly changing climate and the forestry industry's reseeding practices. It is now standard practice for governmental and industrial harvests of trees to be followed by the planting of seeds or seedlings in the harvested areas. Hence, an opportunity automatically arises post-harvest to select seeds (and sometimes different species of trees) from areas with climates that are expected to arrive in the harvested sites decades into the future. The government of British Columbia in Canada was the first federated state on the continent to make the decision to change their seed transfer guidelines accordingly in 2009.
Longer distance forms of assisted migration were not, however, considered prior to climate modelling and within-forest evidence of the increasing pace of climate change. Serious discussion and debate ensued in the forestry profession beginning around 2008. The debate focuses around the ethical implications of artificially migrating ecosystems, the risks and benefits of such endeavors, and the values at the heart of assisted forest migration projects.
There are also recorded instances of inadvertent assisted migration of North American trees. Beginning in the early 20th century, two trees famously endemic to California, the giant sequoia and coast redwood, have been planted for urban forestry purposes northward in cities along the Pacific coast of Oregon, Washington, and British Columbia. Today these specimens are not only thriving; they are prominent along urban skylines and often outrank the native trees in sizes achieved. As well, several kinds of Magnolia native to the southeastern United States have dispersed into the forest understory, thanks to ornamental plantings producing seeds beyond their native ranges.
Climate change as impetus
Climate change is increasing the average temperatures of North American forests. By the 2020s the forestry profession in North America no longer debated whether human assistance in helping native trees might be necessary. A 2023 scientific article began, "Global change is reshaping climatic conditions at a tempo that exceeds natural migration rates for most tree species." The implications are that "this mismatch may cause catastrophic losses of key forest ecosystem services such as carbon sequestration, habitat provisioning, and forest products."
Range shifts upslope to higher topographic altitudes require less distance than shifts tracking climate over relatively flat terrain poleward. Species within the southern limits of their range are already showing decline or extirpation, including some of North America's most iconic trees (Joshua Tree and giant sequoia). Drier conditions in the American Southwest are leading to shrubby grassland replacing pinyon-juniper woodland, following drought-induced die-off of trees. Foresters in Minnesota are predicting that prairie ecosystems will expand, replacing forests as their region becomes warmer and drier. Regional climate change in the Sierra mountains of California made it possible for native bark beetles to kill drought-stressed conifer trees, which in turn led to expansive and catastrophic fires. Although the giant sequoia is more resistant to bark beetles than are the pines and firs intermixed in and surrounding its groves, the regional fires consuming the standing dead conifers in 2020 and 2021 proved fatal to giant sequoias whose living canopies were also engulfed.
The northern populations of temperate tree species ranges in coastal Washington and Oregon began to evidence abrupt decline and mortality over such large regions that news articles written for the public, as well as academic papers, reported on this phenomenon in 2021 and 2022. Bigleaf maple, Western redcedar, Douglas fir, and several species of true firs (genus Abies) were featured in reports of sudden decline. While proximate causes included evidence of native insects and fungal pathogens, the reports attributed ultimate cause to the unprecedented summer droughts and heat waves in the region. In 2023, the tree species dominating the lower western slopes of California's Sierra Nevada mountains were reported to be so lagging in tracking warming climate upslope that one-fifth of the total area was labelled as "zombie forests" — that is, forests that can no longer establish new seedlings and thus will not revive after a canopy-destroying fire.
A synthesis paper published in 2022 by a team of governmental and university forestry scientists in the United States warned that "climate change may represent the greatest challenge ever faced by forest managers, conservation biologists, and ecologists." Catastrophic tree mortality was among the challenges listed by the team as not only projected but already underway.
Definitions
A 2011 discussion paper in the British Columbia Journal of Ecosystems and Management offered this definition of assisted migration: "the purposeful movement of species to facilitate or mimic natural population or range expansion to help ensure forest plantations remain resilient in future climates." A 2016 review article defines forestry assisted migration as "the physical realignment of natural populations to the climate for which they are adapted, by reforestation in sites where their suitable climate is predicted to occur in the future, as an active management option with the aim of maintaining healthy tree ecosystems in the future." The article says that human assistance in helping trees migrate is necessary because "geographic shifts of tree populations will have to be 10 to 100 times faster than they have been in the past or are at present."
Three types
There are three types of forestry assisted migration, which Natural Resources Canada describes in this way:
Assisted population migration: the human-assisted movement of populations within a species' established range. Assisted range expansion: the human-assisted movement of species to areas just outside their established range, facilitating or mimicking natural range expansion. Assisted long-distance migration: the human-assisted movement of species to areas far outside their established range (beyond areas accessible through natural dispersal).
The United States Forest Service uses the same three types, but sometimes refers to the third type as assisted species migration. In 2018 the U.S. Forest Service published a longer policy document, which listed and defined the three types this way:
Assisted population migration (also assisted genetic migration or assisted gene flow) – moving seed sources or populations to new locations within the historical species range. Assisted range expansion – moving seed sources or populations from their current range to suitable areas just beyond the historical species range, facilitating or mimicking natural dispersal. Assisted species migration (also species rescue, managed relocation, or assisted long-distance migration) – moving seed sources or populations to a location far outside the historical species range, beyond locations accessible by natural dispersal.
Terminology to distinguish forestry applications of assisted migration from the more controversial practices being debated within conservation biology were suggested in a lengthy 2012 report on climate adaptation within the U.S. Forest Service. The authors proposed "forestry assisted migration" for their agency's endeavors and "rescue assisted migration" for the species-specific extinction concerns of conservation biologists. That same year nine forestry scientists (all but one of whom is Canadian) coauthored a paper that likewise recommended "forest assisted migration" as preferred terminology for helping their profession stay clear of the controversy that they referred to as "species rescue assisted migration". As of 2021, the original classification names of the three types is in public as well as professional use, with foresters focusing on the two most moderate forms.
Early scholarship and debate
Beginning in 2004 and accelerating in 2007, researchers in conservation biology published papers on the pros and cons of supplementing traditional management practices for preventing plant and animal extinctions with species translocation tactics to accommodate the range shifts already becoming evident as a result of climate change. Conservation biologists also published arguments for and against the three main terms used to name the same management tool: assisted migration, assisted colonization, and managed relocation.
The focus among forestry professionals and researchers was different. Paleoecologists had already concluded that there were significant lags in northward movement of even the dominant canopy trees in North America during the thousands of years since the final glacial retreat. In the 1990s, forestry researchers had begun applying climate change projections to their own tree species distribution models, and some results on the probable distances of future range shifts prompted attention. As well, translocation terminology was not controversial among forestry researchers because "migration" was the standard term used in paleoecology for natural movements of tree species recorded in the geological record. Discussion in forestry journals therefore pertained more to questions of how and when and for which species climate-adaptive plantings and range expansions should begin.
Debate as to the need for and ethics of assisted migration was present among forestry researchers. But compared to the debate among conservation biologists, it was muted and short-lived. One of the strongest statements urging caution that appeared in a forestry journal was published in 2011. Isabelle Aubin and colleagues stated that "assisted migration is being proposed to reduce the impacts of human-induced climate change, an unprecedented situation in human history that brings with it entirely new environmental, societal and ethical challenges."
But also in 2011, researchers within the Canadian Forest Service expressly distinguished forestry assisted migration from the concept debated in conservation biology. Preventing species extinctions was not a forestry focus. Rather, forestry forms of assisted migration would be undertaken for the purposes of maintaining the forests' ecosystem services as well as the extractive resource values of timber and pulp production. The Canadian Forest Service also produced research showing forest health and productivity could benefit from relocating forest understory plant species along with the dominant canopy tree species and that this would help with the successful establishment of a forest at a new location.
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