Assisted Migration

Overview

Trees, like humans, have adaptations that allow them to thrive and reproduce under certain environmental and climatic conditions. Climate change is bringing rapid changes to the environment, including new temperature ranges, drought, and new precipitation regimes. For some trees, the rate of climate change far exceeds their ability to adapt. Assisted migration - the human-assisted movement of species to more suitable habitats in response to climate change - is being used to address this challenge.

When comparing human adaptations with those of trees, the key difference lies in the timescale of adaptation. Humans can quickly adjust behaviors and technology in response to climate conditions, but trees adapt over generations. Trees do migrate naturally through dispersal of seeds, but the process is very slow — with distance limited to how far seeds can travel by wind, water, or hitching a ride with another creature. Assisted migration “speeds up” the migration of trees and gives species a chance to thrive and regenerate in locations where projected conditions are more suitable.

Forms of Assisted Migration

Assisted migration includes several different forms which vary in terms of risk, ecological implication, and policy. The most common forms of assisted migration are shown in figure 1.

Figure 1: Types of Assisted Migration. Dependent on conditions, location and other factors, different approaches are used to help migrate species.

• Assisted Population Migration: moves populations to new locations (often higher elevations) within their historical species range.
• Assisted Range Migration: moves populations to areas just at, or beyond their historical range, mimicking natural dispersal.
• Assisted Species Migration: moves populations to a location far outside the historical species range, to locations inaccessible by natural dispersal.

The idea of moving species beyond their current range may cause hesitation, but in the era of climate change the idea of “native” species is more nuanced. Ranges, species, and entire forest types are shifting, thus our understanding of native species is also changing. Typically, native species have been defined by their geographic range, but current ranges of species are not the same as they once were. As the climate warms rapidly, each snapshot in time results in a slightly different list of species native - or currently present - in each region. Matching species to their ideal conditions in order to avoid extinction is more important than adhering to strict boundaries of historical geographic range as regional climates change.