Helping trees adapt to climate change possible but a huge task

The world is warming pretty fast, and many species are already on the move, heading toward the poles or up mountainsides to areas where the climate matches what they’re used to. But many won’t be able to move fast enough, or they will find cities, farms or other disturbances in their way.

To help some of those species, scientists have proposed a method called assisted migration (or assisted colonization) that would give plants or animals a boost by transplanting individuals from one place to another.

Eli Lazarus of Cardiff University in Wales and Brian McGill of the University of Maine in Orono wondered if such a project would work for temperate, wind-dispersed trees in North America (a category that includes willow, birch and Scots pine trees). The researchers built a simulation that would let them examine how trees would naturally migrate in the face of climate change, what would happen when they were confronted with a disturbed landscape and whether a tree-planting project could change things. They published their findings August 27 in PLOS ONE.

To keep up with climate change, trees in temperate North America would have to migrate north at a rate of about 350 meters per year. But on their own, those trees moved only 130 meters each year in the simulation, a rate on par with what previous studies have calculated.

That rate of movement slowed down in the presence of a disturbed landscape (one that had been interrupted with the trappings of human civilization) and reached zero when at least 40 percent of the land area was disturbed. Again, this was not all that surprising. Scientists who have studied migration during past eras of climate change — such as the time when Earth was warming after the end of the last Ice Age — observed that such movement in Europe was halted by mountain ranges and the Mediterranean Sea. Like cities, those features of the landscape act as barriers to movement — and resulted in many extinctions.

Such barriers can be overcome, however, with the strategy of assisted colonization, according to the simulation. By deliberately planting trees in a targeted program, ensuring that they only go in places that are suitable, the migration rate can be forced to reach that key rate of 350 meters per year.

Such a program would come at a great cost, though. Trees would have to be raised in nurseries to sizes suitable for planting. And then time and labor would be required for the actual planting. The researchers don’t say how many trees would be needed but do mention that an intentional planting program “would have to occur on a massive scale” if it were to be effective.

But there are a couple of models for such programs, they note. The Green Belt Movement in Kenya has planted more than 51 million trees since 1977. And more than 12 billion trees have been planted worldwide as part of the United Nation Environment Programme’s Billion Tree Campaign. So it may be possible to make assisted migration a success — if we’re willing to put in the effort to make it one.