Why We Move Trees

About assisted migration, 10 min

All tree species have places where they grow well, where they grow reasonably, and where they don't grow at all. This is because different places have different environmental variables that influence how well certain types of trees grow, one of these variables is temperature. For example a tree species that grows in the tundra would die quickly if it was planted in the tropics, the tundra tree prefers a cold environment.

Historically temperatures change slowly over many generations, the tundra tree could survive these slow changing temperatures by adapting. One way it can adapt is by evolving different genetic traits, in a hotter climate, freeze resistance may not be so useful, but drought resistance would be. Another way is by migration, physically moving where the species is. Trees of course can't move so their seeds are the ones that do the travelling.

However recently, temperature changes have greatly accelerated. The mentioned ways that trees adapt are really slow processes, too slow to keep up with the changing climate.

This post will focus on migration, it will use a visual to show how different species with different environmental preferences react. In addition to temperature, we'll include three unchanging variables that impact trees.

Elevation - An area's height above sea level. This variable influences many other variables (which won't be considered here) like amounts of rain and snow.

Slope - The steepness of an area. The roots of species grow in different ways, ones with shallow roots may have troubles growing on steeper slopes.

Aspect - Which direction the slope is facing. This influences how much sun an area gets. In the Northern hemisphere, South facing slopes get more sun than North facing slopes.

The variables will be represented by colour intensity:

Temperature

Cold

Elevation

Low

Slope

Flat

Aspect

Dark

There will be four species, each represented by a colour, shown in the leftmost column. Their variable preferences are according to the colours just above.

Species Temp Elev Slope Aspect

Below, from the elevation map in the bottom right corner you can see a mountain to the North, steep slopes in the middle, and a valley to the South. Given an unchanging temperature, species will migrate to areas that match their variable preferences, then stop moving since no variables are changing. You can view this by pressing play, and view it again by pressing reset.

Legend

Slope

Temperature

Aspect

Elevation

This scenario has a curvy ridge going from North to South, with a narrow valley on the right side. This time temperature will slowly increase towards the North, slowly like it changed historically. Focus on how the green-yellow species moves in the bottom right corner.

Legend

Slope

Temperature

Aspect

Elevation

Above, you saw the green-yellow species move through the valley to the North, and as the temperature got hotter, the darker species which like heat continued to expand. If the temperature kept increasing the darker species would likely take over the whole map, so it's a good thing that the green-yellow species could continue to move North to colder temperatures.

Below, We'll see the same scenario but with fast temperature change, more similar to the current pace of change.

Legend

Slope

Temperature

Aspect

Elevation

In this scenario, the heat increased too quickly for yellow-green to move North since the light green species now blocks the pathway. This means the species would be unable to move to its prefered areas in the future and will likely get outcompeted by the darker species as temperatures continue to increase, which would eventually cause it to disappear from the area.

A tree species disappearing isn't good for many reasons. One is that we rely on them for resources, different tree types are good for different production reasons, losing a type can result in economic losses. Another reason is lost biodiversity, many different organisms rely on each other in ecosystems, taking away a tree species can break the balance of the ecosystem and negatively impact even more organisms.

There is a strategy to help alleviate this problem, by helping with migration ourselves. Below is the same scenario as above, but now there's an edit tab where you can place species anywhere you want on the map. This time press play and wait until the temperature stops changing, then pause. Now there's two ways we can induce the migration, press the edit tab and add more of the green-yellow species to the map in one of two ways. Either draw a corridor between the Northern and Southern parts of the valley, or just add the species to the Northern portion of the valley. Once done, press play again.

Legend

Edit

Slope

Temperature

Aspect

Elevation

If done correctly, the species now establishes in the North and there is now less risk of it disappearing. This is the concept of assisted migration, humans can aid in the process of migration by planting trees either where they'll be able to grow in the future or where it will help make a migration pathway.

Assisted migration can be a very useful conservation tool but it is a complex process, a lot of research needs to be done before implementing it. One important piece of research is provenance testing where many individuals of the same species are planted in many different areas to find out which trees do well in different places. Due diligence is done to avoid risks like spending lots of money to move trees where they do not grow well.

I'll leave you with a sandbox mode where you can make different scenarios yourself. The temperature change slider will change how quickly species can migrate. The edit tab has some extra options like being able to edit the environmental variables. You can change the temperature's behaviour, and draw a different elevation from which, slope and aspect will be generated.

Legend

Edit

Slope

Temperature

Aspect

Elevation

Temperature Change

Slow

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