Forest Migration

Migration is for the birds! And a large number of other taxa, like dragon flies, buffalo, gazelles, and bats. These all sound like reasonable migratory candidates. Would you believe that trees deserve to be added to this list?

Though trees don’t have legs, they are still capable of traveling long distances. They may not travel thousands of miles to winter in Mexico (though it would be a sight if they did), but they do still move. Even though an individual tree dwells in a fixed place for the entirety of its life time, tree populations are capable of migrating across landscapes over the course of multiple generations.

This movement is caused by the expansion and compression of forest borders, a type of movement regulated by two ecological forces. The first of the two is sexual reproductive dispersal. This is a positive force that causes forests and all the plants within them to spread, to cover more ground. Biotic factors, like the presence of beneficial soil microbes, or abiotic factors, like adequate rainfall, contribute to the effectiveness of this force. The second, a negative force, is suppression by the environment of seed and plant survival. Cold snaps in early spring or drawn out droughts in summertime can cause buds and seeds to die and forest borders to retreat. When the dispersal potential rises above the suppressive forces, the forest range border expands. But when the plants experience more suppression than dispersion, the border contracts. The constant push and pull between these two forces is how we get forest migration.

Now, let’s step from the theoretical processes into real-world application. We can examine the phenomenon of forest migration through the lens of climate change, as it is constantly altering environmental factors that affect plant survival success across the planet. As temperatures get warmer, plants that once survived in a region may now be unable to be successful there. This differential success causes migration.

Let’s use a mountain as an example. Coniferous trees, like pines, usually grow in the areas of higher altitude on the mountain face, while deciduous trees, like maples, prefer the balmier, lower altitudes. As global temperature steadily increases, warm air creeps its way up mountains, and so do deciduous trees. The higher altitude areas have become warmer and thus more habitable for trees that prefer warmer temperatures. The boundary between deciduous and coniferous is constantly shifting upward, which poses a direct threat to high elevation forests.

The effects of climate change are not only felt in montane areas. If any large grouping of plants in unable to migrate northward to zones of suitable temperature fast enough to keep up with global temperature change, we start to get effects like genetic bottlenecking, population fragmentation, or species extirpation. All of these phenomena lead to a decrease in genetic diversity within plant populations.

Maintaining a high degree of genetic diversity is important in any population of organisms because the more genes that are in the gene pool, the greater chance offspring have to succeed. There are more chances to inherit genes that code for beneficial traits like cold tolerance, attractive flowers, and efficient chloroplasts when there are more genes to choose from. Genetic diversity is also desirable because it reduces the occurrence of undesirable traits, like pathogen susceptibility, in a population.

When a population of organisms experiences genetic bottlenecking, the genetic diversity of the population is greatly reduced. This reduction is usually caused by cataclysmic, one time events, like fire or deforestation, or it can happen more gradually, like with climate change. Fragmentation occurs when a population is separated by the sudden appearance of physical boundaries (like large bodies of water or man made cities), leaving the fragments subject to inbreeding depression, another phenomenon that greatly reduces genetic diversity. And there’s also extirpation, or local extinction, where a species goes entirely extinct in one area.

What to do, what to do…

There are ways for humans to intervene. Assisted migration is a possibility, a way to help trees get to suitable environments quicker. It involves taking seeds or existing saplings and planting them in a new habitat where they can succeed. This solution has the potential to be effective both economically and environmentally, allowing us to preserve biodiversity, despite the warming planet. There are, of course, certain precautions to be taken before moving trees all over the place. Robust predictive models must be formulated and utilized to determine which species actually require assisted migration. The problem with these models is that they generally only have a 20 year predictive scope. That leaves us with a lot of future uncertainty.

There has been consistent controversy over moving tree species around within their native range and moving them outside of their native range into other climatically suitable areas. This practice does not align with traditional conservation strategies because plopping a whole population of trees into a new environment poses certain risks to the the population that is receiving the migrants, or the donor population. There could be hybridization between migrant and native plants, introduction of new pathogens, or the migrant species could become invasive. Translocated populations could also experience the founder effect. This is a phenomenon that occurs as a result of breeding within a small founding population and leads to diminished genetic diversity. And that’s the problem assisted migration was supposed to solve in the first place.

Perhaps the potential to preserve biodiversity outweighs the plethora of potential risks, but how can we be sure? In reality, it would be impossible to relocate every population of at risk trees. We would invariably revert to relocating populations of the trees most useful to us for forestry purposes, which would still leave us with greatly diminished levels of biodiversity. We could stop burning fossil fuels and raising livestock to reduce green house gas emissions to slow climate change. Or we could perform assisted migration and radically change the forest dynamics of entire continents. Unfortunately, neither of these propositions are viable options.

We are facing grave environmental issues at this moment in Earth’s history. We have many ideas, but few answers. This can be seen as an unsolvable problem, or as a challenge. Scientific discoveries do not give a succinct answer, rather each one turns over a stone to reveal twenty new paths of inquiry, and twenty new problems to solve. This exponentially proliferating effect of active questioning need not be seen as a daunting phenomenon. We need not feel at odds with ourselves and our environment. Not knowing the solution now does not invalidate our current knowledge, as science is plastic and ever growing. Gaps in our our understanding should not be a deterrent. Rather, they should be a call to action, to scientific endeavor.

As far as climate change and assisted migration go, we don’t quite know what to do yet, but that’s ok. We are now saddled with the task of matching the height of our effortful zeal to the mountainous issues towering all around us.

We have so much to do my friends. Let’s get to work.

Sources

Beckage, B., B. Osborne, D. G. Gavin, C. Pucko, T. Siccama, and T. Perkins. “A Rapid Upward Shift of a Forest Ecotone during 40 Years of Warming in the Green Mountains of Vermont.” Proceedings of the National Academy of Sciences 105.11 (2008): 4197–202. Web.

Gray, Laura K. et al.. “Assisted Migration to Address Climate Change: Recommendations for Aspen Reforestation in Western Canada”. Ecological Applications 21.5 (2011): 1591–1603. Web.

John H. Pedlar et al.. “Placing Forestry in the Assisted Migration Debate”. BioScience 62.9 (2012): 835–842. Web.

Sauer, Jonathan D. Plant Migration: The Dynamics of Geographic Patterning in Seed Plant Species. Berkeley: University of California Press, c1988 1988. http://ark.cdlib.org/ark:/13030/ft196n99v8/