Tree migrations: a matter of scale

By Benjamin Seliger

In everyday life, trees may appear as stationary, permanent fixtures of the landscape, but in reality they are far from it. I will argue that all trees are constantly migrating, and summarize the means by which they do so in this post.

When I tell people I study tree migration, I often get funny looks. After all, how can an organism that cannot even move migrate? The answer to this, like many other phenomena in ecology, is time. Every year, mature trees release thousands of seeds in all directions, and when one of those seeds grows into an adult in a place where others of its own kind were not growing before, the species is migrating. Note that this perspective of migration is not limited to the cyclical movement of animals with the seasons, but rather an adjustment of a species’ range to preferred conditions as climate changes. I will refer to the latter as a range migration and the former as a seasonal migration. The two definitions are really explaining the same process at different scales; seasonal migration being the movement of individuals within their lifespan, and range migration as the movement of populations over the course of multiple generations. The end result for either definition is all the members of a species living in a different place than they did previously due to a change in environmental conditions.

Examples of seed dispersal, from top to bottom: wind-dispersed winged seeds of maple trees; floating-water-dispersed coconut fruit; attractive and tasty animal-dispersed apples.

Let us again consider a tree that is releasing its seeds. Like any parent, the tree’s goal is to put its children (seeds) in a position in which they can prosper. The first challenge the young seed faces is to get far away from its parent. Unlike animals, the parent is a competitor, and not a nurturer, for the seed. Underneath its parent’s shadow the seed cannot collect enough sunlight to grow, and above the parent’s roots the seed cannot gather enough water or nutrients from the soil to live. The best interest of both parties then is for the seed to get far away. This movement of progeny away from parent is called dispersal.

In order to successfully disperse seeds requires clever adaptations by trees. Without any means to move, seeds would simply fall from the branch to the ground directly below, and would be largely doomed to fail for the reasons mentioned. This is called gravity dispersal, and is not a very successful strategy. However, trees have evolved several interesting solutions to this problem, the most basic of which is wind dispersal. In this dispersal strategy seeds are made lightweight and with wings for wind to catch. Examples of this are dandelions (Taraxacum), and the helicopter seeds of maples (Acer) and ashes (Fraxinus) in North American trees. The disadvantage to this dispersal strategy is that the seeds must be small, meaning each seed has less energy to start its life with, and wind dispersal cannot be effective in places with little wind (like the tropics).

Another natural force trees can exploit for dispersal is water. As Charles Darwin traveled around the world on the Beagle wondered how coconut trees seemed to be on every tropical island he visited. Upon returning back to England, he conducted a study in his basement where which he put coconuts  in pools of saltwater. After several months, what he found was not only did they stay afloat, but even after months of direct contact with salt the coconut seeds were able to germinate. A coconut that is swept away in the tide could then travel thousands of miles to another beach and start new population. Trees that grow alongside rivers often use water dispersal as well. The downside to water dispersal is that the species is confined to forever live within close proximity to either a river or the ocean, as the seeds can never move back upland to drier conditions. However, both coconuts (Cocos nucifera) and mangroves (Rhizophora) have employed water dispersal to become some of the most wide ranging tropical trees in the world.

The last way trees have devised to migrate is by tricking the animals around them. The earliest records of seeds go back nearly 400 million years (Tiffney 2004), and presumably early animals have been eating seeds since they became aware of them. The few seeds that managed to survive a trip through an animal’s gut would have enjoyed a place to grow complete with nutrient rich dung for fertilizer. The tricky part for the seed is surviving the journey. Grinding teeth and strong stomach acids are both capable of making a seed, or what is left of it, unable to germinate, in which case the animal functions as a predator, instead of a seed disperser.

Ranges of beech (Fagus grandifolia) from 12,000 to 2,000 years before present. Reanges interpreted from pollen in lake sediment cores. Figures from Pollen Viewer 3.2 by Phil Leduc.

In order to increase chances of germination, plants then enclose their seeds in fleshy fruit to attract animals. Thinking they are snagging a tasty meal, animals end up swallowing the seeds unharmed, and unknowingly move the seeds as long as it takes for the meal to pass through their digestive system. This is on the order of minutes for birds, and hours or days for mammals (Willson 1993). For a wide ranging mammal, a day could feasibly transport the seed dozens of kilometers. Seeds dispersed in this manner often have protective coatings that wear off as they are being digested, in some cases even requiring being eaten before the seed can germinate. This dispersal strategy is the most complex, as it is a mutual interaction between two organisms. Creating fruit that attracts and tastes good to animals costs a lot of energy, but the reward of long distance dispersal is great. Examples of trees utilizing this dispersal mode are likely present in your own refrigerator, with apples (Malus) and oranges (Citrus) being two of many.

Knowing how trees are able to migrate begs the final question as to why they would have undergone such trouble to evolve these adaptations in the first place. Paleoecology offers a great deal of perspective to this question. As we look back in time, we see that climate is constantly changing. In fact, given how many variables are involved, it is argued that climate is never the same twice (Jackson and Overpeck 2000). Much like the seasonal migration in which some environmental cue tells the animal it is time to move on, the changing climate is always causing range migrations as the last individuals die in an inhospitable area and seedlings take root in new frontiers. So the next time you look at a firmly rooted tree, remember that sometimes all it takes to understand the natural world around us is thinking on a different scale.

References

Darwin, C. (1909). Voyage of the Beagle. Courier Dover Publications.

Jackson, S. T., & Overpeck, J. T. (2000). Responses of plant populations and communities to environmental changes of the late Quaternary. Paleobiology,26(sp4), 194-220.

Tiffney, B. H. (2004). Vertebrate dispersal of seed plants through time. Annual review of ecology, evolution, and systematics, 1-29.

Willson, M. F. (1993). Mammals as seed-dispersal mutualists in North America. Oikos, 159-176.