Millions of years earlier, another migration crossed the Bering Land Bridge in the opposite direction. However, it wasn’t a migration of people. It was a migration of Douglas fir trees. The story of this migration starts in Mexico. The first Douglas fir originated in northern Mexico some 50 million years ago. Douglas fir is closely related to the larch, which evolved from the pines. Douglas fir either branched from the same pine ancestors or from the larch. In either case, the two took quite different forms, the larch becoming deciduous with bundles of needles, and Douglas fir growing evergreen needles that resemble those of the spruce and fir.
|Bigcone Douglas fir|
Now we must stop and face this question: How can trees migrate? Unlike the strange, magical trees that destroyed the Orcs in “Lord of the Rings,” trees in our world remain rooted in one spot their entire lives. Yes, but their winged seeds can travel! They are adapted to do just that. Let’s suppose that in 100 years a tree grows to be 100 feet tall and produces seeds that are blown a conservative 200 feet from the tree. Thus, the forest would move 200 feet in 100 years, or on average, 2 feet per year. At that rate, it would take about 2500 years for the trees to migrate one mile. It may seem impossible for this forest to migrate all the way to Oregon, until you consider that it had millions of years to complete the trip. At this rate of one mile every 2500 years, the offspring of these seemingly immovable trees could be growing in Portland in less than 3 million years. Not content to linger in Portland, they continued right up the Pacific Coast to Alaska and across the Bering Land Bridge and south into Asia
Before they migrated very far, they faced another difficulty that might have stopped them before they even made it to Portland. The problem is this: As they migrated farther north, the climate changed. With these changes in the climate, the trees must adapt to the new environment. Douglas fir adapted to many changing conditions in their journey north to Alaska and then south into Asia. The journey was not without its setbacks. For example, they encountered a changing climate and several ice ages. And they required the right conditions for their seeds to germinate and survive. Douglas fir seeds germinate best in a seed bed of bare soil. Finally, they may have been stopped by open ocean at the Bering Strait, but at least once the Bering Land Bridge formed and allowed them to cross into Asia.
|Douglas fir seeds with wings|
As they moved south through eastern Asia, they adapted to the conditions in different locations, and developed into several new species, including one in Japan, one in Taiwan, and several in China:
- Japanese Douglas fir (Pseudotsuga japonica), is limited to Japan. Like all of the Asian species of the genus, it is smaller than the Douglas fir of the Pacific Northwest, growing to 100 feet. The cones are also smaller, only 2 in. or less. This species grows only rarely on steep slopes up to 3600 feet elevation.
- Formosan Douglas-fir, (Pseudotsuga wilsonia), grows mostly in Taiwan with a few locations in China. It is closely related to the Chinese species.
- Several species of Douglas fir grow in China. Like the Japanese Douglas fir and Formosan Douglas fir, these species are smaller than those of North America and have smaller cones.
Many other conifers have taken similar migrations from North America to Asia across the Bering Land Bridge including some of the true firs (Abies genus), spruce, and pines. And remember the ancient relatives of the Douglas fir, the larch? I suspect the larches took a similar journey, not only migrating to Asia, but to Europe as well. Here in North America, it appears that while migrating north, they also made a right turn and migrated to the East Coast and all across Canada. Some remained in central Alaska.
More recently and closer to home, Douglas fir has been adapting to different conditions here in western North America. Most notably, they have migrated north and south as well as up slope and down slope, with changes to the climate. Cold weather and glacial ice forced them south many times in the past thousands of years. When it warmed and the ice melted, they were able to migrate north again. Note that the glacial till left by the retreating glaciers was the ideal seed bed for the germination of Douglas fir seeds.
The Cascade Mountains have imposed an interesting split in the population of Douglas fir in the Pacific Northwest. As the saying goes in biology, “Absence makes the genes wander.” Whenever the members of a species are separated by a barrier, they will develop genetic differences. And if the environment is different in the separate populations, we will be able to see these differences as they adapt to the different conditions.
As everyone from Oregon and Washington knows, the climate west of the Cascades is mild and damp, well in truth, wet. On the other hand, east of the Cascades, it is dry and prone to temperature extremes and drought. As the separate populations of Douglas fir adapted to these different environments, they evolved into two distinct varieties, and we can now see the differences between them:
Douglas-fir: The Genus Pseudotsuga, Denis P. Lavender and Richard K. Hermann. You can download the PDF here.
Intercontinental disjunctions between eastern Asia and western North America in vascular plants highlight the biogeographic importance of the Bering land bridge from late Cretaceous to Neogene by Jun Wen, Ze-Long Nie, and Stefanie M. Ickert-Bond. Journal of Systematics and Evolution 54:469–490.
See the following topics at The Gymnosperm Database by Chris Earl: