We all know about the magnificent redwood forests of the West Coast, which boast the world's largest living trees, giant sequoias (Sequoiadendron giganteum). These trees are so large that they would have given their prehistoric cousins a run for their money. But have you ever wondered how these trees got there? Where did they come from? Are they native to the United States, or are they a non-native that expanded its range millennia ago?
Nebraska has far fewer trees than either coast, but still, I can’t help but wonder how our trees, like the tough-as-nails bur oak and ponderosa pine, got here. And more specifically, how did they get here so long ago that they’ve had thousands of years of evolutionary relationship-building?
These are the questions a tree nerd like me thinks about almost every day. One of the most interesting aspects of tree biology is the evolutionary ecology that has shaped our forests into how they look today. Both biological and anthropogenic factors have influenced our forests, and one could argue that neither of these limiting factors is more important than the other.
What’s the Woolly Mammoth Got to Do with It?
Roughly 200,000 years ago, the woolly mammoth was theorized to have crossed the Bering Strait land bridge from Asia, making its way southeast onto the northern plains of what is now modern-day Canada. Other megafauna, such as giant sloths, are theorized to have migrated to North America roughly 8 million years ago.
Megafauna were an integral part of seed dispersal in the prehistoric landscape of North America. Today, one mature elephant can eat up to a thousand pounds of vegetation in a single day, spreading seeds up to 35 miles from the parent tree. You can imagine the impact of the much larger woolly mammoths in increasing the range of several plant species in prehistoric North America.
Glacier movement also played a profound role in reshaping prehistoric forests. Glaciers advanced and retreated over thousands of years, changing the climate and forcing species to adapt to variations in soil temperature and moisture availability. A very good example of these changes can be seen right in our own backyards. The fertile Great Plains soil was created when ancient oceans and ice sheets deposited till, or sediment, as they receded.
Another good example can be seen in the borealis forests in the northern region of North America. When glaciers retreated, tundra was replaced by spruce forest, which then was replaced by deciduous forest in places where the climate was significantly warmed.
East Asian Cousins
A 2015 study by Duke University investigated the evolutionary origins of plant species within the southern Appalachians. The researchers specifically chose this region because it has more tree species than anywhere else in the United States. Several of the plant species included in this study also happen to be considered “native” or “naturalized” in Nebraska. The researchers concluded that over half of the 250 tree species studied could trace their roots back to East Asia.
For example, the alternate-leaved dogwood (Cornus alternifolia) was shown to have split from its East Asian cousins approximately 22 million years ago, around the same time that most of the deciduous forests in Laurasia (one of Pangaea’s landmasses) started to decline. On the other hand, this research also suggests that roughly 25% of all species in our eastern forests—including hawthorn and most oaks—arose within eastern North America. The remaining 25% of trees are theorized to have arisen within the western part of North America, with very few tracing their origins back to Europe.
The Human Impact
Between 16,000 and 25,000 years ago, early humans made their way into North America using land-bridge structures similar to those used by the megafauna before them. Research suggests that over the next thousand years, the early humans hunted as many as 19 herbivorous animals larger than modern elk to the point of extinction. As their main seed distributors were hunted to extinction, trees struggled to expand their ranges.
The formation of our native North American forests is a complex geoecological issue cloaked in enigma. Research points to several causes, and while there is no one single explanation to the question of how or why North American forests changed over time, we do know that the migration of megafauna across the Bering Strait, glacial movements and historical human impact all played a critical role.
Sources:
https://today.duke.edu/2015/05/easternforests
https://www.americanforests.org/article/north-american-forests-in-the-age-of-man/
https://treescharlotte.org/tree-education/a-brief-history-of-trees/
https://www.americanforests.org/article/the-trees-that-miss-the-mammoths/
