Eva Legge, Biological Sciences, Fall 2020
Figure: Over the past 56 million years, oaks have come to dominate American forests. In fact, North America is home to 60 percent of the existing 435 quercus (oak) species
(Source: Pixabay)
Growing up in central Texas, live oak trees have been a defining part of my childhood. With their twisted branches, knobbled trunks, and tiny leaves, many oaks seem to come straight from a fairy tale. When I was young, I was so taken by their majesty that I actually did believe fairies lived in the trunks of these trees. Live oaks are not only striking in their figure, but also surprisingly numerous. Accounting for 8.4 percent of all trees in Austin, live oaks are the third most populous tree in the city (Nowak et al., 2016).
Austin is not unique in this abundance of oaks. Quercus, the oak tree genus, makes up more biomass than any other genus of woody plant in the United States and Mexico, and 60 percent of the existing 435 oak species reside in the Americas (Hipp et al., 2020). It is perhaps unsurprising, then, that oaks are a keystone species in the Northern Hemisphere, meaning that their presence is essential to the forest’s proper function. Botanists Hipp, Manos, and Cavender-Bares would go so far as to assert that oaks are “the single most important group of trees in [North American] forests” (Hipp et al., 2020). But how did oak trees become so diverse and widespread?
Before 2008, it was impossible for botanists to accurately decipher the oak’s evolutionary history. Their high rate of hybridization (i.e. mating between two oak species to make a third, novel species) has created a muddled evolutionary picture, and when botanists tried to use conventional biomolecular techniques, one hybrid species would often trace multiple lineages (Hipp et al., 2020).
But in 2008, three botanists discovered a genomic technique that revolutionized oaks’ evolutionary tree. This new technique, “restriction-site associated DNA sequencing,” allows botanists to read short pieces of DNA at a time, targeting one evolutionary history (Hipp et al., 2020). By pairing this analysis with data from the fossil record, they could now draw a detailed pathway of the oak’s ancestry, helping researchers understand how oaks became a keystone species in North America and predict what their future may hold in the face of the climate crisis.
Fifty-two million years ago, global temperatures dropped and glaciers expanded, forcing oak trees to migrate southwards towards the continental United States (Hipp et al., 2020). The oaks were met with a virtually blank slate, as the tropical forest that once dominated North America were forced south due to dropping temperatures. Met with unlimited possibilities, the oak trees made their way into every corner of the United States and into Mexico. As the oaks moved south, they split into separate lineages: one on the western side of the Rockies and one on the eastern side. The eastern line split further into a northeastern lineage, a southeastern lineage, and a Texan lineage, which gave rise to the oaks that now dominate my neighborhood (Hipp et al., 2020). One of the keys to the oaks’ success was the pace of its southward migration — they migrated and diversified perhaps faster than any other deciduous tree growing today, allowing them to quickly become established in their new territory (Davis, 1983).
As botanists traced the oak’s evolution, they came across some fascinating patterns. Distantly-related oaks coexisted in the same habitats, while closely-related oaks were separated by subtle differences in altitude and moisture (Hipp et al, 2020; Fallon, 2018). This pattern of “oak co-occurrence” was found in oak communities across the country, from the rocky bluffs of the eastern United States to the mountains of southern Arizona. The reason for this strange pattern is simple: oaks from the same lineage (i.e. closely-related oaks) tend to be specialized to slight climatic differences. Distantly related oaks, however, displayed convergent evolution, and would adapt to the similar climatic conditions as their distant relatives (Hipp et al., 2020). Growing far away from closely related species provides a key benefit: protection from pandemics. For trees, it is harder to catch a pathogen from a distant relation than from a close cousin, an advantage that has helped oaks dominate specific regions for centuries (Hipp et al, 2020).
Oaks have other qualities in their toolkit for success. Evidence suggests that oaks help other species take root by creating a soil environment that is favorable for the growth of symbiotic mycorrhizal fungi, an organism that helps trees acquire nutrients. Once oaks are established in a forest, they frequently outgrow and outnumber every other species (Hipp et al., 2020). In fact, oaks may be able to help their relatives by passing their genes over to neighboring oaks (primarily through pollen and seed), which allows them to adapt to local environmental conditions (Gerber, 2014). This so-called “adaptive gene flow” may have played a pivotal role in the oak’s evolutionary success, and could be vital for their survival in upcoming years. As climate change exposes plants across the country to drought and disease, the genetic diversity of oaks and the resilient features they exhibit may help these trees continue to thrive (Kremer & Hipp, 2019).
References
Davis, M. B. (1983). Quaternary History of Deciduous Forests of Eastern North America and Europe. Annals of the Missouri Botanical Garden, 70(3), 550. doi:10.2307/2992086
Gerber, S., Chadœuf, J., Gugerli, F., Lascoux, M., Buiteveld, J., Cottrell, J., . . . Kremer, A. (2014). High Rates of Gene Flow by Pollen and Seed in Oak Populations across Europe. PLoS ONE, 9(1). doi:10.1371/journal.pone.0085130
Fallon, B., & Cavender-Bares, J. (2018). Leaf-level trade-offs between drought avoidance and desiccation recovery drive elevation stratification in arid oaks. Ecosphere, 9(3). doi:10.1002/ecs2.2149
Hipp, A., Manos, P., & Cavender-Bares, J. (2020). Ascent of the Oaks: How they evolved to rule the forests of the Northern Hemisphere. Scientific American.
Kremer, A., & Hipp, A. L. (2019). Oaks: An evolutionary success story. New Phytologist, 226(4), 987-1011. doi:10.1111/nph.16274
Nowak, D. J., Bodine, A., Hoehn, R., Edgar, C., Hartel, D., Lister, T., & Brandeis, T. (2016). Austin’s Urban Forest, 2014 (Rep.). Newtown Square, PA: US Forest Service.