I look forward to the blooming of the redbuds (Cercis spp.) every spring. They paint entire swaths of forest and roadside with a gentle pink haze. It’s this beauty that has led to their popularity as an ornamental tree in many temperate landscapes. Aside from their appeal as a specimen tree, their evolutionary history and ecology is quite fascinating. What follows is a brief introduction to this wonderful genus.
The redbuds belong to the genus Cercis, which resides in the legume family (Fabaceae). In total, there are about 10 species disjunctly distributed between eastern and western North America, southern Europe, and eastern Asia. The present day distribution of this genus is the result of vicariance or the geographic separation of a once continuous distribution. At one point in Earth’s history, The genus Cercis once spanned from Eurasia to North America thanks to land bridges that once connected the continents. At some point during the Miocene, this continuous distribution began to break apart. As the climate changed, various Cercis began to diverge from one another, resulting in the range of species we know and love today.
All of them are relatively small trees with beautiful pink flowers. Interestingly enough, unlike the vast majority of leguminous species, redbuds are not known to form root nodules and therefore do not form symbiotic relationships with nitrogen-fixing bacteria called rhizobia. This might have something to do with their preference for rich, forest soils. Until more work is done on the subject, its hard to say for sure why they don’t bother with nitrogen fixers.
One of the most interesting aspects of the redbuds are their flowers. We have already established that they are quite beautiful but their development makes them even more interesting. You have probably noticed that they are not borne on the tips of branches as is the case in many flowering tree species. Instead, they arise directly from the trunks and branches. This is called "cauliflory," which literally translates to "stem-flower." In older specimens, the trunks and branches become riddles with bumps from years of flower and seed production.
It's difficult to make generalizations about this flowering strategy. What we do know is that it is most common in dense tropical forests. Some have suggests that producing flowers on trunks and stems makes them more available to small insects or other pollinators that are more common in forest understories. Others have suggested that it may have more to do with seed dispersal than pollination. Regardless of any potential fitness advantages cauliflory may incur, the appearance of a redbud covered in clusters of bright pink flowers is truly a sight to behold.