Tree hybrids describe individual trees that appear to share characteristics of two different species. Hybrid animals are familiar to everyone, from dog-wolf hybrids to ligers (lion-tigers). It may not be as easy to notice hybrid trees, but they are quite common in nature. In the Bluegrass, oak hybrids are so common that it is often difficult to identify individual species. The red oaks, including Shumard oak, southern red oak, pin oak, Northern red oak and black oak are especially prone to hybrid formation. In many areas of the Bluegrass, it is impossible to assign individual trees to particular species.
At Quiet Trails State Nature Preserve, for example, I have tried in vain to identify many of the trees to a known species. After over 30 years looking at trees in Central Kentucky, I am convinced that the red oaks of the Bluegrass form a complex genetic mix among trees of the Cumberland Plateau and trees of the Mississippi Valley. I have as yet no genetic evidence that this is true, but hope to get some soon. Complex, stable mixes of species are sometimes called hybrid swarms.
If hybrid breeding is that easy among oak species, why do the species still exist? You might think that easy hybridization would blur the lines among species, and that new species would arise. This is certainly true over long periods of time (evolutionary time), and is one way new species arise. However, the vast majority of oak trees are still recognizable to species. This shows us that hybridization is not only rare, but tends not to lead to new species formation very often.
Part of the reason for this is that some hybrid oaks do not reproduce – they are as sterile as a mule. Others may reproduce but the progeny look like one of the grandparents. For example seeds I took from a Deam’s oak (Quercus x deamii, also known as Quercus muehlenbergii x macrocarpa) produced seedlings that looked exactly like bur oak. This is probably because the male parents of these trees were all bur oaks.
Modern molecular genetics tools can help us understand the nature of hybrids in trees, although only a few studies have been done. In Europe, English oak, Q. robur, and sessile oak, Q. petraea, commonly hybridize. The range of the two species overlaps (they are sympatric), and they cross-fertilize easily.
In spite of this, Graham Muir, Colin Fleming and Christian Shlotterer (Nature 2000, 405:1016) found that at the molecular genetic level, the two species retain their distinct identities. This is true in spite of the fact they they are often difficult to distinguish in the field.
So, despite the ease of hybrid formation in this cross, the individual species are stable.
We don’t yet fully understand why species can easily hybridize yet retain their species identities over long periods of time. Molecular genetics is a fairly new science, especially when compared with the lifetime of a tree. So a lot more interesting information is yet to be revealed.
Among the Bluegrass Venerable Trees, there are some known hybrids listed here. These are natural hybrids first identified in the field, and does not include hybrids produced in horticulture. We will add more information as we find more hybrids.
Bur oak hybrids
- Hybrids with white oak, Quercus x bebbiana, Bebb oak
- Hybrids with swamp white oak, Quercus x schuettii, Schuetti oak or swamp bur oak.
- Hybrids with bur oak oak, Quercus x deamii, Deam’s oak. These are occasionally found throughout the Bluegrass, including one fine specimen in Lexington.
Bebb oak has been found in Kentucky, but Scheutti oak has not.
Chinkapin oak hybrids
- Hybrids with bur oak oak, Quercus x deamii, Deam’s oak. These are occasionally found throughout the Bluegrass, including one fine specimen in Lexington.
Shumard oak hybrids
Shumard oak hybridizes with all the other red oaks, including (in Kentucky), Northern red oak, Southern red oak, pin oak, shingle oak, willow oak, water oak, black oak and blackjack oak. In the Bluegrass, hybrids of uncertain parentage are common. The taxonomic status of Shumard oak in the Bluegrass in uncertain, and it may be a part of a multi-species hybrid swarm. We are planning further research on this subject.
Kingnut hybrids
- Hybrids with sweet pecan, Carya x nussbaumeri.
- Hybrids with pignut hickory, Carya x dunbarii, Dunbar oak
We have not seen any of these in the Bluegrass. Pecan is not native to the Bluegrass, so the first hybrid would probably only be found in west of the Bluegrass. The second hybrid may exist in our forests but might be hard to recognize. It has only been reported in Monroe and Livingston Counties in western Kentucky.
Blue ash
Hybrids are not known for blue ash