Genetic (DNA) taxonomy of three species of oaks at Pleasant Valley Conservancy

It is easy to identify a tree as a member of the genus Quercus (“oak”), yet often difficult to distinguish a specimen at the species level. This is especially true for oaks of the section Lobatae, which includes red, black, and Hill’s oak, three species that are prolific at Pleasant Valley Conservancy. To keep things simple, I will call this the “red oak group”.

Key taxonomic characteristics of oaks include bud and leaf morphology, acorn shape (including the cap), and bark thickness and structure. If the specimen is a “good species”, the characteristics fit well, and a species name can be attached. But confusion often arises, and this is often due to hybridization.

It has been known for some years that hybridization is common in the red oak group, which explains why these species are hard to pin down taxonomically. It seems reasonable that if two or more of these species are growing in the same general area, hybridization might occur.

Present day plant taxonomy makes extensive use of DNA analyses, which provide “ground truth” for traditional morphological taxonomy.

We have been fortunate that botanist Andrew Hipp, Senior Researcher at the Morton Arboretum, has taken an interest in the oaks at Pleasant Valley Conservancy. (Andrew worked as a naturalist at the UW-Madison Arboretum, and received his Ph.D. working on sedges. He wrote an outstanding book on the Sedges of Wisconsin.)

Since Andrew took up his post at the Morton Arboretum, he has been working on the taxonomy of oaks, using DNA techniques.

We had a visit from Andrew in 2010, when he first confirmedthe presence of Hill’s oak at PVC. 

Later he returned with a research team and sampled a number of oak specimens for DNA analysis.

The details of the DNA studies are too complicated to present here, but can be found at the following link: 

Owusu, Sandra A., Sullivan, Alexis R., Weber, Jaime A., Hipp, Andrew L. and Gailing, Oliver. 2015. Taxonomic relationships and gene flow in four North America Quercus species (Quercus section Lobatae). “Systematic Botany”, Vol. 40(2): 510-521.

Andrew’s group studied red oak-group specimens from 17 separate geographic sites in the Midwest. My post here deals just with the oaks he sampled from PVC.

The map here is from Andrew’s paper, with a few labels added to indicate the approximate locations where the samples were taken. It shows the distribution of genetically pure, hybrid, and misclassified individuals. Each specimen is shown with the classification originally made based on traditional taxonomic criteria. Symbols with open centers indicate that the DNA data agreed with the taxonomy. If the symbol has a black circle, it means that the DNA indicated that specimen was a black oak but had been misclassified. Those with a black star indicate black X red oak hybrids. Those with a white plus sign are black X Hill’s oak hybrids.

Figure showing distribution of members of the red oak group from the research paper,
with labels added to show the approximate location at Pleasant Valley Conservancy.

The table below summarizes the data from this figure.

Analysis of hybridization in the red oak group, based on an analysis of 64 specimens from the north and east side of Toby’s Prairie.

Tree identity	How many?
Hill’s oak (Q. ellipsoidalis)	5
Red oak (Q. rubra)	20
Black oak (Q. velutina)	23
Hills X Black	2
Red X Black	5
Black misclassified as Hill’s	8
Black misclassified as Red	1

In sites such as PVC, where all three species are living close together, it is perhaps not surprising that hybrids (based on DNA) are common. A significant number of specimens of Hill’s and black oak had been misclassified (based on DNA). However, the DNA analysis indicated that quite a few tree specimens at PVC were not misclassified.

Andrew has now moved on to a study of the bur oak/white oak group, and will be back this summer to do DNA sampling from some of these trees.