Oak or oak?

Anyone who thinks that an oak is simply an oak could be wrong. And often, only genetics can help to identify them.

The three most common oak species in Switzerland – pedunculate oak, sessile oak, and downy oak – have overlapping leaf characteristics and are therefore difficult to tell apart. On top of that, they interbreed, producing hybrids that carry traits of both parent species or show intermediate features. Site conditions such as soil moisture or altitude can also alter leaf appearance. However, for practical purposes, whether for planting or forest management, it is crucial to know which species you are dealing with. The same applies to research.

Genetic analyses can identity species without relying on penotypic characteristics. At WSL, we developed an approach (using SNPs) that does not only reliably distinguishes among the three oak species but also estimates the degree of admixture resulting from hybridization and backcrossing. Hybridization occurs when two species mix. If this is followed by further mating with one of the parental species, this is referred to as backcrossing.

Genes don't lie

Ternary plot showing the distribution of three oak species: 'Sessile oak', 'Pedunculate oak', and 'Dwarf oak'. Data points are represented in green across the triangular grid.
Each of the 827 points represents a tree that has been examined. The axes show the probability (in %) that it belongs to the pedunculate oak, sessile oak, or downy oak species – or somewhere in between.

We genetically analyzed over 800 oaks from Switzerland – with a clear result: the so-called oak species complex includes both pure species and all kinds of hybrids.

A comparison between an independent species identification based on leaf characteristics and genetic data showed that, while the two methods generally agree well, the classification remains sometimes uncertain for sessile oak, downy oak, and their hybrids.

Which oak is it?

In these three leaves, the leaf petioles have different lengths. However, this characteristic alone is not enough to identify the species. 

Drei Eichenblätter nebeneinander, A: braun, B: grün, C: gelblich-braun, jeweils mit unterschiedlichen Texturen und Formen.

Experts use a magnifying glass and an identification key similar to this cheat sheet:

And the answer is:

A= Downy oak

B= Pedunculate oak

C= Sessile oak

In our online game, you can try to identify the species like the pros with close-up shots of the leaves. After a short warm-up round with pure species, hybrids also appear on the scene – oaks with mixed genetic material and leaves that are almost impossible to classify.

At that point, only genetics can provide certainty.

Contact