

(click on the image to enlarge) Distribution of Quercus sadleriana and Q. pontica(click on the image to enlarge) [2]

At the 8th International Oak Society conference at The Morton Arboretum we learned in one of the many interesting presentations[1] that the origin of the Eurasian ‘White Oaks’ (also called the roburoids, such as Q. robur, aliena, faginea, petraea, macranthera, fabri, griffithii, etc.) lies in the Americas. Predecessors of the current species migrated from this area eastwards (and westwards?) and thus the present day distribution of the Eurasian White Oaks gradually evolved. In some cases, relict populations occurred. For example, among the Californian White Oaks Quercus sadleriana proved to be something special. Its closest relative is Q. pontica, growing in the South Caucasus and in North Eastern Turkey, some 10,000 km apart and on another continent! The theory is that a common ancestor traveled via a land bridge some 10 million years ago to what is now called Europe and later became extinct. The two mentioned species developed from this parent and survived in

Leaves of Q. 'Tromp Deerpon'

two relict populations on two continents: North America and Europe.

Both species show some similarities. Q. sadleriana is a low-growing shrubby oak, 1 – 3 m in height, stoloniferous, evergreen, and it has a serrate leaf margin. It grows on open, rocky slopes, ridges, in conifer forest; at an altitude of 600–2200 m. in the Siskiyou region of Northern California and the Klamath Ranges in Southwestern Oregon. Q. sadleriana hybrids exist with Q. garryana var. breweri, also a White Oak but with lobed leaves.

The vernacular name for Q. sadleriana is deer oak and it was discovered in 1852 by plant hunter John Jeffrey. The oak was not officially described till 1871, by Robert Brown (R. Br. ter.).

Its closest relative, Q. pontica, is also a shrubby oak but growing up to 8 m. The leaves are much larger than those of Q. sadleriana and it also has a serrate leaf margin. The tree is not evergreen (in contrast to Q. sadleriana), but it also grows in mountainous habitat, in this case in the Caucasus and Pontic Mountains. Q. pontica was described by K. Koch in 1849. This species is known to hybridise

New leaves and catkins on Q. 'Tromp Deerpon'

with other species: Q. ×hickelii is the offspring of Q. pontica × Q. robur, and Q. ‘Pondaim’ originated in Trompenburg as the offspring of Q. pontica × Q. dentata.

So it is likely that both these species are able to hybridise with other oak species as well. The existence of a hybrid between Q. sadleriana and Q. robur is mentioned in the UK. In Trompenburg, Rotterdam, The Netherlands, we have Q. sadleriana seedlings growing from a tree in our collection. They are different from the mother plant in several characteristics that show that the pollen parent must have been a Q. pontica, which is quite possible because that tree was located not very far away. The leaves of the finest offspring of this cross are larger and the winter buds are closer, both characteristics indicating that the pollen plant was Q. pontica. From the female parent our selection inherited denser branching and on some branches the foliage persists (i.e., remains green) over winter. Following Dick van Hoey Smith’s tradition for naming Trompenburg introductions, this selection was named Quercus ‘Tromp Deerpon’ (deer oak × Pontic oak). Already several nurseries have received propagating material of this extraordinary hybrid and registration is in preparation.

Quercus 'Tromp Deerpon' at Trompenburg Arboretum in The Netherlands Q. pontica, pollen parent of Q. 'Tromp Deerpon'