Corylus has around 14–18 species. The circumscription of species in
eastern Asia is disputed, with
World Flora Online and the
Flora of China differing in which taxa are accepted, within this region. WFO accepts 17 species while Flora of China accepts 20 species (including
Corylus mandshurica). Only those taxa accepted by both sources are listed below. The species are grouped as follows: • Nut surrounded by a soft, leafy
involucre, multiple-stemmed, suckering
shrubs to 12 m tall • Involucre short, about the same length as the nut •
Corylus americana – American hazel, eastern
North America •
Corylus avellana – Common hazel,
Europe and
western Asia •
Corylus heterophylla – Asian hazel,
Asia •
Corylus yunnanensis – Yunnan hazel, central and southern
China • Involucre long, twice the length of the nut or more, forming a 'beak' •
Corylus colchica – Colchican filbert,
Caucasus •
Corylus cornuta – Beaked hazel, North America •
Corylus maxima – Filbert,
southeastern Europe and
southwest Asia •
Corylus sieboldiana – Asian beaked hazel,
northeastern Asia and
Japan (syn.
C. mandshurica) • Nut surrounded by a stiff, spiny involucre, single-stemmed
trees to 20–35 m tall • Involucre moderately spiny and also with glandular hairs •
Corylus chinensis – Chinese hazel, western China •
Corylus colurna – Turkish hazel, southeastern Europe and
Asia Minor •
Corylus fargesii – Farges' hazel, western China •
Corylus jacquemontii – Jacquemont's hazel,
Himalaya •
Corylus wangii – Wang's hazel,
southwest China • Involucre densely spiny, resembling a chestnut burr •
Corylus ferox – Himalayan hazel, Himalaya,
Tibet and southwest China (syn.
C. tibetica). Several
hybrids exist, and they can occur between species in different sections of the genus, e.g.
Corylus × colurnoides (
C. avellana ×
C. colurna). The oldest confirmed hazel species is
Corylus johnsonii found as
fossils in the
Ypresian-age rocks of
Ferry County, Washington. Chilean hazel (
Gevuina avellana), despite its name, is not related to this genus. == Ecology ==