Hazel D. Dean

The hazel (''Corylus'') is a genus of deciduous trees and large shrubs native to the temperate Northern Hemisphere. The genus is usually placed in the birch family Betulaceae,Germplasmgobills Information Network
''Corylus''
Rushforth, K. (1999). ''Trees of Britain and Europe''. Collins .Huxley, A., ed. (1992). ''New RHS Dictionary of Gardening''. Macmillan . though some botanists split the hazels (with the hornbeams and allied genera) into a separate family Corylaceae. The fruit of the hazel is the hazelnut.

Hazels have simple, rounded leaves with double-serrate margins. The flowers are produced very early in spring before the leaves, and are monoecious, with single-sex catkins. The male catkins are pale yellow and long, and the female ones are very small and largely concealed in the buds, with only the bright-red, 1-to-3 mm-long styles visible. The fruits are nuts long and 1–2 cm diameter, surrounded by an involucre (husk) which partly to fully encloses the nut.

The shape and structure of the involucre, and also the growth habit (whether a tree or a suckering shrub), are important in the identification of the different species of hazel.

The pollen of hazel species, which are often the cause for allergies in late winter or early spring, can be identified under magnification (600×) by their characteristic granular exines bearing three conspicuous pores.

Species

''Corylus'' has 14–18 species. The circumscription of species in eastern Asia is disputed, with ''WCSP'' and the ''Flora of China'' differing in which taxa are accepted; within this region, only those taxa accepted by both sources are listed below.WCSP
''Corylus''
/ref>Flora of China
''Corylus''
/ref>Flora of North America
''Corylus''
/ref> 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 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.

Ecology

At least 21 species of fungus have a mutualistic relationship with hazel. ''Lactarius pyrogalus'' grows almost exclusively on hazel, and hazel is one of two kinds of host for the rare '' Hypocreopsis rhododendri''. Several rare species of Graphidion lichen

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