Several rare species of Graphidion lichen depend on hazel trees. Lactarius pyrogalus grows almost exclusively on hazel, and hazel is one of two kinds of host for the rare Hypocreopsis rhododendri. Ecology Īt least 21 species of fungus have a mutualistic relationship with hazel. The oldest confirmed hazel species is Corylus johnsonii found as fossils in the Ypresian-age rocks of Ferry County, Washington. Several hybrids exist, and can occur between species in different sections of the genus, e.g. Corylus ferox – Himalayan hazel, Himalaya, Tibet and southwest China (syn.Involucre densely spiny, resembling a chestnut burr.Corylus wangii – Wang's hazel, southwest China.Corylus jacquemontii – Jacquemont's hazel, Himalaya.Corylus fargesii – Farges' hazel, western China.Corylus colurna – Turkish hazel, southeastern Europe and Asia Minor.Corylus chinensis – Chinese hazel, western China.Involucre moderately spiny and also with glandular hairs.
Involucre short, about the same length as the nut.Nut surrounded by a soft, leafy involucre, multiple-stemmed, suckering shrubs to 12 m tall.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. 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. 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 fruits are nuts 1–2.5 cm ( 1⁄ 2–1 in) long and 1–2 cm diameter, surrounded by an involucre (husk) which partly to fully encloses the nut. The male catkins are pale yellow and 5–12 centimetres (2– 4 + 3⁄ 4 inches) 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 flowers are produced very early in spring before the leaves, and are monoecious, with single-sex catkins.
Hazels have simple, rounded leaves with double-serrate margins. The genus is usually placed in the birch family Betulaceae, though some botanists split the hazels (with the hornbeams and allied genera) into a separate family Corylaceae. The hazel ( Corylus) is a genus of deciduous trees and large shrubs native to the temperate Northern Hemisphere.