Plant 214

Juglans regia L. (Juglandaceae)


Common walnut

Common walnut is often grown in the United Kingdom as an ornamental, specimen tree although walnut tree roots release an enzyme inhibitor that suppresses the growth of other plant species. The species is widely cultivated in the temperate regions of the world for its edible crop and exceptionally hard timber. Common walnut originates from Central Asia, where the walnut forests of Tien Shan in Kyrgyzstan are considered a globally-important biodiversity hotspot. There are approximately 47,000 hectares of walnut forest in Kyrgyzstan, although large areas are under threat.

Walnut trees are very sensitive to climate change, despite surviving annual temperature ranges of between -24 and +36 degrees Celsius. Erratic weather conditions, particularly late spring frosts, damage shoots and flowers and can produce serious walnut crop failures. Drought and forest fires are also significant threats, both of which are exacerbated by climate change.

Following independence from the Soviet Union in 1991, Kyrgyzstan's economy crashed and the state-owned walnut forests became vital sources of income for local people; providing a valuable food crop, as well as fuel, timber and land for farming. As a result, human pressure on the forests increased; few walnuts were left to germinate and young trees were damaged by grazing animals. Forest management became unsustainable. Over recent years, conservationists and local people have been involved in decision making and long-term lease agreements have been developed. As local people have gained a greater stake in the future of these forests and their productivity, the quality of the management and sustainability of these forest resources has increased.

Walnuts are generally associated with positive health benefits. In parts of southern France it was noted that heart disease was rare despite a diet rich in saturated fat. This was initially linked to the consumption of red wine. However detailed investigation revealed that low levels of cholesterol were probably due to eating daily green salads dressed with chopped walnuts and walnut oil. Walnut oil is also rich in vitamin E, omega 3 and cancer-preventing antioxidants.

As walnut trees have been part of human cultures for millennia, it is unsurprising they are associated with many traditional beliefs. For example, the fruit is a symbol of fertility and prosperity. Through the Doctrine of Signature, the brain-shaped seed, inside a hard shell, led to the belief walnuts were good for the brain. Yet resting beneath the tree was thought to 'dull the brain' and lead to illness.

Further reading

Gauthier M-M and Jacobs DF 2011. Walnut (Juglans spp.) ecophysiology in response to environmental stresses and potential acclimation to climate change. Annals of Forest Science 68: 1277-1290.

Hemery GE and Popov SI 1998. The walnut (Juglans regia L.) forests of Kyrgyzstan and their importance as a genetic resource. Commonwealth Forestry Review 77: 272-276.

Sarah Lloyd