Morus is one of a group of genera whose species are divided between China and eastern North America. These so-called Tertiary relicts are evidence that a vast temperate forest stretched from the Mediterranean through Central Asia into eastern North America approximately 15 to 65 million years ago. White mulberry is distributed in a fragmented arc that approximates the Silk Roads. However, the precise distribution of white mulberry is unknown; thousands of years of anthropogenic change have blurred its native range. The most important human use of white mulberry is as food for silkworms.
Given the vast range of white mulberry, and its long history of association with humans, there is considerable variation in the habit, leaf shape and fruit colour and form of the species. Individual trees may also be very long lived, for example, the white mulberry in the Oxford Botanic Garden is a remnant of John Sibthorp's rearrangement of the Garden at the end of the eighteenth century. Silk farmers are likely to have unconsciously selected trees with low tannin and high protein since silkworms fed on tannin-rich, protein-poor leaves grow slowly and produce little, low-quality silk.
As an Imperial luxury the mysteries of silk manufacture were jealously guarded by the Chinese. For at least 2,500 years, classical European and North African civilizations imported silk in ignorance of its nature and source; Rome haemorrhaged gold to acquire silk. If 'woven wind' was to be affordable and not bankrupt western Exchequers, western populaces had to curb their desires for silk or wrest the secrets of sericulture from the Chinese.
Sericulture gradually spread along the Silk Roads to Constantinople and throughout Asia Minor. Once the secrets of silk production were out of Chinese hands, silk became cheaper. Attempts to grow silkworms in Britain and North America were prolonged disappointments. With the usual awe at the profits to be made, schemes to introduce sericulture started from the early 1600s and continued into the nineteenth century; all failed. In the 1830s, North American farmers' lost fortunes during 'multicaulus mania' speculating that sericulture could be established. An unforeseen legacy of mass white mulberry planting has been the species' establishment as a widespread weed.
Across Central Asia white mulberry fruits, whether fresh or dried, have been consumed for millennia and numerous forms unconsciously selected. Furthermore, white mulberry has become important as a protein-rich, livestock fodder, perhaps because of selection as a protein-rich silkworm food.
Feltwell J 1990. The story of silk. St. Martin's Press, London.
Zeng Q et al. 2015. Definition of eight mulberry species in the genus Morus by internal transcribed spacer based phylogeny. PLoSONE 10: e0135411.