A Closer Look at the Red Mulberry (Morus rubra L.)

Issue: 
January-February 2003

RED MULBERRY IS A WIND POLLINATED understory tree species that grows to 15 m and occasionally reaches into the canopy of deciduous forests. This monoecious/dioecious tree species flowers at leaf emergence in early spring and sets multiple purplish-red fruit in late summer.

Red mulberry is considered an integral component of the rare Carolinian forest ecosystem contributing to the biodiversity of the region by providing an important food source for migratory birds and various animals. Historical records indicate that red mulberry was an important source of lumber, being used for furniture and fence posts. Additional uses include medicinal compounds from the bark and leaves and the use of the fruit for human consumption in pies, jams and wine. Recently, red mulberry has enjoyed increased use as a landscape tree as the public has become more interested in gardening with native species. There is also some indication the red mulberry has special root characteristics that make it suitable for bioremediation.

Red mulberry is considered the most endangered tree species in Canada.

The genus Morus consists of approximately 12 species native to the temperate and sub-tropical regions of the northern hemisphere. Red mulberry is only found from Florida to Texas, and north to Vermont, southern Ontario, and South Dakota. In Canada, it is restricted to the Carolinian Zone of southern Ontario. Growing in moist forested habitats such as floodplains, bottomlands, sand spits and along the slopes of the southern portion of the Niagara Escarpment, red mulberry is currently found in only eight confirmed locations in Canada in which the populations contain five or more trees. These eight critical sites include Hamilton’s Royal Botanical Gardens, Ball’s Falls Conservation Area (the Niagara Peninsula Conservation Authority), Niagara Glen (Niagara Parks Commission), Rondeau Provincial Park (Ontario Ministry of Natural Resources, OMNR), Point Pelee National Park (Parks Canada), Fish Point Provincial Nature Reserve, Pelee Island (OMNR), Middle Island (Parks Canada) and East Sister Island (OMNR). Historical records indicate that the abundance and distribution of red mulberry has been reduced substantially in the last century due to urban and agricultural expansion.

Red mulberry is considered the most endangered tree species in Canada. It has been recognized as "endangered" in Canada by COSEWIC, the Committee on Status of Endangered Wildlife in Canada (G5 S2), and is listed as "threatened" or "rare" in three northern US states. Efforts to conserve the species have received strong support from land managers and naturalists and a recovery plan has recently been developed for the species. Red mulberry is receiving considerable conservation attention not only because of its low numbers (there are only 234 trees left in Canada across a total of 18 sites) but also because it is suspected of being under pressure from hybridization with a more abundant introduced species, white mulberry (Morus alba L., Moraceae). However, there is no scientific data to confirm the magnitude of hybridization in red mulberry populations or the impacts that it may have on persistence of the species.

Researchers at the University of Guelph are currently examining the effects of hybridization on endangered populations of red mulberry using molecular and morphological markers. Here, biotechnological tools are providing genetic markers that are specific to either red or white mulberry, which are then used to detect the occurrence of hybrids. DNA fingerprinting of red, white and hybrid mulberry trees is required as a first step in the recovery process because tree identification is extremely difficult based on leaves or fruit alone, as white mulberries also have red fruit and leaf morphology is extremely variable among both species. Molecular tools are also being used to determine the impact of white removal and the success of transplanting efforts.

This research not only contributes to an understanding of the consequences of hybridization in rare species but also to the conservation of red mulberry populations in southern Ontario. The research further yields insights of direct benefit to Ontario’s national and provincial parks, the National Recovery Plan established for this species, as well as the greater scientific community. Research is funded by the University of Guelph, the World Wildlife Fund, the Canadian Forestry Service and the Canadian Wildlife Service.

By Kevin Burgess, Ph.D., a candidate in the Plant Population and Evolution Research Laboratory at the University of Guelph. Please feel free to contact him for more information at burgessk@uoguelph.ca or 519-824-4120 (x6009). Photos from Trees of Ontario, Lone Pine Publishing, 2001.

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