Norm C. Kenkel
Department of Botany, University of Manitoba
Winnipeg, Manitoba, Canada
E-mail: kenkel@umanitoba.ca
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Northern range extension of Tilia americana L.
(Basswood, Linden) at Delta, Manitoba
Norm C. Kenkel Department of Botany, University of Manitoba Winnipeg, Manitoba, Canada E-mail: kenkel@umanitoba.ca |
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Tilia americana L. (known as basswood, linden, lime or whitewood) is indigenous to North America. A large tree (in Manitoba, to 25-30 m in height and 1 m in diameter), it forms a straight trunk that is branch-free on its lower portions. The main branches are slender and somewhat arching, forming a pyramidal to rounded crown. The species commonly regenerates from the base of old stumps, and it is not uncommon for individuals to occur as a clump of trunks with a common base. The dark grey-brown bark is smooth on young trees but forms into long, narrow scaly ridges on mature specimens. The root system is wide-spreading and deep. Leaves are relatively large, alternate, and heart-shaped. The fruit is globose, indehiscent, and nut-like, and is attached by a long rachis to the back of a large bract that acts like a samara wing (Boivin 1967).
Tilia americana normally grows in mixture with other hardwood species, and rarely forms pure stands. In Canada, it is characteristic of deep, fertile soils (Hosie 1979), although it may be found on less fertile soils in parts of the United States. According to Fowells (1965), inadequate precipitation limits the distribution of the species in the mid-western United States. Few established seedlings are found where the species forms a major component of the canopy. It is thought that seedling loss is mainly the result of herbivory by rabbits and deer (Fowells 1965). The ability of the species to regenerate from stump sprouts has ensured its survival in areas where it has been logged.
Tilia americana is native to the Northern Deciduous and Great Lakes - St. Lawrence forest regions of North America. However, it also extends into grassland areas along river courses in Manitoba and the mid-western United States, where it forms a component of riverine gallery forests. In Canada, it is found from western New Brunswick into southern and central Québec and Ontario, extending as far west as north-western Ontario (along the U.S. border) and southern Manitoba. In the United States, the species occurs as far south as the mountainous regions of North Carolina, Tennessee, and northern Arkansas. The western limit for the species is south-central Manitoba and North Dakota, and along the Niobrara River in north-central Nebraska.
Tilia americana reaches its northern limit in the province of Manitoba (Scoggan 1957), where it occurs sporadically in gallery forests along the Red and Assiniboine rivers and their tributaries. According to Scoggan (1957), the species occurs as far west as Brandon (WIN specimen no. 17996), though Oswald and Nokes (1988) give its western limit as Spruce Woods (about 40 km east of Brandon). Along the Red River, the species occurs naturally as far north as the city of Winnipeg. Scoggan (1957) lists the most northerly locality as Portage la Prairie (49°58'N, 98°15'W, WIN specimen no. 17994).
In Canada, basswood has been widely planted outside its range. Boivin (1967) reports that the species has naturalized in the Moose Jaw, Saskatchewan area.
In May 1994, while undertaking a reconnaissance survey of Oxbow Woods (50°09'N, 98°22'W, about 2 km south of the southern shore of Lake Manitoba), I found a single specimen of basswood (two trunks with a common base, DBH » 80 cm). The tree is an 'emergent' (taller than the main canopy), and occurs near the shore of the Blind Channel, a former (3000 year old) channel of the Assiniboine River. The tree appears to be very healthy and has no large dead branches. Associated tree species are Acer negundo (Manitoba maple), Fraxinus pennsylvanica (green ash), Quercus macrocarpa (bur oak), Ulmus americana (American elm) and Populus balsamifera (balsam poplar). An extensive search of Oxbow Woods (an area > 70 ha) revealed no other individuals of the species. No seedling regeneration was observed, although the tree was observed to be fruiting in the fall of 1994. A fruiting specimen has been deposited in the University of Manitoba herbarium (WIN specimen no. 56298), and another in the herbarium of the University Field Station.
The species has not been previously recorded from the area. Löve and Löve (1954) do not mention the occurrence of basswood, nor is it included in the list prepared by Shay (1975). It seems likely that the specimen, despite its size, was missed by previous investigators since it is the only one in the area.
The occurrence of Tilia americana L. at Oxbow Woods, Delta represents a 22 km northern range extension of the species. This single tree is the most northerly naturally-established specimen of the species on the planet.
Tilia americana L. has long occurred in Manitoba, as evidenced by the presence of small amounts of basswood pollen in sediment cores taken from the southern basin of Lake Manitoba (Nambudiri and Shay 1986). The occurrence of the species along the Blind Channel is perhaps not surprising given the Holocene evolution of southern Manitoba. Teller and Last (1981) report that the Blind Channel is a former main channel of the Assiniboine River that drained into Lake Manitoba between 4500 and 3000 B.P. About 3000 years ago, the Assiniboine changed course to flow into the Red River as it does today. Rannie et al. (1989) undertook a more detailed Holocene study based on radiocarbon dating and paleochannel morphology. They concluded that at least part of the Assiniboine channel flowed into Lake Manitoba between 6000-7000 and 3000 years B.P., with the Blind Channel phase occurring between 4520 and 2980 years B.P. Given that basswood is today found sporadically along the Assiniboine River between Brandon and Winnipeg, its occurrence along the former river channels might have been anticipated. A botanical exploration should be undertaken along the Blind Channel between Lake Manitoba and Portage la Prairie to determine whether the species occurs elsewhere in the region.
Boivin, B. 1967. Flora of the prairie provinces. Part I. Provancheria 2. Mémoires de l'Herbier Louis-Marie. Faculté d'Agriculture, Université Laval.
Fowells, H.A. 1965. The silvics of forest trees of the United States. U.S.D.A. Agricultural Handbook No. 271. Washington, D.C.
Hosie, R.C. 1979. Native trees of Canada. 8th edition. Fitzhenry and Whiteside, Don Mills, Ont.
Löve, A. and D. Löve. 1954. Vegetation of a prairie marsh. Bull. Torrey Bot. Club 81: 16-34.
Nambudiri, E.M. and C.T. Shay. 1986. Late pleistocene and holocene pollen stratigraphy of the Lake Manitoba basin, Canada. Palaeontographica Abt. B 202: 155-177.
Oswald, E.T. and F. H. Nokes. 1988. Field guide to the native trees of Manitoba. Canadian Forestry Service, Publ. MG-10041.
Rannie, W.F., L.H. Thorleifson and J.T. Teller. 1989. Holocene evolution of the Assiniboine River paleochannels and Portage la Prairie alluvial fan. Can. J. Earth Sci. 26: 1834-1841.
Scoggan, 1957. Flora of Manitoba. National Museum of Canada, Ottawa. Bull. No. 140, Biological Series No. 47.
Shay, J.M. 1975. Oxbow species list. Unpublished document, University of Manitoba Field Station.
Teller, J.T. and W.M. Last. 1981. Late Quaternary history of Lake Manitoba, Canada. Quat. Res. 16: 97-116.