Jennifer Barker and Norm C. Kenkel
Department of Botany, University of Manitoba
Winnipeg, Manitoba, Canada
E-mail: kenkel@umanitoba.ca
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Biotic inventory and development of a low impact, self-guided trail in Oxbow Woods, Delta, Manitoba
Jennifer Barker and Norm C. Kenkel Department of Botany, University of Manitoba Winnipeg, Manitoba, Canada E-mail: kenkel@umanitoba.ca |
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The term 'gallery forest' (a fringe forest) is often used in reference to river bottom forests in the prairie regions of North America (e.g., Killingbeck and Wali 1978). Weaver (1960) offers an overview of these forests in the Missouri-Nebraska-Kansas-South Dakota region of the United States. A number of additional papers have described the structure and floristic composition of gallery forests in the midwestern United States (e.g., North Dakota: Wikum and Wali 1974, Killingbeck and Bares 1978, Keammerer et al. 1975, Johnson et al. 1976, Reily and Johnson 1982; South Dakota: Wilson 1970; Illinois: Bell and del Moral 1977, Hosner and Minckler 1963; Oklahoma: Ware and Penfound 1949; Wisconsin: Barnes 1985, Dunn and Stearns 1987; Kansas: Abrams 1986; Minnesota: Noble 1979; Nebraska: Weaver et al. 1925). In Canada, well-developed prairie gallery forests are restricted to the province of Manitoba. General descriptions of forests bordering the south shore of Lake Manitoba, which show a strong floristic affinity with riverine gallery forests, include Lšve and Löve (1954), MacKenzie (1982) and Kenkel (1986). Essenburg (1991) offers a floristic description of gallery forests in southern Manitoba.
Prairie gallery forests develop on eroded river sediments. Prairie rivers are continually changing their course, eroding sediments on the outside of a meander while depositing material on the inside. These accumulated nutrient-rich sediments provide a substrate for vegetation colonization. The first woody species to colonize on the moist sediments are willows (Salix exigua, S. nigra, S. amygdaloides) and the cottonwood (Populus deltoides). This vegetation stabilizes the substrate and allows later-successional species to establish and grow (Wilson 1970, Noble 1979, Barnes 1985). Mature gallery forests in North Dakota and Manitoba are generally dominated by Fraxinus pennsylvanica (green ash), Acer negundo (Manitoba maple, box elder), Ulmus americana (American elm) and Tilia americana (basswood) (Kenkel 1986, Killingbeck and Bares 1978). Flooding-intolerant species such as Quercus macrocarpa (bur oak), Populus tremuloides (trembling aspen), and P. balsamifera (balsam poplar) occur in areas less prone to spring flooding. Gallery forests further south generally have a higher tree diversity. Celtis occidentalis (hackberry), which also occurs along the south shore of Lake Manitoba, is common from South Dakota southward. Other tree species occurring further south include silver maple (Acer saccharinum), river birch (Betula nigra), various oak and ash species, and a number of others (for a complete list, see Abrams 1986, Hosner and Minckler 1963, Weaver 1960).
In southern Manitoba, gallery forests occur in the valleys of the Red and Assiniboine rivers and their various tributaries. At the landscape level, these forests represent critical wildlife habitat and are important as corridors for the movement and dispersal of wildlife. Habitat fragmentation and degradation of these forests, the result of cutting for firewood and the clearing of land for agriculture and housing, is a serious problem in the province. In many areas river bottom forests have been completely destroyed, while the majority of the remaining gallery forest has been highly modified from excessive trampling and other human disturbances. These disturbances have resulted in a severe reduction in the floristic diversity of native species in these forests, while at the same time increasing the abundance of Eurasian invasive weedy species.
The objectives of this study were to: (a) complete a floristic inventory of Oxbow Woods, a large undisturbed gallery forest near Delta, Manitoba, and (b) develop an interpretive trail through this gallery forest, so as to provide cultural, natural and ecological information for educational use.
The Oxbow Woods gallery forest is located in southern Manitoba at 50_10'N, 98_20'W, at an elevation of approximately 250 m a.s.l. The forest is located on high ground adjacent to the Delta Marsh, about 2 km south of Lake Manitoba.
Approximately 13,500 years ago, glaciation in north-central Canada led to the formation of glacial Lake Agassiz, the result of water impoundment behind the northern ice sheet. Water levels subsided as the ice retreated northward, creating a flat landscape of clay soils on the former lake bottom. Between 9.5 and 6 thousand years ago the Lake Manitoba basin is thought to have alternately drained and filled in response to changing environmental conditions (Teller and Last 1981). All or part of the Assiniboine River system is thought to have drained into the south end of Lake Manitoba between 6,000-7,000 years ago and 3,000 years ago. Oxbow Wood occurs along one of three major drainage channels. Known as the Blind Channel, it drained the Assiniboine River into Lake Manitoba between about 4,500 to 3,000 years ago. After this time the river changed its course to drain into the Red River, as it does to this day (Rannie et al. 1989).
Southern Manitoba is characterized by a continental climate, with short warm summers and long, cold winters. Mean annual temperature is 1.9°C, with monthly means ranging from 19.2°C (July) to -18.1°C (January). Annual mean precipitation is » 50 cm, about 60% of which falls between May and September (Environment Canada, means for the period 1967-1991, Delta University FS).
The Oxbow Woods is strongly aligned floristically with the deciduous forests of eastern North America. The dominant tree species found in this forest, including bur oak, green ash and Manitoba maple, have an eastern deciduous forest affinity (Rudd 1951, Lšve 1959). Hackberry and basswood, which occurs sporadically in the Delta Marsh area, are also of eastern affinity.
Eastern deciduous forest understory species found in Oxbow Woods include Smilacina racemosa, Trillium cernuum, Menispermum canadense, Rudbeckia laciniata, Phryma leptostachya, Amphicarpa bracteata and Celastrus scandens. Carex assiniboinensis, which is often dominant in the understory of Oxbow Woods, is indigenous to continental North America (Manitoba and Saskatchewan, and south to northern Iowa and South Dakota). Eurasian invasive weedy species are uncommon, indicating minimal anthropogenic disturbance in Oxbow Woods.
In many parts of Oxbow Woods, woody shrubs form a major component of the biomass. Important species include American hazelnut (Corylus americana), choke cherry (Prunus virginiana), saskatoon (Amelanchier alnifolia) and nannyberry (Viburnum lentago).
A number of species common to Oxbow Woods reach their North American northern and/or western distributional limits in southern Manitoba (Löve 1959). The threatened gallery forests of southern Manitoba thus represent a unique community assemblage deserving of protection.
Fire is a natural part of the history of southern Manitoba. Evidence of lightning strikes are apparent throughout Oxbow Woods. Most of these strikes take out a single large oak or ash tree. On occasion, lightning strikes may start localized ground fires which kill trees in larger patches.
Prairie fires may also burn into the forest, as happened during an accidental fire in April 1991. This fire burned into the extreme northern section of Oxbow Woods, killing a number of trees in the transitional forest-grassland region adjacent to the closed forest.
Porcupines are occasionally seen in Oxbow Woods, and may open the forest canopy by pruning the upper branches of large trees. Browsing by rabbits and white-tailed deer, both of which are abundant in Oxbow Woods, is expected to affect the distribution and abundance of tree, shrub and herbaceous species in the forest. Osmorrhiza longistylis (sweet-scented cicely) is most commonly found along the deer trails in the forest, suggesting that the sticky seeds of this species are dispersed by deer. Pocket gophers are important herbivores in the forest, eating both above and below-ground plant parts and turning up fresh mounds of soil. These mounds are nutrient rich and may provide temporary habitat 'islands' for ruderal plant species.
Old and recent maps and aerial photographs were consulted to determine recent changes in land use in the area. Interviews with field station staff and local residents provided information on the activities of the previous owners of the properties. Studies conducted at Oxbow Woods were also consulted. European settlement patterns were found in a book on the history of the Westbourne region.
A preliminary archaeological study was conducted in the north meadow of the Oxbow Woods to determine whether the area was used in pre-European contact times.
Floristic information was acquired from herbarium records and previously compiled species lists (Löve and Löve 1954, Walker 1959, Tande and See 1977). The main inventory was conducted by plant collection, sight identification and the ground truthing survey. Dr. Bruce Ford (Department of Botany, University of Manitoba) helped identify a number of Carex species in the forest.
The forest was gridded with five north-south transects and seven east-west transects using compass-to-compass measurements. Intersections and end points were marked with flagging tape. In areas with high shrub cover, it was necessary to flag the transects every 50 m. These reference points were used for navigational purposes and for trail mapping.
Vegetation types were delineated based on species dominance. Only areas distinguishable on aerial photographs, and of sufficient spatial scale to appear on the vegetation map, were considered. The following general type delineation was used:
Other unique features of the area were also noted, such as the location of the only basswood tree found in the forest, and the location of the elm trees.
Provincial topographic maps (1:50,000 scale) and recent aerial photographs were consulted to determine the scale and alignment of the transects. A contour map of the Delta Marsh region (Ducks Unlimited, 1980) was also consulted.
Infrared aerial photographs from 1988 of Oxbow Woods were scanned and exported into the Macintosh program Map II™. After processing, the image was then imported into Canvas™ 3.05 to produce the final vegetation and trail maps and figures.
Reconnaissance helped determine criteria for sighting the trail. It was decided to utilize previously established deer trails to minimize anthropogenic disturbance to the forest. Since deer trails meander through the forest, a systematic approach was necessary to determine which trails to utilize. The transects established for the ground truthing survey were also used in sighting the trail and obtaining a floristic inventory.
It was decided to establish a trail system consisting of two loops (short and long trails) beginning and ending in the same place. The longer trail was established so as to traverse the entire forest perimeter, and to exit into the ecotone between the marsh and the forest. The short trail encompasses the beginning and end of the long trail, but cuts across the central region. Interpretive stops along the trails highlight the major ecological communities and other interesting features in Oxbow Woods. The trail avoids particularly sensitive areas (e.g., Ostrich Fern stands) and areas where excessive clearing of vegetation would have been required.
Preliminary paths were temporarily flagged and various trail options 'tested' by walking along the deer trails and noting landscape and floristic features. Areas of the forest particularly susceptible to trampling were avoided.
Parks Canada and Manitoba Provincial Parks personnel were contacted to determine how to best mark hiking trails. From the various options suggested, a low-impact approach was felt most appropriate. After visiting several provincial trails, it was decided to use corrugated plastic signs attached to trees (as used in Duck Mountain Provincial Park) to mark the Oxbow trail. We utilized triangular markers (20 x 15 cm) cut from commercially-obtained sheets of bright orange corrugated plastic. Interpretive stop numbers were stenciled onto markers with automotive spray paint. Markers were fastened to trees at eye level, using galvanized metal nails.
In some areas, a small amount of clearing was necessary to remove trail hazards. Most hidden logs were removed from the path, and shrub branches at eye and ground level were trimmed. In a few areas, branches obscuring the visibility of markers were trimmed. It was not felt appropriate to 'pave' the trail with wood chips or other material, since a minimal impact result was desired.
The Assiniboine River flowed into Lake Manitoba via the Blind Channel between 4,520 and 3,000 years before present (Rannie et al. 1989). Old maps indicate that several additional former channels of the Assiniboine River occur in the area, but most have been drained and cleared for cultivation.
Cultural material recovered from the archaeological survey suggests that the Oxbow Woods area may have been used as a temporary camp by the indigenous peoples. La Verendrye was the first European to visit the region, building Fort La Reine north of present-day Portage la Prairie in 1738. Permanent European settlement in the region began after 1851 with the establishment of a mission by Reverend Cockran.
The Inkster family built their original homestead on the west side of the Blind Channel, in the small copse to the north-west of the Oxbow Woods. Remnants of this settlement can still be seen today. There was a small native settlement on the east side of the Blind Channel (north of Oxbow Woods). For a period of time the Inksters raised their cattle on this land, with the natives tending the herds, but this relationship soured. The next generation of Inksters built a successful farm east of the southern portion of Oxbow Woods. Sheriff Inkster had built a home on the south-west side of the channel, but no evidence of it remains (he perished in the fire that burned the structure to the ground). Donald Bain purchased the property which became the University Field Station (including the Inkster farm) in the 1920s, building the hunting lodge and outbuildings along the Lake Manitoba shore. The Inksters became his tenants for a time, but they eventually abandoned the farm after rents became unreasonable. The remaining farm buildings are used today for storage.
There is evidence that the Delta Marsh area was an important encampment and hunting area for North American indigenous tribes. Early Europeans camped and hunted in the area after the La Verendrye explorations. Cattle from the Inkster farm grazed the grasslands in the area, and may have grazed the forest understory as well. Donald Bain apparently used the Oxbow Woods for deer hunting, but it appears to have been otherwise undisturbed. There is some very limited evidence of selective logging, but the stumps are only about the size of fence posts.
The University of Manitoba acquired the property in 1966, but Oxbow Woods was not used intensively for research purposes because of poor access. The Assiniboine River diversion, constructed between the University Field Station and Oxbow Woods, was completed in 1976. Today access to Oxbow Woods is most easily made by crossing the diversion and proceeding along the east dike road.
In 1988, the Province of Manitoba established Oxbow Woods and surrounding area as an Ecologically Significant Area. It is rumored that illegal hunting still occurs in Oxbow Wood, but the area is otherwise undisturbed.
The floristic inventory of Oxbow Woods (Appendix A) includes several species not previously listed for the region. Of particular interest is the discovery of a large basswood (Tilia americana) tree in the forest. This specimen represents a slight northern extension for the species (the species occurs along the current Assiniboine River at Portage la Prairie and as far west as Spirit Sands, south of Carberry).
Dominant species in Oxbow Woods have mainly an eastern deciduous forest floristic affinity (Löve 1959). Rudbeckia laciniata often dominates the ground cover of open forest stands in mid to late summer. Amphicarpa bractaeata often forms a 'carpet' in well-shaded areas beginning in mid-June. In other areas the boreal species Aralia nudicaulis dominates the ground cover in early summer. Ozmorrhiza longistylis, Zizia aurea and Galium sp. are common understory components in the spring. Carex sp. (mostly C. assiniboinensis) are also very common. They are practically 'evergreen', and as a result are most conspicuous in the early spring and late fall. Matteuccia struthiopteris (ostrich fern) is locally abundant, forming large monodominant clonal populations in open forest. Showy spring ephemeral species include Trillium cernuum, Aquilegia canadensis, Viola pubescens and Cypripedium calceolus.
Tall shrubs are commonly encountered in many regions of Oxbow Woods. Corylus americana (hazelnut, sometimes found with C. cornuta) is often the dominant shrub. Amelanchier alnifolia (saskatoon) and Prunus virginiana (chokecherry) occur in more open areas of the forest. Viburnum lentago (nannyberry) and Crataegus rotundifolia(hawthorn) are less abundant, and generally occur along forest margins. Symphoricarpos occidentalis, Rubus idaeus and Rosa sp. are common low shrubs in open meadows and glades. Other shrub species occasionally encountered include Salix discolor (pussy willow), Ribes sp. (gooseberries), Prunus pennsylvanica (pin cherry), P. americana (wild plum), Cornus stolonifera (red-osier dogwood), Alnus rugosa (speckled alder), Sambucus pubens (elderberry), various species of Viburnum (including V. trilobum (highbush cranberry), V. rafinesquianum (downy arrow-wood) and V. edule (lowbush cranberry)).
Dutch Elm Disease has killed most of the American elm in Oxbow Woods, but a few mature trees have survived. Some of the largest oaks occur at the extreme south end of the forest, along high banks near the Blind Channel. These may reach 150 years of age, though the mean age of the mature oaks in the area is about 100 years. The oaks typically grow on the higher, drier ground, while the Manitoba maple and green ash are more common in lower, wetter areas. The largest Manitoba maples are no more than about 80 years in age, while most mature green ash are between 70 and 100 years old (though some of the largest ash are about 120 years old).
In the balsam poplar stand, trees average about 60 years in age. The species is successfully regenerating in the area, though there is some evidence of invasion by green ash. This stand is surrounded on three sides by meadow vegetation, and the humus layer is not as well developed as in the mature oak/maple/ash forests.
Very old, dying specimens of trembling aspen (Populus tremuloides) are occasionally encountered in the forest, particularly in open forest areas dominated by green ash and sedges. These trees are not regenerating.
Meadow areas occur at the north end of the forest. Grass species include Poa pratensis, Agrostis stolonifera and Elymus virginicus. The dominant shrubs are snowberry, rose and raspberry. A number of herb species also occur, including Equisetum pratense (meadow horsetail), Thalictrum dasycarpum (tall meadow rue), Glycyrrhiza lepidota (wild licorice), Convolvulus sepium (morning glory) and Circium arvense (Canada thistle). The few scattered trees and tall shrubs in these meadows were killed by a fire that burned through the area in 1991.
This vegetation represents a transition zone between the open meadows and closed forests. These areas have typical meadow vegetation interspersed with forested patches. The shrubs associated with the forested patches are primarily Prunus virginiana (chokecherry) and Corylus americana (hazelnut). Small tree species such as Salix petiolaris and Crataegus rotundifolia are found here as well.
Stands of Matteuccia struthiopteris occur as two large colonies and a few smaller ones mainly on the west side of Oxbow Woods, in open forest areas. This species almost completely dominates the ground cover beginning in June. These stands are easily trampled and should be enjoyed from a distance.
Populus balsamifera almost completely dominates the canopy in this small area. The ground cover is not as diverse as the ash/maple/oak forest, and there is limited soil organic matter accumulation. On the south and east sides of this stand, a transition zone dominated by Salix species occurs adjacent to the open meadows.
These areas occur mainly in the north-central part of the forest. The forest consists of a closed canopy of bur oak and green ash and a subcanopy of Manitoba maple. The shrub layer is poorly developed, and this has resulted in a diverse herb layer. Common understory species include Carex assiniboinensis, Amphicarpa bractaeata (hog peanut), Viola pubescens (yellow violet), Maianthemum canadense (wild lily-of-the-valley), Arenaria laterifolia (grove sandwort), Corydalis aurea (golden corydalis), Aralia nudicaulis (wild sarsparilla), Zizia aurea (golden alexanders) and Scutellaria galericulata (skullcap mint). A few tall shrubs such as Prunus americana (wild plum) and Viburnum lentago (nannyberry) are occasionally encountered. The fern stands are occasional in these open forests.
These areas occur mainly in the central portion of the Oxbow Woods. The open wooded areas are dominated by species typical of the open forest area, but scattered clumps of shrub (mostly hazelnut) are also common.
These areas are found mainly along forest edges and at the south end of Oxbow Woods. The canopy is often dominated by bur oaks, while tall shrub stands form a dense subcanopy. Low light levels result in a poorly developed understory. The dominant shrubs are Corylus americana, Amelanchier alnifolia and Prunus virginiana.
In the spring of 1994 we discovered a single mature specimen of basswood (Tilia americana) in Oxbow Woods. It appears to be the only specimen of the species in this forest, and does not seem to be successfully regenerating. This is the furthest north that naturally-occurring basswood has been found in North America. The trail passes by the tree near the north-west edge of the forest (stop 3).
Appendix C is the map of the trail system and interpretive stops. Appendix D is a map of the major vegetation types described above. These maps are available from at the field station office, or from Dr. Norm Kenkel (Department of Botany, University of Manitoba).
Appendix C shows the trail system (short and long trails) and the locations of interpretive stops. The trail begins at the north meadow and continues south into the transition between forest and meadow. The next highlighted feature is the basswood tree, followed by the fern stands. The trail continues past a stand of American elms, where the short trail splits off to the north-east and the long trail continues south into the more shrubby areas of the forest. The long path loops around at the south end of the forest, exits into the marsh-forest ecotone on the south-east edge, then continues along the east edge of the forest. The path turns north to meet up with the short trail just before entering the balsam poplar stand. Both trails go through the poplar stand and continue to the meadow-forest transition, where the loops are completed.
A major concern in citing and utilizing the trails was to minimize present and future environmental impacts, specifically impacts related to trampling of the vegetation. Dale and Weaver (1974) demonstrated that both woody plants and certain 'delicate' species (e.g., Aquilegia, Thalictrum) are brittle and therefore do not tolerate trampling. They also noted that trailside species receive more light and rainfall, and have less root competition from trees. In an earlier study, Bates (1934) found the cover of genera such as Galium, Viola and Agrostis are unaffected by trampling except in wet conditions. Liddle (1975) found that monocots are generally more resistant to trampling than dicots, but that most dicots can survive moderate trampling. It should be noted that these studies were conducted under trampling intensity levels of >1,000 people per season, which is well above the anticipated use of the Oxbow Wood trails. Even so, the following precautionary measures should be followed to maintain ecosystem and trail integrity:
To facilitate further study of the effects of trampling and trail use, it is recommended that records be kept of the number of persons using the trail each season. The following procedures for continued monitoring of trail use is recommended:
While the trail has been designed to be 'low-maintenance', limited seasonal clearing of obstructions and undergrowth may be required. In the future, markers will require replacement as they are damaged or lost.
The goal of the interpretive trail is to provide cultural, natural and ecological information on Manitoba gallery forests, primarily for educational use. The information provided by this study will be useful to naturalists, educational groups, summer university courses, elementary, secondary and post-secondary students, and researchers.
The interpretive trail pamphlets are a compilation of information gathered over the course of this study. The short pamphlet is designed for general use, as a handy guide through the forest trail and the interpretive stops (Appendix E). It should be useful to various school and naturalist groups. The long pamphlet includes detailed maps, and provides a more in-depth discussion of the interpretive stops and historical information (Appendix F). Both pamphlets are intended for educational use, and to raise public awareness of the status of gallery forests in Manitoba.
From this study, it is clear that a number of additional studies in the Oxbow Woods could be undertaken. Forest tree regeneration is a particularly interesting area of research. What is the fate of basswood in the forest, if the species fails to regenerate? Is secondary succession occurring in the forest, particularly in the balsam poplar stand? Why are some areas of the forest almost shrub-free, while others have a very dense shrub cover? What role does herbivory play in determining floristic composition and species abundance? How important are fire and lightning strikes?
Additional information on the fauna of Oxbow Woods is also required. Bird and mammal species lists (Appendix B) were compiled from information provided by Drs. Spencer Sealy and Rick Riewe (Zoology, University of Manitoba) and from The Manitoba Naturalists Society, but a more detailed survey is required. The invertebrates, mosses and fungi are also deserving of greater study.
Funding for this project was provided by the University of Manitoba as an Environmental Science Cooperative work-term. The partnership and aid of Anke Kirch with field, research and the computer mapping was indispensable. Dave Walker provided expertise with the geographical information systems analyses. Additional field assistance from Kelly Graham, Curt Horning and Kevin Brownlee was invaluable. The hospitality and cooperation of the University Field Station staff was greatly appreciated.
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