Chapter 4: The Aspen Parkland

About 200 years ago, millions of plains bison spent the summer on the northern part on the Great Plains, in southern Manitoba, Saskatchewan and Alberta and the Central United States. They were grazing on mixed grassland and short grassland (dry steppe). In autumn, most, if not all of these herds, left their summer range on the Great Plains and went onto their winter range. The western bison herds went westward into the aspen parkland of the foothills of the Rocky Mountains of Alberta and Montana. Further west, in the higher parts of the Rocky Mountains, the wood bison was living, also known as the Mountain Bison.

The bison herds on the summer pasture, in the middle of the northern Great Plains, wandered in autumn northward into the Aspen Parkland and Boreal Mixed Forest of Alberta, and Saskatchewan. They stayed during the winter between the Southern and Northern Saskatchewan River. Further north, the wood bison was grazing. It did not move out onto the Great Plains further south, but had its summer and winter range on the grasslands within this vast forest. And the bison herds in the eastern part of the Great Plains went in autumn eastward, to their winter range in Central Manitoba.

Millions of plains bison were living during the winter in the Aspen Parkland and the southern part of the Boreal Aspen Forest of Western Canada. Why were they able to live there all winter? How much fodder was growing there on their winter range? How large was there the large mammal biomass? In other words: How many bison were able, to live on this winter pasture per square mile? And in what kind of a climate has this food on the winter pasture of the bison grown? What have scientists found out about this?

Edmund S. Telfer and George W. Scotter, Canadian Wildlife Service, Edmonton, Canada report about the "Potential for Game Ranching in Boreal Aspen Forests of Western Canada" in Journal of Range Management 28(3), May 1975 pp. 172-180: "In the boreal mixedwood regions of western Canada, mixed spruce and aspen stands are interspersed with stands of willow and of pure aspen. (1975:172) Extensive wet meadows, dominated by sedges, occur in the boreal mixedwood region of western Canada. Aspen forests have a well-developed shrub understory, yielding on the order of 100 pounds of browse per acre per year. Relatively mild snow conditions in the boreal aspen forests permit general use of all browse-producing areas by cervids during most winters. Grasslands, and wet meadows form excellent range for bison in the boreal mixedwood forest." (1975:172, 173, 179)

"The region contains large native ungulates, including bison (Bison bison), moose (Alces alces), elk (Cervus canadensis), mule deer (Odocoileus hemionus), and white-tailed deer (O. virginianus). The landscape is an undulating plain with low hills, interspersed with small lakes and marshy areas. As a consequence, their winter range (in this boreal mixedwood region) is intimately interspersed with their summer range." (Comment: They cannot move south anymore in spring onto Great Plain, as most of them did about 200 years ago.)

"The aspen forests are rather thin-crowned, when compared to coniferous forests, thus permitting the development of an understory of shrubs, forbs, grasses, and sedges. The aspen is often interspersed with willow (Salix spp.), an excellent food for browsing mammals. In addition, the understory species in the forests and other species of the interspersed openings are also highly palatable. They include sedges, such as Carex atherodes for bison, shrubs, such as Saskatoon serviceberry (Amelanchier alnifolia) and aspen saplings for deer, and grasses, such as bearded wheatgrass (Agropyron subsecundum), slender wheatgrass (A. trachycaulum), northern reedgrass (Calamagrostis inexpansa), fringed brome (Bromus ciliatus), and bluegrass (Poa spp.)." Telfer, E.S. et al. (1975:173)

"Elk Island National Park is located in central Alberta on a dead ice moraine, known as the Beaver Hills. The region is considered to be an outlying area of the mixedwood forest region and is surrounded by aspen parkland. The park is thus a rather typical section of the boreal aspen forests.

"The present area of Elk Island is 75 miles² (194 km²), of which approximately 62.9 miles² (163 km²) are available ungulate range. It consists of two portions, one of approximately 50 miles² (130 km²) north of Highway 16 ...and the other section of over 25 miles² (65 km²) south of the highway. The area to the south of the highway is currently used as a rearing enclosure for rare wood bison (Bison bison athabascae)." Telfer, E.S. et al. (1975:174)

Range Surveys and Carrying Capacity Estimates

How much aboveground vegetation (g DM/m²) dry weight is growing there per year? And how many hoofed animals are able, to live there? How much large mammal biomass does the pasture of Elk Island National Park support?

E. S. Telfer et al.: "A carry-over of 50% of the net yield of the grasses and sedges at the end of the previous growing season was considered desirable. A large amount of grass and sedge was available in the wet meadow type. However, many meadows around the fringes of sloughs and in glacial kettles are small and are seldom used by bison. On the other hand, some of the large wet meadows are heavily used. It therefore seemed advisable, to reduce the weight of usable herbage in the wet meadow type to 25% of the total.

"Winter browse was considered to be the twigs of woody plants, found between the heights of 2 ft (61 cm) and 8 ft (2.44 m) above the ground. Twigs below 2 ft (61 cm) in height would be buried in the snow during an average winter, while little feeding occurred above 8 ft (2.44 m). A utilization factor of 60% for browse was employed in the carrying capacity estimates. The food requirements of the ungulates at Elk Island were assumed to be 2.2 lb/100 lb (998 g/45.36 kg) of estimated body weight. The values for annual yield for meat per mile² in Table 2 represent a harvest of 20% of the biomass." (1975:174, 175)

Standing stock of forage remaining after summer grazing. North of Highway 16,

Elk Island National Park, August 1972, (oven-dry)

Herbage yield

Upland grass 171.91 g/m²

Wet meadow 371.84 g/m²

Shrub meadow 138.99 g/m²

Dense Populus forest 25.09 g/m²

Telfer, E.S. et al. (1975:174) Table 1

Carrying capacity of ungulates on portion of Elk Island National Park,

north of Highway 16.

From: Telfer, E.S. et al. (1975:175) Table 2. *In autumn before harvesting. 1.00 animal unit (AU) = 450 kg average body weight: bison. The projected carrying capacity for Elk Island National Park of 5708 kg/km² is relatively high, even when compared to such famous big game areas as East Africa and the more southerly part of the Great Plains of the United States. Telfer, E. S. et al. (1975:178)

Climate at Edmonton, west of Elk Island National Park,

53°34´ N, 113°31´W, elevation 206 m:

473 mm annual precipitation

33 kcal/m² mean annual solar net radiation at earth’s surface.

Bryson, R.A. and F.K. Hare (1974:116)

Edmonton, Elk Island National Park, compared with other bison ranges and other areas, further north.

After Climatic World Atlas, 1972, Moscow, Leningrad, I. A. Golzberg (1972:67).

Days above 10°C per year

Edmonton, Elk Island National Park, central Alberta, 130 days

Annual 10°C temperature sum per year

Climatic World Atlas (1972:32), Moscow, Leningrad, I. A. Golzberg

Edmonton, Elk Island National Park 1600° 10°C temperature sum per year.

Mean annual solar net radiation at earth’s surface

                    R.A. Bryson and F. H. Hare (1974:116)

Edmonton, Elk Island National Park, central-eastern Alberta, 35 kcal/cm² yr

Edmonton, average annual precipitation 473 mm, Elk Island National Park.

After Bryson and Hare 1974:147.

Critical Plant Mass for Bison

How much fodder is growing in the Aspen Boreal area east of Edmonton, Alberta? How much aboveground vegetation must grow at least per year, so that the bison will be able, to live there? - We find out more about this in, "Foraging Ecology of Bison in Aspen Boreal Habitats" by R. J. Hudson and S. Frank. in Journal of Range Management 40(1), January 1987 p. 71-75 report. They are both professors at the University of Alberta, at Edmonton.

"In this study, forage intake rates and feeding times of North America’s largest wild grazer, the bison (Bison bison), were related to forage biomass during summer and autumn in aspen boreal forest habitats. Irrespective of season, maximum feeding rates of 68 g/min declined by 50%, as forage biomass was reduced to 780 kg/ha (78.0 g/m²). This reduction was due primarily to smaller bite sizes. However, bison compensated by increasing cropping bite rates to more than 60 bites/min on heavily grazed swards. Grazing times increased from 9 h/day in summer to 11 h/day in autumn, offsetting slight decreases in average foraging efficiency. Upland meadows were preferred habitats for grazing, despite relatively low pasture biomass and potential dry matter intake rates." (1987:71)

"We evaluated activity budgets, habitat selection, and foraging behavior of free-ranging bison during June-July, when pastures were green and in September-October, when vegetation had cured. The 2 periods were separated by the peak of the rut during August." (1987:72). The July scans were conducted, when maximum temperatures were 21-25° C. Maximum temperatures during October scans were 10-13°C. Most foraging time was spent on grassy upland meadows and least in poplar forests." (1987:72)

Proportion of grazing time

spent by bison in each habitat in relation to forage biomass

After R. J. Hudson and S. Frank (1987:72) Table 2. Kg/ha changed into g/m².

Foraging Rates

"The maximum consumption rate of bison, feeding on herbaceous vegetation was 67.5 g/min. There was no obvious difference between summer and autumn (Fig. 2). When pasture biomass was 779 kg/ha (77.9 g/m²), consumption rates fell to one half of this maximum. Bite sizes increased logarithmically with forage biomass to almost 1.2 g/bite on pastures, supporting approximately 3,000 kg/ha (300 g/m²)(Fig. 3). Cropping bite rates declined moderately from an observed maximum of 65 bites/min to 46 bites/min over this range of bite sizes" (when only 77.9 gDM/m² was left). 

Foraging Efficiency

"Compared with 68 g/min for bison, maximum rates determined on summer pastures in the same study area were 18 g/min for wapiti (Hudson and Nietfeld 1985) and 23 g/min for moose (Renecker and Hudson 1986). Estimates of over 60 g/min have been obtained for beef cattle in other parts of central Alberta (Arthur 1984)." R. J. Hudson and S. Frank (1987:72)

"Larger herbivores are also expected, to require higher forage biomass, to achieve these maxima (Wickstrom et al. 1984). In this study, bison foraged more efficiently on sparse pastures, than might be expected. In spite of their considerably larger size, the critical biomass of 779 kg/ha (77.9 g/m²) for bison fell within the range of 542 kg/ha (54.2 gDM/m²) (Wickstrom et al. 1984) and 1000 kg/ha (100 gDM/m²) (Hudson and Nietfeld 1985), reported for wapiti in grassland habitats. The high feeding rate of bison is achieved by large bite sizes and rapid bite rates." (1987:73)

"High foraging efficiency permitted bison, to meet their forage requirements without prolonged foraging. Grazing time, observed in this study (8.7-10.7 h/d), were no greater, than those of wapiti (8-13 h/d) on the same pastures (Gates and Hudson 1983). Average foraging rates declined slightly from summer (43 g/min) to autumn (39 g/min). However, extending foraging times by 2 h, permitted bison, to actually increase daily dry matter intakes." R. J. Hudson and S. Frank (1987:75).

"Bison bison, like its immediate ancestor (Bison occidentalis), is a grassland specialist, that seemed to rapidly fill a niche, left vacant by megafaunal extinctions at the Pleistocene-Holocene boundary (McDonald 1981). Modern bison evolved as recently as 5000 years BP, when forests and savannas were replaced by grasslands. They dominated this environment, as evidenced by the abundance of fossilized bones and dense populations, which existed at the time of European exploration and settlement. High foraging efficiency would have permitted the attainment of such high population densities, and indeed the adaptation could have been honed by prolonged existence at high densities." R. J. Hudson and S. Frank (1987:75).

Result: The average adult bison is only able to live, were at least 77.9 g/m² aboveground dry matter has grown per year. The larger the bison is, the larger must also the aboveground plant cover be, on which it is grazing. Because the animal is only able to graze a little faster, than normal, when the pasture is too poor.

Large Bison Bull

How much aboveground fodder must grow at least per year, so that a large bison bull will be able to live there?

G. E. Belovsky is at The University of Michigan School of Natural Resources Ann Arbor, MI, USA. He explains, where the bison is still able to live and where not, in: Oecologia, (Berlin) (1986) 70:35-52, 1986: Study site: The necessary data, to construct linear programming models of herbivore foraging, were collected during the summers (May-Sept.) of 1978-1982 at the National Bison Range, Montana. The Bison Range is an intermountain Palouse prairie, ranging in elevation from 800 to 1600 m above sea level. Over the Bison Range, the average plant biomass of new growth is 189 g/m². - Belovsky, G. E. (1986:36).

Bison Range herbivores during summer: Bison bison, average body weight 636.0 kg. Average energy requirement (2x basal): 74,250.4 KJ/day. (17,738 kcal/day). Belovsky, G. E. (1986:38) Table 3.

"Monocot and dicot gross energy contents were respectively 18.4 KJ/g-dry wt and 20.1 KJ/g-dry wt. For species in a given plant class, these values were found to be fairly constant. ... Crude protein averaged 12.3% over the summer and 17.7% over the period of peak parental investment in reproduction; approximately 6-7% is required for maintenance and 13-16% for growth and reproduction (French et al. 1955). Only during the period of peak parental investment (lactation) might protein be constraining." Belovsky, G. E. (1986:40).

"A maximum size, at which energy demands are not satisfied, may also be found. ... A maximum size, however, does emerge for endotherms (warm-blooded animals) at a size, slightly greater than 10³ kg (= 1000 kg), corresponding with the largest mammal at the Bison Range, a large bull bison at 1250 kg. ... If foraging also sets a maximum size for endothermic herbivores, we must question arguments, that large mammalian herbivores went extinct at the end of the Pleistocene from human hunting (Martin 1973).

"Many of the extinct herbivorous species (= like the mammoth and mastodon) were larger than Bison bison, and the above analysis indicates, that larger herbivores are not energetically capable of surviving in this grassland, which is one of the more productive regions in North America for herbivores. Therefore, the existence today of the larger extinct herbivores would be impossible; i.e., no niche space exists for them. Perhaps, the environment changed at the end of the Pleistocene by becoming more xeric (= drier) (Hester 1967), which might have made the foraging energetics of the larger species untenable." Belovsky, G. E. (1986:46).

Bison cow with calves, a few days old, born about 1, May 1970. There was then still snow on the ground, later as usual. This cow is pushing these calves onto their feet, so that they will not under-cool and die. This has not been observed in other years. Photo by Mike Sample. From: Margaret Mary Meagher, The Bison of Yellowstone National Park (1973:76) Figure 28.

Water and Heat

Besides nutrients, the plant also needs a certain amount of water and heat, to grow properly. How much aboveground dry matter (g DM/m² yr) does 1 mm of precipitation produce per year at the National Bison Range in Montana, in the northwestern USA?

The New Encyclopaedia Britannica Vol. 29 (1997) p. 419 says under "Montana": Average annual precipitation is scant, about 13 inches (330 millimeters)." Over the Bison Range, the average plant biomass of new growth is 189 gDM/m² per year. Belovsky, G. E. (1986:36).

189 gDM/m² yr : 330 mm ppt yr = 0.269696969 gDM/m² yr/1 mm ppt year, on National Bison Range, Montana. This means: 1 millimeter of annual precipitation produces 0.269696969 gram aboveground plant matter (dry weight).

The British zoologist R. M. Laws, University of Cambridge (1970:3) states: The elephant is still able, to live in the semi-desert, where there is at least 300 mm of rain per year. These 300 mm of rain per year are producing in East Africa (Kenya) 255.33 g DM/m² per year. That is 0.8511 g DM/mm yr. 0.8511 : 0.269696969 = 3.15 times.

This means: 1 mm of rain in Kenya, East Africa, produces 3.15 times more aboveground dry matter, than 1 mm of annual precipitation in Montana. Because in Kenya it is much warmer, than in Montana. This shows me: When the mammoth was living on the Great Plains of North America, there had to grow much more food, than now. Much more food (aboveground plant matter) will only be able to grow there, when it is much warmer than now. And the climate of the Great Plains in North America had to be then much wetter than now (not drier!). In other words: When the mammoth was living on the Great Plains of Southern Canada and the northern United States, annual precipitation had to be then much larger than now!

How much aboveground plant matter (g DM/m²) dry weight must grow at least per year, so that the bison would be able to live there the year round? How much do the larger bison bulls need, and how much the bison herd as a whole, with its bulls, cows, and calves? How much above ground plant matter (dry weight) must grow at least, so that the 100 kg bison calf is able to live there, the 200 kg, 300 kg bison? How much aboveground dry matter must grow at least, if we take the whole bison herd? And how much food must grow at least per year, so that the larger bulls would be able to live there? I have drawn the following two values onto millimeter paper, to find out.

Lowest aboveground plant matter (dry wt) per year

for maintenance for large hoofed animals

Body weight of hoofed animals (bison, rhino, elephant, horse, wild ass). The lowest amount of aboveground plant matter dry weight (gDM/m² yr), where the animals is still able to live, where it is still able to maintain its body weight. Bison, average weight, 77.9 gDM/m² (Hudson, R. J. and S. Frank (1987:72, 73). 1250 kg bison bull in National Bison Range in Montana (Belovsky, G. E. (1986:36, 46) needs at least 189 gDM/m² year average aboveground plant matter per year. From 1250 kg body weight upwards to14 000 kg, the critical aboveground plant production I have calculated from the metabolic body weight.

This shows us, where the bison is still able to live, how much aboveground plant matter (g DM/m² yr) must grow there at least per year (marginal bison habitat). If the average of the bison herd is 400 kg (50 percent grown animals, 50 percent calves), then it will need at least 61 gDM/m² per year. If the average body weight of the animals of the bison herd is 500 kg per animal, it will need at least 76 gDM/m² per year. And the 850-kg bison bull needs at least 128 gDM/m² per year.

This agrees quite well with the 77.9 g DM/m², the critical biomass of the bison pasture, (for a mixed herd of bulls, cows and calves), which R. J. Hudson and S. Frank (1987:71, 73) have reported. This agrees also quite well with the amount of annual aboveground plant production (dry weight) for the African elephant: where it still is able to live, and where it will starve to death.

Mountain bison cows with small calves in Hayden Valley, Yellowstone National Park sometime before 1894. That is a southwestern form of the Wood bison, before it was mixed up with the Prairie bison. Photo by John Folsom, a winter-keeper at Canyon. From: Margaret Mary Meagher, The Bison of Yellowstone National Park (1973:15) Fig. 9. This rare photo was taken in spring, shortly after the calves were born, when the last patches of snow on the north-facing mountain slope are melting away. The small calves are playing around.

Large Plant Eaters and Plant-cover in Africa

How much aboveground vegetation must grow at least per year in Africa, so that the larger hoofed animals are able to live there?

David Western reports under the heading, "Linking the ecology of past and present mammal communities", in Fossils in the Making, Anna K. Behrensmeyer and Andrew P. Hill (editors). The University of Chicago Press, Chicago, London. He has studied some large African hoofed animals, of different body weight, and how much aboveground vegetation (dry weight) must grow there at least, so that they will be able to live there.

Animal body weight and plant mass in Africa

After David Western (1980:51), Figure 3.4. Relationship of an animal’s body weight to the average vegetation mass, on which it feeds. Data from the Amboseli ecosystem (Western, D. 1973). *This is the aboveground plant mass (dry weight), on which the animal feeds, like grasses, sedges, terminal twigs, leaves, and so on. That is, the annual aboveground plant production and the vegetation, left over from the year(s) before that, which it is still able to eat, is able to digest. It does not include the wood of the trees and bushes and "dead" plant mass, too low in protein, and too high in fiber.

Body Size and Food Handling

The functional significance of variation in body size among mammals. By T. H. Clutton-Brock and Paul H. Harvey. Advances in the study of mammalian behavior. Edited by John F. Eisenberg and Devra G. Kleiman. Special publication No. 7, The American Society of Mammalogists. Published 11 March 1983

Among herbivores, increased size has several disadvantages in relation to food handling. The larger mouths of bigger species of ungulates limits the ability to select the most nutritious parts of plants (Jarman, 1974) and, as a consequence, they cannot compensate so effectively for seasonal changes in food quality by increasing selectivity. For example, in the Serengeti, the grass grows long and rank by the end of the dry season (= and dries up) and the proportion of leaf available is low. Impala (Aegypceros melampus) and, to a lesser extent, topi (Damalisens corrigum), are able to pick green leaves from short grass or bushes, whereas buffalo cannot and thus eat a higher proportion of coarse grass. Thus, the quality of the food of buffalo is more strongly influenced by seasonal changes in the average quality of the standing crop (Fig. 4).

The large mouths of bigger herbivores apparently also make difficult grazing on very short-cropped swards and this, too, may restrict the ability to select high quality food during periods of shortage. Experimental evidence from sheep supports this effect. On swards with a tiller length of 10 cm, yearling sheep show a faster rate of dry matter intake than do lambs (probably because they can take larger bites), whereas on shorter swards, the intake rate of lambs is greater than that of yearlings because their smaller mouths allow them to grasp short grass more effectively (Allden and Whittaker, 1970). – Clutton-Brock, T. H. et al. (1983:641).

Assuming that throughput rate does not change with body size, Owen-Smith (pers. comm.) has calculated that the minimum level of crude protein in the diet required to maintain body weight would be 5% for a 500 kg ruminant, 9% for a 50 kg animal, and 16% for a 5 kg one, to maintain its body weight. - Clutton-Brock, T. H. et al. (1983:647, 648)

Body Size and Interspecific Competition

When large species require higher food density to maintain intake rate, large size may be a disadvantage in indirect competition. If two grazing species of different sizes with minimum food availability requirements of 0.5 and 2.0 kg/dm/m² are introduced to a fenced paddock containing a food supply with a standing crop of 3 kg/dm/m², both populations will graze until overall biomass is reduced below 2 kg/dm/m². After this, the intake rate of the larger species will not meet life-maintaining requirements unless they can exclude the smaller animals by direct interaction.

Some observations suggest that where food supplies are indefensible, smaller grazers tend to exclude larger ones in times of food shortage. On the Serengeti plains, the larger herbivores are the first to leave preferred graze types and move to areas with greater food but lower food quality as standing crops decline during the dry season, whereas at the beginning of the wet season they are the last to return to feed on the first flush of annual grasses. Similar sequences of use of preferred grazing areas have been recorded in other guilds of herbivores (Nicholson et al., 1970). Clutton-Brock, T. H. et al. (1983:653, 654)

Evidence that smaller herbivores can exclude larger ones on a more permanent basis is also provided by other studies. When wapiti (Cervus canadensis) were introduced to New Zealand, they failed to spread in areas where the smaller red deer (Cervus elaphus) were numerous, whereas when the still smaller sika deer (Cervus nippon) was introduced to Ireland, it replaced natural red deer populations in parts of the country (R. Harrington, pers. comm.).

In eighteenth century England, sheep were usually excluded from deer forests because, ‘being such close grazers, they would pick out all the finest grasses and hinder the deer from thriving’ (White, 1789), and in areas of Scotland where sheep and red deer compete, the latter spend little time on areas of Agrostis-Festuca greens for which they show a strong preference where sheep are absent.

Indian rhinoceroses (Rhinoceros unicornis) suffer from competition with domestic buffalo (Laurie, 1978), whereas African buffalo populations in the Ruwenzori National Park increased when the density of hippopotamuses (which, despite their larger size, are notoriously close grazers) were reduced (Eltringham, 1974). And increasing numbers of wildebeest (Connochaetes taurinus) in the Serengeti may have been partly responsible for checking the growth of buffalo populations during the last decade (Sinclair, 1979).

Comparisons of a variety of grazing species also indicate that larger individuals may be less able to tolerate long periods of acute food shortage than smaller ones. In many dimorphic north temperate ungulates [where the male is much larger than the female], males are more likely to die than females during periods of food shortage associated with high population density.

For example, when the reindeer population of St. Mathew Island (= in the Bering Strait) crashed from 6,000 to 42 during a single year, only one of the survivors was a male (Klein, 1968). These effects are presumably a consequence of the larger individuals being unable to gather sufficient food after fat deposits have been exhausted. Clutton-Brock, T. H. et al. (1983:655, 656)