Frozen Fauna of the Mammoth Steppe: The Story of Blue Babe

by Guthrie, R. Dale

  How the Mammoth Steppe Differed from Tundra and Boreal Forest

  We can use the above discussion to explain why Pleistocene mammals in Alaska were so different. The subsurface reserves of grasses growing on richer soils allow them to get by with less protection against large-mammal herbivores. Such grass can tolerate being grazed. Large grazing mammals are adapted to eating these kinds of plants; leaf tissue with few toxins and comparatively high digestibility is readily converted into large-mammal tissue. Even the dead winter tissue of these plants is energy rich and can be used by horses and bison as winter range.

  It is important to discuss biological adaptations of grass in order to understand the biology of northern Pleistocene bison; the two go together. One characteristic of monocotyledonary plants (grasses and some grasslike plants) is that they grow not from the tip, but from the base, that is, their meristematic tissue is near the ground. These meristems are precious; they contain the highest concentrations of nutrients and are major investments for each plant. A herbivore can bite off a mature stem and leaves without damaging the grass’s meristem. This is why the remaining blade of grass that has been mown or grazed keeps its flat top; new growth is from below. Dicotyledonary plants (forbs and broad-leaf woody plants) do not work this way. Their meristematic tissue is on the twig tip. It is difficult for them to tolerate herbivore grazing as a herbivore almost always eats new growth on the periphery of the shrub or forb. Dicots must either grow so high as to be out of a browser’s reach or they must be rich in toxins, pharmacologically active compounds that disrupt the herbivore’s metabolism. Alkaloids such as caffeine, nicotine, and opium are some familiar examples of toxins plants make to protect themselves from insect and mammalian herbivores.

  Fig. 8.3. Below-ground stores, defense strategy, productivity, and biomass. Plant groups vary in the percentage of above-ground and below-ground biomass. Because grasses have extensive root reserves, they do not have to have such extreme antiherbivory defenses. Woody plants are much more exposed, having a considerable portion of their tissue above ground, and are, therefore, usually better defended.

  Extensive subsurface reserves (fig. 8.3) enable grasses to grow rapidly and replace damaged above-ground tissue, so they do not require elaborate chemical defenses. Grasses sometimes do use silica from the soil as abrasive phytoliths to discourage grazers. Phytoliths produce the sharp roughness felt when running one’s hand the wrong way on a blade of grass; silica is in all soils and is a relatively cheap compound for grasses to produce. Silica deters most plant-eating insects because their chitonous jaws are torn apart by phytoliths. The tough, grass-nipping jaws of grasshoppers are one exception.

  Grasses take the chance of exposing their photosynthetic tissue to many potential herbivores during the short summer season. This evolutionary daring-do occurs mainly because grasses thrive in a seasonal environment; nutrients are exposed above ground for only a short time. In the dry season or winter the plant pulls back nutrients to the roots. Tissue left above ground is dehydrated and of poor quality or is abandoned altogether as standing dead material. In the former case, being grazed is not much of a loss, and in the latter it is a benefit. Removing dead tissue before the next growing season allows the soil to warm quicker and speeds recycling of minerals and nutrients left above ground.

  Many grasses have evolved concurrently with large herbivores, such as bison, and actually use herbivores as their seed dispersers. Grass seeds are produced in profusion on a seed head. Individual seeds are never very large, as anyone knows who has ever planted lawn grass or hay, but grass seeds do contain energy-laden starches and protein-rich germ cells. They are sought after; however, because the seed coat is so resistant, a herbivore must crush it with its teeth or the seed will not otherwise be digested. Invariably the herbivore misses some seeds. Those that survive are deposited in a warm pile of fertilizer, ready to sprout in a choice microhabitat. Thus grasses give something to large herbivores, and the large herbivores, in turn, aid seed dispersal. Some grass species even have strains adapted to intense grazing (lawn forming), which switch to vegetative reproduction, and others that do well with light grazing. Steppe bison lived in a habitat characterized by plants adapted to large grazers, and vice versa. Today vegetation in these same regions is comprised of plants that virtually exclude large herbivores.

  The tundra and boreal forest landscape is thus not simply a product of average annual rainfall and degree days. Vegetation itself affects soil character. The largely toxic insulating plant mat, shielded from high evaporation, promotes permafrost, or at least very cool soils, and limits available nutrients (fig. 8.2). This, in turn, favors the same plants that created those soil conditions. The cycle propels itself; conservative plants on low-nutrient soils must defend themselves against herbivory by large mammals. This largely toxic vegetation limits the species diversity and biomass of the large mammal community.

  Aridity: Key to the Mammoth Steppe

  The factor that could break this cycle of toxic litter, cold summer soils, and conservative plants is moisture. The Mammoth Steppe was more arid. Aridity, like cold or poor soils, is harder on some plants than others. Aridity is hardest on the very plants that dominate Alaska today—conservative forms with most of their mass above ground. Today, few Alaskan plants are stressed by lack of moisture during the growth season. Aridity is a factor only on some steep south slopes, one of the few places grasses and aridity-tolerant forbs now grow.

  As it is, interior Alaska receives very little moisture, around 11 inches (275 mm), technically near the definition of a desert (fig. 8.4). Interior Alaska receives the same annual moisture as west Texas, or the Kalahari Desert, yet one cannot hike comfortably during the summer without rain gear or without getting wet feet. And Alaska has many lakes, muskegs, and rivers in spite of its annual rainfall. The main reason west Texas and the Kalahari are much drier is moisture loss. Desert areas receive tremendous solar input on clear summer days; this combines with wind to dry the surface and pump heat into the soil. In fact, most of these areas have a net loss of moisture over the year, losing more moisture than that gained by rainfall.

  Interior Alaska, like most tundra and boreal forest around the globe, loses little moisture in comparison to that received. Some of this retention of moisture can be explained by the long Fairbanks winter (fig. 8.5) which ties up water as ice and reduces both evaporation and runoff. But long winters are not all the story. In the far north, light summer showers are frequent and skies are often overcast. When skies are clear, thunderheads soon return evaporated moisture to the landscape. Also there are few drying winds. A more proximate factor is shallow permafrost, which restricts percolation and fosters the accumulation of litter and mosses that catch and hold moisture. Evapotranspiration by plants in the present environment of the far north is comparatively low (Chapin 1980).

  A gravel or dirt road that breaks this damp mat dries easily between showers and soon becomes a ribbon of powder dust. It is strange to walk down the dirt road leading to our house, padding in deep, soft dust, and see in the woods on either side the soggy understory of horsetail rash and moss.

  Throughout the far north occasional summer dry spells create ideal situations for forest fires. When woody vegetation above the moss dries out, it is subject to easy ignition. Most fires are started by lightning. Big thunderheads, produced by moist updrafts of warm air, crackle and boom, igniting the toxic resins and terpine hydrocarbons these plants produced to defend themselves against herbivores. Undone by fire, acres and acres of boreal forest and tundra burn when weather is dry and sunny, often giving the best summer days a smoky flavor.

  Fig. 8.4. Precipitation curve for Fairbanks, Alaska. At present, and probably throughout the Holocene, precipitation peaks during July and August, while most snow falls before January. (Data prepared by S. Bowling, Geophysical Institute, for years 1949–80.)

  Fig. 8.5. Fairbanks area temperatures. Temperature varies markedly with altitude and aspect in the Fairbanks, Alaska r
egion; the official record from the weather station at just under 130 m provides this seasonal pattern. Present-day temperatures at Pearl Creek, where Blue Babe was found, would be similar. Glacial temperatures were probably only a few degrees different, on average, but the windier glacial climate would have created exceptionally severe winter wind-chill factors. (Data prepared by S. Bowling, Geophysical Institute, for years 1949–80.)

  Mountain slopes are frequently drier than the lowlands, not because they receive less rainfall or snow, usually they receive more, but because steeper slopes improve drainage even when underlain by shallow permafrost. Mountain slopes have more wind and barren patches that allow evaporation, but there is almost no tradition in Alaska of carrying extra water when hiking. Most rolling ridges and meadowlands are wet.

  Unlike the grassland localities of Johannesburg, South Africa, or Salina, Kansas, rain in Fairbanks, Alaska, usually comes as fine mist droplets; it rains many days each summer, but without much accumulation. Locals call it “dry rain.” Pelting rainstorms of the midcontinent grasslands are seldom seen in interior Alaska. One Fairbanks Alaskan tour company had a “money-back guarantee” against rainy weather. Rainy weather was defined as at least one-half inch of rain a day. It sounded good to tourists who did not realize that few of our rainy days are wet enough to qualify.

  In addition to absolute decreases in rainfall, aridity can also be effectively increased by shifting rain to a different time of year and increasing evaporation. Most plains grasslands get their moisture in the spring; virtually none has late summer and fall rains. August is the rainy month in Fairbanks (fig. 8.4). A more windy climate would also promote aridity. Wind and sun can dry soil more than the lack of rain. Together, wind, sun, and dry weather create a dry ground litter.

  Aridity could break this cycle of low nutrients and toxic plants, and dryness could result in a richer soil. Picture interior Alaska at the start of another glacial. The average sun angle is a little lower, the weather a little cooler. Mountains to the south nearer the Pacific coast catch most of the year’s moisture. In the interior it hardly rains or snows and the wind is stronger, probably throughout the entire year. Plants that dominate the north today are stress-tolerant species, but they are tolerant of low amounts of nutrients, not low amounts of water. They are not summer-aridity tolerant. As such, mesic- and hydric-adapted plants would begin to die and cease reproduction, while plant species tolerant of aridity would become more common. At first these plants would have to manage on nutrients from the dead plants they are replacing, but because their above-ground dead tissue is more easily decomposed and easier to bum to the soil surface, the insulating plant litter would eventually thin. The sun, shining all day long in the summer, is making its rounds in normally cloudless skies. As the more exposed soils dried out, summertime soil temperatures would also rise slightly.

  Thus a new cycle begins, one keyed to high evaporation and a deeper summer thaw. Plants now have access to nutrients that were previously frozen in lower soil levels, which favors species with much of their biomass beneath the soil surface. The thinner above-ground biomass of these plants allows the sun to reach the soil; they do not generate such an undigestible, nonburnable litter and humus mat. When fire occurs, and it is more likely to occur and spread widely in such arid conditions, it burns down to the soil. But the new plants have most of their biomass beneath the ground and most can recover from fire. For the same reason they are not so vulnerable to herbivores. Unlike spruce and Labrador tea, these plants are quite edible.

  Grasses of the Mammoth Steppe probably recycled nutrients more rapidly. Nutrients were not tied up for decades or centuries in woody above-ground tissue or undecomposable litter. Despite less above-ground biomass the productivity of such a grassy landscape would be greater. In addition to access to deep nutrients, without the acidic mantle of undecomposed litter, soils could be more basic, shifting ionic availability. Also, nutrients sprinkled over the landscape—loess deposition itself—would enrich soils with freshly ground minerals. Combined with wind, eolian erosion and substrate redistribution would, in turn, have created “new” soils, that is, soils with unleached nutrients, basic in pH, and readily available cations.

  As glaciers flow out away from the mountains, they actively abrade valley walls and bottom, carrying the mountains with them as they come. This newly ground stone is dumped at the terminis where it is washed by summer meltwater into broad river flats stretching for many miles. Strong winds produced by the ice masses whip over these outwash flats, picking up silt-sized particles and carrying them up to hundreds of miles away in thick dust clouds.

  Without cataclysmic change, that is, with the same or slightly more oblique sun angles, with temperatures only a few degrees cooler, and with even less standing biomass but vegetation of a different kind, it is possible to have a much more productive natural rangeland. Such a northern rangeland apparently was usable by a much larger biomass of large mammals and by many different kinds of large mammals, allowing grazers such as mammoths, horses, and bison to become widespread.

  Today, one can see some effects of aridity on well-drained, south-facing slopes of river bluffs, where evaporation is high. (Water moving along the river flats creates a relatively windy condition.) These steep bluffs are often grassy with a peppering of sage, Artemesia, or other arid-tolerant species, which are often uncommon or nonexistent away from these special habitats (fig. 8.6). If the south-facing slopes and windswept grassy river flats were more extensive, they could support elk (Cervus) and bison today. Interestingly, the last sign of the bison and elk in interior Alaska comes from such areas (Guthrie, unpub.). Decreasing summer and autumn rainfall, increasing wind, and clearer summer skies (all interrelated) would allow the vegetation of these south-facing river slopes to expand back up into the hills and across the flats, because it would break the toxic vegetation mat and the shallow annual thaw cycle.

  Fig. 8.6. South-facing slope near Fairbanks, Alaska. The scattered grass, sage, forbs, and exposed soil found on many south-facing buffs in interior Alaska are similar to arid grasslands much farther south. The steep slope increases insolation and allows water to drain rapidly, making the soil warmer and drier.

  I alluded earlier to the different evolutionary strategy of these plant groups. A spruce tree, which stands exposed for a hundred years, is very likely to be eaten unless it can deter herbivores with poisons. More ephemeral plants face a different balance of pressures, a different game of probabilities. A grass with an extensive root system can more easily extract nutrients and can use the same roots to store reserves out of reach of a bison or horse. Once grazed, it can recover relatively rapidly. Grasses have no woody tissue to care for over winter. Growth starts from the ground surface each year. In a sea of similar grasses, it is likely that any particular grass may reach seed stage without being grazed. Grasses can afford a more “free-wheeling” gambling strategy (fig. 8.3). Natural selection has apparently struck a different balance with grasses, favoring species that do well on fertile soils; grasses devote little energy to defensive toxins and instead push for rapid growth and seed production.

  It is because there are such gambler strategies among plants that large herbivorous mammals have evolved. Grazers and ungulates take advantage of plants with fewer toxic defenses, plants that can recover and even benefit from grazing. Like wildfire, bison remove the standing dead sward, allowing sun to reach new plant tissue in the spring. In many cases large grazers also do the plant a favor by dispersing seeds in their feces.

  Pleistocene reconstruction has suffered from an inadvertent misapplication of the “uniformitarianism principle” of vegetational control. As one climbs a mountain, one passes through vegetation zones. These differ from place to place, but in interior Alaska they go something like this: spruce forest and its successional facies at the base, grading into a deciduous zone of alder, on into dwarf birch, then into taller tundra, and on to dwarf tundra plants on the highest felfields. In general, this gradation is c
ontrolled by temperature, that is, by degree days. As such, this same altitudinal zonation is repeated on a latitudinal scale. There is a northern treeline of spruce, after which one finds mainly shrubs, and farther along northern tall tundra, and into the high Arctic a felfield cold desert roughly comparable to the highest alpine vegetation.

  If one sees the Pleistocene differing from the present mainly in degree of cold, or degree days, it is easy to imagine the effects wrought by the reduced insolation during full glacials as simply shifts in altitude and vegetation zones (Ritchie 1984). This is a misconception, however, because it ignores aridity. If temperature alone controls vegetation zones, one must imagine North America as a ladder (fig. 8.7) of temperature isobars, each of which determines vegetational zones as one passes critical thresholds from tundra in the north to tropical forest in the south. However, the actual vegetation map of North America shows major deviations from such a horizontal patterning. Grasslands throw a wrench into this neat system because they run at right angles to this ladder (fig. 8.7). This occurs because grasslands are controlled mainly by aridity rather than temperature. Their north-south alignment corresponds to the rain shadow of the Rocky Mountains, with the shortgrasses abutted against the mountains on the High Plains, with midgrasses farther east and tallgrasses even farther away, breaking up a temperature isobar pattern.

  Fig. 8.7. Steppe orientation in Asia and North America. Vegetation zones are not only affected by temperature; moisture is equally important. These diagrammatic maps of the Soviet Union and the United States show temperature isobars (above) and moisture isobars (below). One can see from these comparisons that the distribution of steppe vegetation is better explained by precipitation than by temperature. Because of the Rocky Mountains, moisture isobars are almost north-south in North America. The east-west tending mountains in southern Asia create the opposite pattern.