A Brief Ecological History of Interior Alaska
Although it is not immediately apparent on today’s landscape, open, grassland vegetation is a venerable part of the ecological history of interior Alaska and the Bering Strait region in general (known as Beringia).
During the periodic glacial advances of the Pleistocene Epoch (over the past two million years or so) the climate has periodically turned colder and sea level has dropped considerably as water was transferred from the ocean through the atmosphere into to terrestrial glaciers the size of continents. With these episodic and major drops in sea level, the Bering Strait receded which opened a vast land connection between Alaska and northern Asia (the Bering Land Bridge). The effect of the concurrent formation of gigantic glaciers to the east and south of Alaska, and a parting of the sea to the west, was a strong increase in the "continentality" in the climate of interior Alaska, as compared to the relatively warm and wet climate of the inter-glacial periods (which was generally similar to today’s climate).
This shift in the climate to cold and dry conditions during glacial advances favored different elements of our flora, as compared to the inter-glacial periods. Scientists who study paleoecology (or ecological history) have learned that the set of plants that dominated interior Alaska’s landscape during these ice-age periods was, in fact, considerably different from the plants we see here today. For example – during each glacial period trees were almost entirely absent from the landscape. Paleoecologists inform us that the vegetation mosaic was much more open, with considerable areas of bare soil, and that grasses, sedges and forbs were predominant over woody plants throughout the landscape. So, instead of tundra, meadows and similar low-statured vegetation being confined to alpine areas and small patches in particularly dry areas in the lowlands as they are today, these vegetation types blanketed the landscape as far as the eye could see during periods of glacial advance. The evidence used to draw these conclusions comes from analysis of fossil pollen and macrofossils (plant parts) taken from cores extracted from lake sediments.
The group of animals that frequented the open ice-age landscape was quite different in many ways to the boreal fauna that we see here today. There were many more grazers (such as Saiga antelope, steppe bison, and wooly mammoth, horses) as compared to today, where we have a preponderance of "browsers" in our mammalian fauna. Large grazing mammals are generally associated with grass-dominated ecosystems such as the great plains of western North America (as they once were) or the cold steppes of Siberia and Mongolia. Bones of these large mammals are still being found, washing out of permafrost along arctic and interior Alaska River terraces. These bones have been dated to full glacial time periods.
So, several lines of evidence including fossil pollen and evidence of the faunal communities tell us that grasses of dry areas have been a significant part of our flora for a very long time, and that they have periodically been very abundant on the landscape of interior Alaska. By examining the geographic and ecological distribution of members of our current flora, we can make educated guesses about which species might have been important components of these very different Pleistocene plant communities. Several plants in the modern flora of Alaska show unique patterns of "disjunction", (or gaps in their geographic range) that suggest that they were formerly much more widespread on the landscape than they are today.
For example, Festuca lenensis is a grass that is relatively rare in Alaska today but was likely much more widespread on the Pleistocene landscape of Alaska. This plant occurs in dry rocky tundra and steppe on both sides of the Bering Strait (in other words in both Russia and Alaska-Yukon) but does not occur in the lower 48 states or southern Canada. We know from it’s habitat preferences that it apparently requires relatively open soils and high light environments (it is intolerant of shade from trees or shrubs) to persist, and it is quite tolerant of drought, cold and windy conditions. This plant is mostly absent from maritime areas and is more abundant in the highly continental areas of Beringia. It is quite likely that this species (whose evolutionary roots are in the flora of north Asia) dispersed into North America on the Bering Land Bridge during glacial intervals. Because Beringia was blocked off from the rest of North America by large ice sheets during the glacial periods, this plant species was not able to easily cross into the rest of North America until the glaciers receded. Once the glaciers did recede, however, the increase in abundance of shrubs and trees that occurred the lowlands across this region in response to the increased warmth and moisture probably kept this species restricted to open habitats in alpine areas where conditions and the structure of the vegetation were generally more favorable for its growth and reproduction.
Did You Know?
The vast landscapes of interior Alaska are changing. Large glaciers are receding, permafrost is melting and woody plants are spreading. Comparison of "then-and-now" photographs and data from major vegetation monitoring should allow detection, understanding and potential management of these changes.