Biological effects of glaciation on an Alaskan white spruce forestMichael G Loso
Alaska Pacific University
Continent: North America
Country: United States of America
City/Town: Wrangell-St. Elias National Park
UTM coordinates and datum: WGS-84 6816192.5 Northing, 400533.75 Easting
Climate Setting: Boreal
Tectonic setting: Continental Collision Margin
Type: Process, Stratigraphy
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In southern Alaska, the potent combination of an ongoing oceanic / continental collision and abundant snowfall gives rise to some of the most spectacular glaciated mountain scenery in the world. For over a century, geologists interested in geomorphology, tectonics, glaciology, and stratigraphy have struggled to understand and interpret this landscape, but only recently have they begun to consider the impacts of these mountains and glaciers on biological systems. The results of these newer interdisciplinary studies are sometimes surprising.
Consider the effects of glacial geomorphology on white spruce, the most common and most commercially valuable kind of tree among the few species that comprise southern Alaska's boreal forest. White spruce forests cling to the lower slopes of Alaska's mountains, occupying slivers of habitable land hemmed in by swamps, rivers, beaches, and cascading glaciers (Figure 1). Because almost all of southern Alaska was covered by glacial ice during the Last Glacial Maximum, around 20,000 years ago, the soils that white spruce grow on are commonly developed from glacial parent materials. The differing properties of glacial till, outwash gravels, and glaciolacustrine sediments, for example, are well recognized by forest ecologists who commonly stratify their study areas on the basis of landforms mapped by glacial geomorphologists.
But the glaciers have more subtle and surprising effects on white spruce than just the soils and landforms they leave behind. These effects are, perhaps surprisingly, good for the forest.
The first example involves fire. White spruce forests commonly burn regularly in Alaska, with a recurrence interval typically less than a century. Lightning strikes are common, humans have a much shorter and less intensive history of fire suppression than in the forests of the lower 48, and for these reasons, "old growth" white spruce forests are quite naturally uncommon in Alaska. But some white spruce forests are lucky enough to be tucked up next to large valley glaciers like the Kennicott, a 45 km long glacier in the Wrangell Mountains. There, researchers cored hundreds of trees to document the age structure of white spruce growing in a narrow strip between the alpine zone and the glacier margin (Figure 2). They found trees some very old trees, but more important than the sheer age of the trees was the wide range of ages they found. This mixed age distribution (Figure 3) contrasts strongly with the narrow range of germination dates in forests that regenerate from fire, and attests to a long-term lack of forest fires in the valley. Why? The most likely explanation is the presence of persistent, downglacier winds (called katabatic winds) that characterize the valleys of large glaciers. Cold dense air, chilled by the ice, flows downvalley almost every day of summer, protecting the narrow isolated isthmus of trees in this study from the spread of fires that burn regularly in the larger forested areas of the river valley below.
The second example involves a tiny beetle. The spruce bark beetle, Dendroctonus rufipennis, is a native insect that eats the living tissue of white spruce trees. Commonly, beetles live off the tissue of dying or recently dead trees, and have little detrimental impact on white spruce forests. But on some occasions, the beetle populations grow to epidemic proportions and begin to kill healthy, living trees. A statewide epidemic of this nature began in 1990, and during the 1990s killed up to 80% of the mature white spruce on millions of acres of land in southcentral Alaska. But the devastation was not universal: beetles are better at attacking and killing some trees than others, and it turns out that one important factor is the age and growth rate of each individual tree. The beetles tend to avoid young, vigorously growing trees. What does this have to do with glacial geomorphology? A lot, if you consider that most of the glaciers in Alaska expanded slightly during a cool period, known as the Little Ice Age, that ended around A.D. 1850. As a consequence, glaciers like the Kennicott commonly have a thin ring of recently exposed glacial moraine around the margins of their lower reaches, and through primary succession these moraines have been colonized by what are now young, vigorous stands of white spruce. The beetles, it seems, know their glacial history: a map of the beetle outbreak in the Kennicott Valley looks surprisingly similar to a map of the Little Ice Age maximum extent of the Kennicott Glacier, in reverse: beetles spared the young forests on these late Holocene moraines (Figure 4).
The effects of glaciation on spruce forests are of course not universally positive. The bulldozing of spruce forests during periods of glacial expansion is only one obvious example of the sometimes difficult relationship between glaciers and trees. The point, rather, is that biology and geology are related in many ways, and that the relationship is rarely simple. The spruce forests of the Kennicott Valley are only one example. Because we are concerned primarily with the surface of the Earth, where biological processes are most active, geomorphologists, perhaps more than most geologists, must be concerned with this relationship. It is not enough to understand mountain building and glaciation: a modern geomorphologist must understand climate history, meteorology, botany, and at least on rare occasions, the behavior of little beetles.
- Ecology Steering Committee, 1992, Ecological definitions for old-growth forest types in the Alaska region: USDA Forest Service Technical Report R10-TP-28.
- Hard, J.S., 1987, Vulnerability of white spruce with slowly expanding lower boles on dry cold sites to early seasonal attack by spruce beetles in southcentral Alaska: Canadian Journal of Forest Research, v. 17, p. 428-435.
- Loso, M.G., 1998, Productivity, population structure, and subsistence use of a white spruce forest in the Kennicott Valley, Alaska: Mountain Research and Development, v. 18, p. 285-308.
- Wiles, G.C., Jacoby, G.C., Davi, N.K., and McAllister, R.P., 2002, Late Holocene glacier fluctuations in the Wrangell Mountains, Alaska: Geological Society of America Bulletin, v. 114, p. 896-908.