Southcentral Alaska: a Natural Experiment in Plant Succession

To study how vegetation changes over time in an ecosystem, clear a few acres with a bulldozer, then watch over it for 1,000 years or so. Pencil in the approximate dates when grasses pop up, then chart when the grasses give way to shrubs. Years later, dust off the clipboard to mark when the shrubs are pushed out by trees.

A quicker way is to let mother nature do the work, according to Thomas Ager, a geologist with the Global Change and Climate History Team of the U.S. Geological Survey in Denver.

A chunk of Southcentral Alaska the size of Utah is Ager's experimental plot. Instead of a bulldozer, glaciers scraped the landscape there during the last ice age, from about 25,000 to 12,000 years ago. Later, seeds surfing on the wind fell upon the virgin plot of bare mineral soil. The seeds germinated into plants and initiated the greening of Ager's study area--a rough square from the Alaska Range south to Homer Spit, east to Glennallen and west to Cook Inlet across from Anchorage.

During the last glacial period, 95 percent of Southcentral Alaska was coated with glaciers. The area looked much like today's Wrangell-St. Elias Mountains, which many Alaskans get a glimpse of on flights up from Seattle: mountaintops poke though ribbons of ice that look like vanilla ice cream with chocolate swirls.

About 16,000 years ago, glacial ice began receding from Southcentral, leaving behind lakes, rocky moraines, and pulverized rock with barely enough nutrients to support plant life.

But what plants came first? How can someone in the twentieth century rewind natural history to tell the sequence of events since the sixth century?

Ager uses pollen---airborne grains that contain reproductive cells of plants---as archives of ancient plant life. Although it quickly decays when exposed to oxygen, pollen that lands on water and then sinks to the bottom of a lake can endure for thousands of years.

Ager and a team of other researchers sampled the sediment from the bottom of 30 lakes spread out over Southcentral Alaska. Using a hollow tube about two inches in diameter, the scientists pulled up meter-long cores of lake bottom that contained ancient pollen grains.

Scientists are able to determine the vegetative history of an area by marking a spot on the core where, for example, white spruce pollen begins showing up in abundance. That part of the core is carbon dated, giving researchers an idea of when white spruce trees began sprouting around the lake.

Using this technique, Ager found the following:

• When glaciers were shrinking about 12,000 years ago, tundra plants and grasses were the first species to colonize the mineral soil. Dwarf birch, willows and other shrubs followed.
• About 9,500 to 10,000 years ago, the first trees appeared in Southcentral Alaska. Ager believes they were balsam poplar, a tree often called cottonwood in the Interior.
• About 9,100 years ago, white and black spruce trees made it to Southcentral. Ager said the winged seeds of both tree species probably blew down through mountain passes from the unglaciated Interior.
• Once white and black spruce made it through the Alaska Range, the trees migrated southward at a rate of eight-tenths of a kilometer per year. At that rate, it took another 1,000 years for the trees to migrate to the Anchorage area.
• Sitka spruce and mountain hemlock, two coastal tree species, moved slowly up the west coast of North America, arriving in Prince William Sound about 4,000 years ago. The trees didn't make it to Kachemak Bay, on the Kenai Peninsula, until as recently as 2,000 years ago. Ager said a formidable stretch of ice between Yakutat and Icy Bay could have presented a difficult hurdle for wind-riding tree seeds.
• Sitka spruce and mountain hemlock didn't make the jump across Cook Inlet from Anchorage until perhaps 700-800 years ago.

While these changes may seem downright snailish to most people, tree species that migrate almost a kilometer a year are moving faster than a charging grizzly bear when viewed from under a geologist's cap. Good thing they've left a pollen trail to document their sprint.