Formerly Frosty Footing Causes Drunken Forests

Science Forum reader Trudy Parcher of Bellingham, Washington, wants to know more about an eye-catching Alaska roadside attraction, the drunken forest.

In a drunken forest, trees--often pipe-cleaner black spruce--tilt in all directions like a group of rowdy revelers stumbling along the street. Unlike pendulous pub patrons, drunken forests aren't caused by beer, but by unique soil conditions found in the north.

Melting permafrost is the most common cause of the drunken forest. Permafrost is ground where the temperature remains below 32 degrees Fahrenheit year-round. According to Tom Osterkamp, a professor of physics with the Geophysical Institute, permafrost is found under 85 percent of Alaska's land area, mostly the northernmost 85 percent. Except for mountain tops, Southeast Alaska is permafrost-free, and Southcentral is nearly so.

Drunken forests can be seen in permafrost-rich areas of the Interior. Osterkamp says drunken forests form when ice-rich permafrost thaws, causing the ground surface to sag. Nearby trees--which have adapted wide, shallow root systems to hold on to what little soil is available above the permafrost table--bow toward the newly formed depressions. Presto, drunken forest.

Actually, it takes a long time for a drunken forest to form. When spruce seeds first drop to the ground and germinate on the future site of a drunken forest, it isn't pock-marked with soupy depressions. The permafrost is still frozen, providing a deceptive foundation. Trees grow normally for perhaps 50 years, until the permafrost gets warm enough to melt and create a thermokarst, the scientific name for a ground slump caused by melting permafrost.

Osterkamp says trees sometimes recover from leaning in a drunken forest by growing back toward the sky. He and his colleagues recently found a spruce tree with a curved, bow-like trunk. By the unique pattern of the tree rings, they determined the tree began its fight to right itself after a thermokarst developed 120 years ago.

The same thing that makes trees tipsy causes problems for those of us who choose to live and work on land underlain by permafrost.

Osterkamp says the Interior is a very unstable region, where much of the permafrost underlying homes and roads is within one degree Celsius of thawing. That's a big deal. If the Interior has a string of years one degree Celsius warmer than normal, houses may begin to tilt. When permafrost below a home melts, it has the effect of eroding away the house's foundation because a big chunk of formerly solid ground is transformed to liquid.

The Interior would need several warmer-than-average years for permafrost to be affected because the ground is a slow conductor of heat. It sometimes takes decades for heat to make its way down to permafrost patches. Once permafrost beneath a road melts, for example, the road reacts by dipping, which creates the roller coaster so familiar to Interior residents.

Osterkamp says humans affect permafrost in many ways, most directly by removing or compacting vegetation, which insulates the ground. Osterkamp says even as slight a disturbance as a snowshoe hare trail through the woods causes the level of the permafrost below the trail to shrink further from the surface than surrounding permafrost.

Unless they happen to be near a road or a rabbit trail, drunken forests are usually the result of a gradual climate warming that melts permafrost. Landslides and earthquakes also can create drunken forests, but if your car's shock absorbers are pumping like pogo sticks when you drive by a drunken forest, chances are melted permafrost is the culprit.