Low-center tundra polygons usually feature a small pond in the center of each, while high-center polygons are well-drained and surrounded by a trough or moat.
My small group and I had just finished paddling down the Aichilik River in the Arctic National Wildlife Refuge. We had set up camp spitting distance from the coast. So close that the pressure ridges of sea ice could be clearly seen from our tents on a patch of dry tundra along the river.
We were being picked up by a bush plane on a small gravel bar across the river from our camp, but dense fogged clogged the air for most of the morning of our pickup. Eventually, and just as the fog was peeling back from our camp, the pilot cruised over, checked out the landing area with couple of slow fly-overs and landed lightly onto the gravel. We broke down camp in record time, paddled our laden packrafts over the river to him, loaded up, and within a few minutes were on our way home.
From the air, the coastal tundra below looked, well, weird. The ground was like a giant, irregular honey-comb.
From the air, the coastal tundra below looked, well, weird. The ground was like a giant, irregular honey-comb. Some combs had low centers filled with small ponds, others had high centers, separated by narrow channels of water. I’d seen it before on previous trips, and even knew a little about what caused the formations (ice and permafrost), but I didn’t have a fully developed idea of the forces behind and ecological importance of this polygon tundra.
Tundra such as the polygons beneath the wings of the plane is one manifestation of what is called simply “patterned ground”. There are others: stripes, circles, and frost boils to name a few, but the most conspicuous are tundra polygons.
So yes, permafrost causes these to form, but how, exactly? I looked to a great book on the natural history of the Arctic, “Land of Extremes” by Alex Huryn and John Hobbie for my answer, or part of it anyway.
Imagine for a moment, dry cracked mud in a vacant lot. The mud breaks into fairly evenly sized pieces with more or less straight sides and angles, right? Well similar forces are at work here. During winter, the water-saturated tundra freezes and cracks form, creating adjacent polygons; same process as our drying mud, just on a larger scale. Some of the polygons below us were 15m or more across. The following spring, more water enters the cracks, then freezes again in autumn, forming an ice wedge that expands as it freezes, pushing the soils out and up forming a low ridge. These low ridges then hold water like a cup, creating a small pond. There were hundreds of these below us.
Some of these polygons are high centered. These are created in the same way, but the inside ponds have drained when the low ridges got breached. The cracks around the edges, then served as a drainage ditch. That water flow melted the ice wedge, and we are left with these mesa-like polygons, surrounded by a moat or trough.
Nifty, and interesting to look at from above, but these polygons are a also a super important part of the arctic tundra ecosystem. But we’ll find out more about that on my next post.