From our February Newsletter. For photos, check out “winter burrows” set on http://www.flickr.com/photos/rclimontana.
Prairie Soils in Winter
Winter is a tricky time for hiking on the prairie; it seems like the soil is either covered in snow and ice or the surface has melted into a sloppy prairie gumbo. Voles and pocket gophers and any number of other species in the subnivean zone make the ground even more treacherous, the ground uneven and pock-marked as a result of their digging. But these activities, both of weather and of animal species, are an important part of prairie ecology, especially when it comes to aerating the soil.
As for weather, you may remember learning in school about erosion of rocks caused by physical weathering. (For those who don’t, physical weathering is the action of physical forces, i.e. pressure, on rocks over time, which leads to erosion, i.e. the breakdown of bigger rocks into smaller rocks). Moisture is a big component of this erosion, as water seeps into cracks in rocks and then expands when it freezes, widening the crack and eventually breaking the rock. This same freeze-thaw cycle breaks up soils, as well, breaking them apart to making space for pockets of air and water that are needed for plants to grow. In fact, this freeze-thaw impact is one of the primary ways soils are aerated.
The small mammals that burrow into the soil are a more visible disturbance in the winter soil. When the snow starts to melt, as it does multiple times in a single winter, you can see the remains of these “vole highways” and pocket gopher casts. The small mammals take advantage of the insulating snow above and dig tunnels at the edge between the soil and the snow, which is often warmer than either the air above or the ground below. When the snow melts, these trails act as low points and collect water. Pocket gopher casts are clearly a way for soil to be churned up. They burrow underground for food and pack the dug-up soil into snow tunnels. When the snow melts, a negative of the tunnels remains in the form of dirt.
Especially on overgrazed rangelands, these actions are important to breaking up soil compaction and helping in the return of native plant species.