This striking pair of photos shows the substantial expansion of white spruce trees (Picea glauca) across a subalpine area near the Teklanika River. The clear outward expansion of these patches of spruce trees illustrates how the vegetation itself can propagate changes once the process is begun. The presence of trees affects the wind conditions in local areas, and patches of trees thus tend to cause snow accumulation by slowing-down the scouring winds. Deeper snow insulates the soil around the forest patches, and also protects young seedlings from the harsh conditions above the snowpack, allowing them to become established. Once the seedlings become establish and grow, they have more reserves to withstand harsh conditions, and in turn trap more snow and produce more seeds, further propagating the transformation of the tundra vegetation into forest, that began with the establishment of a few spruce seedlings during (perhaps) a particularly warm interval conducive to tree establishment in the higher elevation zone.