Recent studies hypothesize that when lake levels are low, sand supply to the dunes is decreased and vegetation communities are able to expand and stabilize the soil. During high levels, sand supply increases and plants are buried. Old buried tree trunks sometimes reappear as 'ghost forests' as sands move and shift. Comparison of aerial photos taken in 1938, 1967, and 1992 suggests a significant expansion of jack pine dune forests occurred in response to diminished sand supply during a 100-year fall in lake level, 1840-1940.